<!-- doc/src/sgml/high-availability.sgml -->

<chapter id="high-availability">
 <title>High Availability, Load Balancing, and Replication</title>

 <indexterm><primary>high availability</></>
 <indexterm><primary>failover</></>
 <indexterm><primary>replication</></>
 <indexterm><primary>load balancing</></>
 <indexterm><primary>clustering</></>
 <indexterm><primary>data partitioning</></>

 <para>
  Database servers can work together to allow a second server to
  take over quickly if the primary server fails (high
  availability), or to allow several computers to serve the same
  data (load balancing).  Ideally, database servers could work
  together seamlessly.  Web servers serving static web pages can
  be combined quite easily by merely load-balancing web requests
  to multiple machines.  In fact, read-only database servers can
  be combined relatively easily too.  Unfortunately, most database
  servers have a read/write mix of requests, and read/write servers
  are much harder to combine.  This is because though read-only
  data needs to be placed on each server only once, a write to any
  server has to be propagated to all servers so that future read
  requests to those servers return consistent results.
 </para>

 <para>
  This synchronization problem is the fundamental difficulty for
  servers working together.  Because there is no single solution
  that eliminates the impact of the sync problem for all use cases,
  there are multiple solutions.  Each solution addresses this
  problem in a different way, and minimizes its impact for a specific
  workload.
 </para>

 <para>
  Some solutions deal with synchronization by allowing only one
  server to modify the data.  Servers that can modify data are
  called read/write, <firstterm>master</> or <firstterm>primary</> servers.
  Servers that track changes in the master are called <firstterm>standby</>
  or <firstterm>slave</> servers. A standby server that cannot be connected
  to until it is promoted to a master server is called a <firstterm>warm
  standby</> server, and one that can accept connections and serves read-only
  queries is called a <firstterm>hot standby</> server.
 </para>

 <para>
  Some solutions are synchronous,
  meaning that a data-modifying transaction is not considered
  committed until all servers have committed the transaction.  This
  guarantees that a failover will not lose any data and that all
  load-balanced servers will return consistent results no matter
  which server is queried. In contrast, asynchronous solutions allow some
  delay between the time of a commit and its propagation to the other servers,
  opening the possibility that some transactions might be lost in
  the switch to a backup server, and that load balanced servers
  might return slightly stale results.  Asynchronous communication
  is used when synchronous would be too slow.
 </para>

 <para>
  Solutions can also be categorized by their granularity.  Some solutions
  can deal only with an entire database server, while others allow control
  at the per-table or per-database level.
 </para>

 <para>
  Performance must be considered in any choice.  There is usually a
  trade-off between functionality and
  performance.  For example, a fully synchronous solution over a slow
  network might cut performance by more than half, while an asynchronous
  one might have a minimal performance impact.
 </para>

 <para>
  The remainder of this section outlines various failover, replication,
  and load balancing solutions.  A <ulink
  url="http://www.postgres-r.org/documentation/terms">glossary</ulink> is
  also available.
 </para>

 <sect1 id="different-replication-solutions">
 <title>Comparison of Different Solutions</title>

 <variablelist>

  <varlistentry>
   <term>Shared Disk Failover</term>
   <listitem>

    <para>
     Shared disk failover avoids synchronization overhead by having only one
     copy of the database.  It uses a single disk array that is shared by
     multiple servers.  If the main database server fails, the standby server
     is able to mount and start the database as though it were recovering from
     a database crash.  This allows rapid failover with no data loss.
    </para>

    <para>
     Shared hardware functionality is common in network storage devices.
     Using a network file system is also possible, though care must be
     taken that the file system has full <acronym>POSIX</> behavior (see <xref
     linkend="creating-cluster-nfs">).  One significant limitation of this
     method is that if the shared disk array fails or becomes corrupt, the
     primary and standby servers are both nonfunctional.  Another issue is
     that the standby server should never access the shared storage while
     the primary server is running.
    </para>

   </listitem>
  </varlistentry>

  <varlistentry>
   <term>File System (Block-Device) Replication</term>
   <listitem>

    <para>
     A modified version of shared hardware functionality is file system
     replication, where all changes to a file system are mirrored to a file
     system residing on another computer.  The only restriction is that
     the mirroring must be done in a way that ensures the standby server
     has a consistent copy of the file system &mdash; specifically, writes
     to the standby must be done in the same order as those on the master.
     <productname>DRBD</> is a popular file system replication solution
     for Linux.
    </para>

<!--
https://forge.continuent.org/pipermail/sequoia/2006-November/004070.html

Oracle RAC is a shared disk approach and just send cache invalidations
to other nodes but not actual data. As the disk is shared, data is
only committed once to disk and there is a distributed locking
protocol to make nodes agree on a serializable transactional order.
-->

   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Transaction Log Shipping</term>
   <listitem>

    <para>
     Warm and hot standby servers can be kept current by reading a
     stream of write-ahead log (<acronym>WAL</>)
     records.  If the main server fails, the standby contains
     almost all of the data of the main server, and can be quickly
     made the new master database server.  This can be synchronous or
     asynchronous and can only be done for the entire database server.
    </para>
    <para>
     A standby server can be implemented using file-based log shipping
     (<xref linkend="warm-standby">) or streaming replication (see
     <xref linkend="streaming-replication">), or a combination of both. For
     information on hot standby, see <xref linkend="hot-standby">.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Trigger-Based Master-Standby Replication</term>
   <listitem>

    <para>
     A master-standby replication setup sends all data modification
     queries to the master server.  The master server asynchronously
     sends data changes to the standby server.  The standby can answer
     read-only queries while the master server is running.  The
     standby server is ideal for data warehouse queries.
    </para>

    <para>
     <productname>Slony-I</> is an example of this type of replication, with per-table
     granularity, and support for multiple standby servers.  Because it
     updates the standby server asynchronously (in batches), there is
     possible data loss during fail over.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Statement-Based Replication Middleware</term>
   <listitem>

    <para>
     With statement-based replication middleware, a program intercepts
     every SQL query and sends it to one or all servers.  Each server
     operates independently.  Read-write queries must be sent to all servers,
     so that every server receives any changes.  But read-only queries can be
     sent to just one server, allowing the read workload to be distributed
     among them.
    </para>

    <para>
     If queries are simply broadcast unmodified, functions like
     <function>random()</>, <function>CURRENT_TIMESTAMP</>, and
     sequences can have different values on different servers.
     This is because each server operates independently, and because
     SQL queries are broadcast (and not actual modified rows).  If
     this is unacceptable, either the middleware or the application
     must query such values from a single server and then use those
     values in write queries.  Another option is to use this replication
     option with a traditional master-standby setup, i.e. data modification
     queries are sent only to the master and are propagated to the
     standby servers via master-standby replication, not by the replication
     middleware.  Care must also be taken that all
     transactions either commit or abort on all servers, perhaps
     using two-phase commit (<xref linkend="sql-prepare-transaction">
     and <xref linkend="sql-commit-prepared">.
     <productname>Pgpool-II</> and <productname>Continuent Tungsten</>
     are examples of this type of replication.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Asynchronous Multimaster Replication</term>
   <listitem>

    <para>
     For servers that are not regularly connected, like laptops or
     remote servers, keeping data consistent among servers is a
     challenge.  Using asynchronous multimaster replication, each
     server works independently, and periodically communicates with
     the other servers to identify conflicting transactions.  The
     conflicts can be resolved by users or conflict resolution rules.
     Bucardo is an example of this type of replication.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Synchronous Multimaster Replication</term>
   <listitem>

    <para>
     In synchronous multimaster replication, each server can accept
     write requests, and modified data is transmitted from the
     original server to every other server before each transaction
     commits.  Heavy write activity can cause excessive locking,
     leading to poor performance.  In fact, write performance is
     often worse than that of a single server.  Read requests can
     be sent to any server.  Some implementations use shared disk
     to reduce the communication overhead.  Synchronous multimaster
     replication is best for mostly read workloads, though its big
     advantage is that any server can accept write requests &mdash;
     there is no need to partition workloads between master and
     standby servers, and because the data changes are sent from one
     server to another, there is no problem with non-deterministic
     functions like <function>random()</>.
    </para>

    <para>
     <productname>PostgreSQL</> does not offer this type of replication,
     though <productname>PostgreSQL</> two-phase commit (<xref
     linkend="sql-prepare-transaction"> and <xref
     linkend="sql-commit-prepared">)
     can be used to implement this in application code or middleware.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Commercial Solutions</term>
   <listitem>

    <para>
     Because <productname>PostgreSQL</> is open source and easily
     extended, a number of companies have taken <productname>PostgreSQL</>
     and created commercial closed-source solutions with unique
     failover, replication, and load balancing capabilities.
    </para>
   </listitem>
  </varlistentry>

 </variablelist>

 <para>
  <xref linkend="high-availability-matrix"> summarizes
  the capabilities of the various solutions listed above.
 </para>

 <table id="high-availability-matrix">
  <title>High Availability, Load Balancing, and Replication Feature Matrix</title>
  <tgroup cols="8">
   <thead>
    <row>
     <entry>Feature</entry>
     <entry>Shared Disk Failover</entry>
     <entry>File System Replication</entry>
     <entry>Transaction Log Shipping</entry>
     <entry>Trigger-Based Master-Standby Replication</entry>
     <entry>Statement-Based Replication Middleware</entry>
     <entry>Asynchronous Multimaster Replication</entry>
     <entry>Synchronous Multimaster Replication</entry>
    </row>
   </thead>

   <tbody>

    <row>
     <entry>Most Common Implementation</entry>
     <entry align="center">NAS</entry>
     <entry align="center">DRBD</entry>
     <entry align="center">Streaming Repl.</entry>
     <entry align="center">Slony</entry>
     <entry align="center">pgpool-II</entry>
     <entry align="center">Bucardo</entry>
     <entry align="center"></entry>
    </row>

    <row>
     <entry>Communication Method</entry>
     <entry align="center">shared disk</entry>
     <entry align="center">disk blocks</entry>
     <entry align="center">WAL</entry>
     <entry align="center">table rows</entry>
     <entry align="center">SQL</entry>
     <entry align="center">table rows</entry>
     <entry align="center">table rows and row locks</entry>
    </row>

    <row>
     <entry>No special hardware required</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
    </row>

    <row>
     <entry>Allows multiple master servers</entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
    </row>

    <row>
     <entry>No master server overhead</entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
    </row>

    <row>
     <entry>No waiting for multiple servers</entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center">with sync off</entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
    </row>

    <row>
     <entry>Master failure will never lose data</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">with sync on</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
    </row>

    <row>
     <entry>Standby accept read-only queries</entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center">with hot</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
    </row>

    <row>
     <entry>Per-table granularity</entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
    </row>

    <row>
     <entry>No conflict resolution necessary</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center">&bull;</entry>
     <entry align="center"></entry>
     <entry align="center"></entry>
     <entry align="center">&bull;</entry>
    </row>

   </tbody>
  </tgroup>
 </table>

 <para>
  There are a few solutions that do not fit into the above categories:
 </para>

 <variablelist>

  <varlistentry>
   <term>Data Partitioning</term>
   <listitem>

    <para>
     Data partitioning splits tables into data sets.  Each set can
     be modified by only one server.  For example, data can be
     partitioned by offices, e.g., London and Paris, with a server
     in each office.  If queries combining London and Paris data
     are necessary, an application can query both servers, or
     master/standby replication can be used to keep a read-only copy
     of the other office's data on each server.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term>Multiple-Server Parallel Query Execution</term>
   <listitem>

    <para>
     Many of the above solutions allow multiple servers to handle multiple
     queries, but none allow a single query to use multiple servers to
     complete faster.  This solution allows multiple servers to work
     concurrently on a single query.  It is usually accomplished by
     splitting the data among servers and having each server execute its
     part of the query and return results to a central server where they
     are combined and returned to the user.  <productname>Pgpool-II</>
     has this capability.  Also, this can be implemented using the
     <productname>PL/Proxy</> tool set.
    </para>

   </listitem>
  </varlistentry>

 </variablelist>

 </sect1>


 <sect1 id="warm-standby">
 <title>Log-Shipping Standby Servers</title>


  <para>
   Continuous archiving can be used to create a <firstterm>high
   availability</> (HA) cluster configuration with one or more
   <firstterm>standby servers</> ready to take over operations if the
   primary server fails. This capability is widely referred to as
   <firstterm>warm standby</> or <firstterm>log shipping</>.
  </para>

  <para>
   The primary and standby server work together to provide this capability,
   though the servers are only loosely coupled. The primary server operates
   in continuous archiving mode, while each standby server operates in
   continuous recovery mode, reading the WAL files from the primary. No
   changes to the database tables are required to enable this capability,
   so it offers low administration overhead compared to some other
   replication solutions. This configuration also has relatively low
   performance impact on the primary server.
  </para>

  <para>
   Directly moving WAL records from one database server to another
   is typically described as log shipping. <productname>PostgreSQL</>
   implements file-based log shipping by transferring WAL records
   one file (WAL segment) at a time. WAL files (16MB) can be
   shipped easily and cheaply over any distance, whether it be to an
   adjacent system, another system at the same site, or another system on
   the far side of the globe. The bandwidth required for this technique
   varies according to the transaction rate of the primary server.
   Record-based log shipping is more granular and streams WAL changes
   incrementally over a network connection (see <xref
   linkend="streaming-replication">).
  </para>

  <para>
   It should be noted that log shipping is asynchronous, i.e., the WAL
   records are shipped after transaction commit. As a result, there is a
   window for data loss should the primary server suffer a catastrophic
   failure; transactions not yet shipped will be lost.  The size of the
   data loss window in file-based log shipping can be limited by use of the
   <varname>archive_timeout</varname> parameter, which can be set as low
   as a few seconds.  However such a low setting will
   substantially increase the bandwidth required for file shipping.
   Streaming replication (see <xref linkend="streaming-replication">)
   allows a much smaller window of data loss.
  </para>

  <para>
   Recovery performance is sufficiently good that the standby will
   typically be only moments away from full
   availability once it has been activated. As a result, this is called
   a warm standby configuration which offers high
   availability. Restoring a server from an archived base backup and
   rollforward will take considerably longer, so that technique only
   offers a solution for disaster recovery, not high availability.
   A standby server can also be used for read-only queries, in which case
   it is called a Hot Standby server. See <xref linkend="hot-standby"> for
   more information.
  </para>

  <indexterm zone="high-availability">
   <primary>warm standby</primary>
  </indexterm>

  <indexterm zone="high-availability">
   <primary>PITR standby</primary>
  </indexterm>

  <indexterm zone="high-availability">
   <primary>standby server</primary>
  </indexterm>

  <indexterm zone="high-availability">
   <primary>log shipping</primary>
  </indexterm>

  <indexterm zone="high-availability">
   <primary>witness server</primary>
  </indexterm>

  <indexterm zone="high-availability">
   <primary>STONITH</primary>
  </indexterm>

  <sect2 id="standby-planning">
   <title>Planning</title>

   <para>
    It is usually wise to create the primary and standby servers
    so that they are as similar as possible, at least from the
    perspective of the database server.  In particular, the path names
    associated with tablespaces will be passed across unmodified, so both
    primary and standby servers must have the same mount paths for
    tablespaces if that feature is used.  Keep in mind that if
    <xref linkend="sql-createtablespace">
    is executed on the primary, any new mount point needed for it must
    be created on the primary and all standby servers before the command
    is executed. Hardware need not be exactly the same, but experience shows
    that maintaining two identical systems is easier than maintaining two
    dissimilar ones over the lifetime of the application and system.
    In any case the hardware architecture must be the same &mdash; shipping
    from, say, a 32-bit to a 64-bit system will not work.
   </para>

   <para>
    In general, log shipping between servers running different major
    <productname>PostgreSQL</> release
    levels is not possible. It is the policy of the PostgreSQL Global
    Development Group not to make changes to disk formats during minor release
    upgrades, so it is likely that running different minor release levels
    on primary and standby servers will work successfully. However, no
    formal support for that is offered and you are advised to keep primary
    and standby servers at the same release level as much as possible.
    When updating to a new minor release, the safest policy is to update
    the standby servers first &mdash; a new minor release is more likely
    to be able to read WAL files from a previous minor release than vice
    versa.
   </para>

  </sect2>

  <sect2 id="standby-server-operation">
   <title>Standby Server Operation</title>

   <para>
    In standby mode, the server continuously applies WAL received from the
    master server. The standby server can read WAL from a WAL archive
    (see <xref linkend="restore-command">) or directly from the master
    over a TCP connection (streaming replication). The standby server will
    also attempt to restore any WAL found in the standby cluster's
    <filename>pg_xlog</> directory. That typically happens after a server
    restart, when the standby replays again WAL that was streamed from the
    master before the restart, but you can also manually copy files to
    <filename>pg_xlog</> at any time to have them replayed.
   </para>

   <para>
    At startup, the standby begins by restoring all WAL available in the
    archive location, calling <varname>restore_command</>. Once it
    reaches the end of WAL available there and <varname>restore_command</>
    fails, it tries to restore any WAL available in the <filename>pg_xlog</> directory.
    If that fails, and streaming replication has been configured, the
    standby tries to connect to the primary server and start streaming WAL
    from the last valid record found in archive or <filename>pg_xlog</>. If that fails
    or streaming replication is not configured, or if the connection is
    later disconnected, the standby goes back to step 1 and tries to
    restore the file from the archive again. This loop of retries from the
    archive, <filename>pg_xlog</>, and via streaming replication goes on until the server
    is stopped or failover is triggered by a trigger file.
   </para>

   <para>
    Standby mode is exited and the server switches to normal operation
    when <command>pg_ctl promote</> is run or a trigger file is found
    (<varname>trigger_file</>). Before failover,
    any WAL immediately available in the archive or in <filename>pg_xlog</> will be
    restored, but no attempt is made to connect to the master.
   </para>
  </sect2>

  <sect2 id="preparing-master-for-standby">
   <title>Preparing the Master for Standby Servers</title>

   <para>
    Set up continuous archiving on the primary to an archive directory
    accessible from the standby, as described
    in <xref linkend="continuous-archiving">. The archive location should be
    accessible from the standby even when the master is down, i.e. it should
    reside on the standby server itself or another trusted server, not on
    the master server.
   </para>

   <para>
    If you want to use streaming replication, set up authentication on the
    primary server to allow replication connections from the standby
    server(s); that is, create a role and provide a suitable entry or
    entries in <filename>pg_hba.conf</> with the database field set to
    <literal>replication</>.  Also ensure <varname>max_wal_senders</> is set
    to a sufficiently large value in the configuration file of the primary
    server. If replication slots will be used,
    ensure that <varname>max_replication_slots</varname> is set sufficiently
    high as well.
   </para>

   <para>
    Take a base backup as described in <xref linkend="backup-base-backup">
    to bootstrap the standby server.
   </para>
  </sect2>

  <sect2 id="standby-server-setup">
   <title>Setting Up a Standby Server</title>

   <para>
    To set up the standby server, restore the base backup taken from primary
    server (see <xref linkend="backup-pitr-recovery">). Create a recovery
    command file <filename>recovery.conf</> in the standby's cluster data
    directory, and turn on <varname>standby_mode</>. Set
    <varname>restore_command</> to a simple command to copy files from
    the WAL archive. If you plan to have multiple standby servers for high
    availability purposes, set <varname>recovery_target_timeline</> to
    <literal>latest</>, to make the standby server follow the timeline change
    that occurs at failover to another standby.
   </para>

   <note>
     <para>
     Do not use pg_standby or similar tools with the built-in standby mode
     described here. <varname>restore_command</> should return immediately
     if the file does not exist; the server will retry the command again if
     necessary. See <xref linkend="log-shipping-alternative">
     for using tools like pg_standby.
    </para>
   </note>

   <para>
     If you want to use streaming replication, fill in
     <varname>primary_conninfo</> with a libpq connection string, including
     the host name (or IP address) and any additional details needed to
     connect to the primary server. If the primary needs a password for
     authentication, the password needs to be specified in
     <varname>primary_conninfo</> as well.
   </para>

   <para>
    If you're setting up the standby server for high availability purposes,
    set up WAL archiving, connections and authentication like the primary
    server, because the standby server will work as a primary server after
    failover.
   </para>

   <para>
    If you're using a WAL archive, its size can be minimized using the <xref
    linkend="archive-cleanup-command"> parameter to remove files that are no
    longer required by the standby server.
    The <application>pg_archivecleanup</> utility is designed specifically to
    be used with <varname>archive_cleanup_command</> in typical single-standby
    configurations, see <xref linkend="pgarchivecleanup">.
    Note however, that if you're using the archive for backup purposes, you
    need to retain files needed to recover from at least the latest base
    backup, even if they're no longer needed by the standby.
   </para>

   <para>
    A simple example of a <filename>recovery.conf</> is:
<programlisting>
standby_mode = 'on'
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
restore_command = 'cp /path/to/archive/%f %p'
archive_cleanup_command = 'pg_archivecleanup /path/to/archive %r'
</programlisting>
   </para>

   <para>
    You can have any number of standby servers, but if you use streaming
    replication, make sure you set <varname>max_wal_senders</> high enough in
    the primary to allow them to be connected simultaneously.
   </para>

  </sect2>

  <sect2 id="streaming-replication">
   <title>Streaming Replication</title>

   <indexterm zone="high-availability">
    <primary>Streaming Replication</primary>
   </indexterm>

   <para>
    Streaming replication allows a standby server to stay more up-to-date
    than is possible with file-based log shipping. The standby connects
    to the primary, which streams WAL records to the standby as they're
    generated, without waiting for the WAL file to be filled.
   </para>

   <para>
    Streaming replication is asynchronous by default
    (see <xref linkend="synchronous-replication">), in which case there is
    a small delay between committing a transaction in the primary and the
    changes becoming visible in the standby. This delay is however much
    smaller than with file-based log shipping, typically under one second
    assuming the standby is powerful enough to keep up with the load. With
    streaming replication, <varname>archive_timeout</> is not required to
    reduce the data loss window.
   </para>

   <para>
    If you use streaming replication without file-based continuous
    archiving, the server might recycle old WAL segments before the standby
    has received them.  If this occurs, the standby will need to be
    reinitialized from a new base backup.  You can avoid this by setting
    <varname>wal_keep_segments</> to a value large enough to ensure that
    WAL segments are not recycled too early, or by configuring a replication
    slot for the standby.  If you set up a WAL archive that's accessible from
    the standby, these solutions are not required, since the standby can
    always use the archive to catch up provided it retains enough segments.
   </para>

   <para>
    To use streaming replication, set up a file-based log-shipping standby
    server as described in <xref linkend="warm-standby">. The step that
    turns a file-based log-shipping standby into streaming replication
    standby is setting <varname>primary_conninfo</> setting in the
    <filename>recovery.conf</> file to point to the primary server. Set
    <xref linkend="guc-listen-addresses"> and authentication options
    (see <filename>pg_hba.conf</>) on the primary so that the standby server
    can connect to the <literal>replication</> pseudo-database on the primary
    server (see <xref linkend="streaming-replication-authentication">).
   </para>

   <para>
    On systems that support the keepalive socket option, setting
    <xref linkend="guc-tcp-keepalives-idle">,
    <xref linkend="guc-tcp-keepalives-interval"> and
    <xref linkend="guc-tcp-keepalives-count"> helps the primary promptly
    notice a broken connection.
   </para>

   <para>
    Set the maximum number of concurrent connections from the standby servers
    (see <xref linkend="guc-max-wal-senders"> for details).
   </para>

   <para>
    When the standby is started and <varname>primary_conninfo</> is set
    correctly, the standby will connect to the primary after replaying all
    WAL files available in the archive. If the connection is established
    successfully, you will see a walreceiver process in the standby, and
    a corresponding walsender process in the primary.
   </para>

   <sect3 id="streaming-replication-authentication">
    <title>Authentication</title>
    <para>
     It is very important that the access privileges for replication be set up
     so that only trusted users can read the WAL stream, because it is
     easy to extract privileged information from it.  Standby servers must
     authenticate to the primary as a superuser or an account that has the
     <literal>REPLICATION</> privilege. It is recommended to create a
     dedicated user account with <literal>REPLICATION</> and <literal>LOGIN</>
     privileges for replication. While <literal>REPLICATION</> privilege gives
     very high permissions, it does not allow the user to modify any data on
     the primary system, which the <literal>SUPERUSER</> privilege does.
    </para>

    <para>
     Client authentication for replication is controlled by a
     <filename>pg_hba.conf</> record specifying <literal>replication</> in the
     <replaceable>database</> field. For example, if the standby is running on
     host IP <literal>192.168.1.100</> and the account name for replication
     is <literal>foo</>, the administrator can add the following line to the
     <filename>pg_hba.conf</> file on the primary:

<programlisting>
# Allow the user "foo" from host 192.168.1.100 to connect to the primary
# as a replication standby if the user's password is correctly supplied.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    replication     foo             192.168.1.100/32        md5
</programlisting>
    </para>
    <para>
     The host name and port number of the primary, connection user name,
     and password are specified in the <filename>recovery.conf</> file.
     The password can also be set in the <filename>~/.pgpass</> file on the
     standby (specify <literal>replication</> in the <replaceable>database</>
     field).
     For example, if the primary is running on host IP <literal>192.168.1.50</>,
     port <literal>5432</literal>, the account name for replication is
     <literal>foo</>, and the password is <literal>foopass</>, the administrator
     can add the following line to the <filename>recovery.conf</> file on the
     standby:

<programlisting>
# The standby connects to the primary that is running on host 192.168.1.50
# and port 5432 as the user "foo" whose password is "foopass".
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
</programlisting>
    </para>
   </sect3>

   <sect3 id="streaming-replication-monitoring">
    <title>Monitoring</title>
    <para>
     An important health indicator of streaming replication is the amount
     of WAL records generated in the primary, but not yet applied in the
     standby. You can calculate this lag by comparing the current WAL write
     location on the primary with the last WAL location received by the
     standby. They can be retrieved using
     <function>pg_current_xlog_location</> on the primary and the
     <function>pg_last_xlog_receive_location</> on the standby,
     respectively (see <xref linkend="functions-admin-backup-table"> and
     <xref linkend="functions-recovery-info-table"> for details).
     The last WAL receive location in the standby is also displayed in the
     process status of the WAL receiver process, displayed using the
     <command>ps</> command (see <xref linkend="monitoring-ps"> for details).
    </para>
    <para>
     You can retrieve a list of WAL sender processes via the
     <link linkend="monitoring-stats-views-table">
     <literal>pg_stat_replication</></link> view. Large differences between
     <function>pg_current_xlog_location</> and <literal>sent_location</> field
     might indicate that the master server is under heavy load, while
     differences between <literal>sent_location</> and
     <function>pg_last_xlog_receive_location</> on the standby might indicate
     network delay, or that the standby is under heavy load.
    </para>
   </sect3>
  </sect2>

  <sect2 id="streaming-replication-slots">
   <title>Replication Slots</title>
   <indexterm zone="high-availability">
    <primary>Replication Slots</primary>
   </indexterm>
   <para>
    Replication slots provide an automated way to ensure that the master does
    not remove WAL segments until they have been received by all standbys,
    and that the master does not remove rows which could cause a
    <link linkend="hot-standby-conflict">recovery conflict</> even when the
    standby is disconnected.
   </para>
   <para>
    In lieu of using replication slots, it is possible to prevent the removal
    of old WAL segments using <xref linkend="guc-wal-keep-segments">, or by
    storing the segments in an archive using
    <xref linkend="guc-archive-command">.
    However, these methods often result in retaining more WAL segments than
    required, whereas replication slots retain only the number of segments
    known to be needed.  An advantage of these methods is that they bound
    the space requirement for <literal>pg_xlog</>; there is currently no way
    to do this using replication slots.
   </para>
   <para>
    Similarly, <xref linkend="guc-hot-standby-feedback">
    and <xref linkend="guc-vacuum-defer-cleanup-age"> provide protection against
    relevant rows being removed by vacuum, but the former provides no
    protection during any time period when the standby is not connected,
    and the latter often needs to be set to a high value to provide adequate
    protection.  Replication slots overcome these disadvantages.
   </para>
   <sect3 id="streaming-replication-slots-manipulation">
    <title>Querying and manipulating replication slots</title>
    <para>
     Each replication slot has a name, which can contain lower-case letters,
     numbers, and the underscore character.
    </para>
    <para>
     Existing replication slots and their state can be seen in the
     <link linkend="catalog-pg-replication-slots"><structname>pg_replication_slots</structname></link>
     view.
    </para>
    <para>
     Slots can be created and dropped either via the streaming replication
     protocol (see <xref linkend="protocol-replication">) or via SQL
     functions (see <xref linkend="functions-replication">).
    </para>
   </sect3>
   <sect3 id="streaming-replication-slots-config">
    <title>Configuration Example</title>
    <para>
     You can create a replication slot like this:
<programlisting>
postgres=# SELECT * FROM pg_create_physical_replication_slot('node_a_slot');
  slotname   | xlog_position
-------------+---------------
 node_a_slot |

postgres=# SELECT * FROM pg_replication_slots;
  slot_name  | slot_type | datoid | database | active | xmin | restart_lsn
-------------+-----------+--------+----------+--------+------+-------------
 node_a_slot | physical  |        |          | f      |      |
(1 row)
</programlisting>
     To configure the standby to use this slot, <varname>primary_slotname</>
     should be configured in the standby's <filename>recovery.conf</>.
     Here is a simple example:
<programlisting>
standby_mode = 'on'
primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
primary_slotname = 'node_a_slot'
</programlisting>
    </para>
   </sect3>
  </sect2>

  <sect2 id="cascading-replication">
   <title>Cascading Replication</title>

   <indexterm zone="high-availability">
    <primary>Cascading Replication</primary>
   </indexterm>

   <para>
    The cascading replication feature allows a standby server to accept replication
    connections and stream WAL records to other standbys, acting as a relay.
    This can be used to reduce the number of direct connections to the master
    and also to minimize inter-site bandwidth overheads.
   </para>

   <para>
    A standby acting as both a receiver and a sender is known as a cascading
    standby.  Standbys that are more directly connected to the master are known
    as upstream servers, while those standby servers further away are downstream
    servers.  Cascading replication does not place limits on the number or
    arrangement of downstream servers, though each standby connects to only
    one upstream server which eventually links to a single master/primary
    server.
   </para>

   <para>
    A cascading standby sends not only WAL records received from the
    master but also those restored from the archive. So even if the replication
    connection in some upstream connection is terminated, streaming replication
    continues downstream for as long as new WAL records are available.
   </para>

   <para>
    Cascading replication is currently asynchronous. Synchronous replication
    (see <xref linkend="synchronous-replication">) settings have no effect on
    cascading replication at present.
   </para>

   <para>
    Hot Standby feedback propagates upstream, whatever the cascaded arrangement.
   </para>

   <para>
    If an upstream standby server is promoted to become new master, downstream
    servers will continue to stream from the new master if
    <varname>recovery_target_timeline</> is set to <literal>'latest'</>.
   </para>

   <para>
    To use cascading replication, set up the cascading standby so that it can
    accept replication connections (that is, set
    <xref linkend="guc-max-wal-senders"> and <xref linkend="guc-hot-standby">,
    and configure
    <link linkend="auth-pg-hba-conf">host-based authentication</link>).
    You will also need to set <varname>primary_conninfo</> in the downstream
    standby to point to the cascading standby.
   </para>
  </sect2>

  <sect2 id="synchronous-replication">
   <title>Synchronous Replication</title>

   <indexterm zone="high-availability">
    <primary>Synchronous Replication</primary>
   </indexterm>

   <para>
    <productname>PostgreSQL</> streaming replication is asynchronous by
    default. If the primary server
    crashes then some transactions that were committed may not have been
    replicated to the standby server, causing data loss. The amount
    of data loss is proportional to the replication delay at the time of
    failover.
   </para>

   <para>
    Synchronous replication offers the ability to confirm that all changes
    made by a transaction have been transferred to one synchronous standby
    server. This extends the standard level of durability
    offered by a transaction commit. This level of protection is referred
    to as 2-safe replication in computer science theory.
   </para>

   <para>
    When requesting synchronous replication, each commit of a
    write transaction will wait until confirmation is
    received that the commit has been written to the transaction log on disk
    of both the primary and standby server. The only possibility that data
    can be lost is if both the primary and the standby suffer crashes at the
    same time. This can provide a much higher level of durability, though only
    if the sysadmin is cautious about the placement and management of the two
    servers.  Waiting for confirmation increases the user's confidence that the
    changes will not be lost in the event of server crashes but it also
    necessarily increases the response time for the requesting transaction.
    The minimum wait time is the roundtrip time between primary to standby.
   </para>

   <para>
    Read only transactions and transaction rollbacks need not wait for
    replies from standby servers. Subtransaction commits do not wait for
    responses from standby servers, only top-level commits. Long
    running actions such as data loading or index building do not wait
    until the very final commit message. All two-phase commit actions
    require commit waits, including both prepare and commit.
   </para>

   <sect3 id="synchronous-replication-config">
    <title>Basic Configuration</title>

   <para>
    Once streaming replication has been configured, configuring synchronous
    replication requires only one additional configuration step:
    <xref linkend="guc-synchronous-standby-names"> must be set to
    a non-empty value.  <varname>synchronous_commit</> must also be set to
    <literal>on</>, but since this is the default value, typically no change is
    required.  (See <xref linkend="runtime-config-wal-settings"> and
    <xref linkend="runtime-config-replication-master">.)
    This configuration will cause each commit to wait for
    confirmation that the standby has written the commit record to durable
    storage.
    <varname>synchronous_commit</> can be set by individual
    users, so it can be configured in the configuration file, for particular
    users or databases, or dynamically by applications, in order to control
    the durability guarantee on a per-transaction basis.
   </para>

   <para>
    After a commit record has been written to disk on the primary, the
    WAL record is then sent to the standby. The standby sends reply
    messages each time a new batch of WAL data is written to disk, unless
    <varname>wal_receiver_status_interval</> is set to zero on the standby.
    If the standby is the first matching standby, as specified in
    <varname>synchronous_standby_names</> on the primary, the reply
    messages from that standby will be used to wake users waiting for
    confirmation that the commit record has been received. These parameters
    allow the administrator to specify which standby servers should be
    synchronous standbys. Note that the configuration of synchronous
    replication is mainly on the master. Named standbys must be directly
    connected to the master; the master knows nothing about downstream
    standby servers using cascaded replication.
   </para>

   <para>
    Setting <varname>synchronous_commit</> to <literal>remote_write</> will
    cause each commit to wait for confirmation that the standby has received
    the commit record and written it out to its own operating system, but not
    for the data to be flushed to disk on the standby.  This
    setting provides a weaker guarantee of durability than <literal>on</>
    does: the standby could lose the data in the event of an operating system
    crash, though not a <productname>PostgreSQL</> crash.
    However, it's a useful setting in practice
    because it can decrease the response time for the transaction.
    Data loss could only occur if both the primary and the standby crash and
    the database of the primary gets corrupted at the same time.
   </para>

   <para>
    Users will stop waiting if a fast shutdown is requested.  However, as
    when using asynchronous replication, the server will not fully
    shutdown until all outstanding WAL records are transferred to the currently
    connected standby servers.
   </para>

   </sect3>

   <sect3 id="synchronous-replication-performance">
    <title>Planning for Performance</title>

   <para>
    Synchronous replication usually requires carefully planned and placed
    standby servers to ensure applications perform acceptably. Waiting
    doesn't utilize system resources, but transaction locks continue to be
    held until the transfer is confirmed. As a result, incautious use of
    synchronous replication will reduce performance for database
    applications because of increased response times and higher contention.
   </para>

   <para>
    <productname>PostgreSQL</> allows the application developer
    to specify the durability level required via replication. This can be
    specified for the system overall, though it can also be specified for
    specific users or connections, or even individual transactions.
   </para>

   <para>
    For example, an application workload might consist of:
    10% of changes are important customer details, while
    90% of changes are less important data that the business can more
    easily survive if it is lost, such as chat messages between users.
   </para>

   <para>
    With synchronous replication options specified at the application level
    (on the primary) we can offer synchronous replication for the most
    important changes, without slowing down the bulk of the total workload.
    Application level options are an important and practical tool for allowing
    the benefits of synchronous replication for high performance applications.
   </para>

   <para>
    You should consider that the network bandwidth must be higher than
    the rate of generation of WAL data.
   </para>

   </sect3>

   <sect3 id="synchronous-replication-ha">
    <title>Planning for High Availability</title>

   <para>
    Commits made when <varname>synchronous_commit</> is set to <literal>on</>
    or <literal>remote_write</> will wait until the synchronous standby responds. The response
    may never occur if the last, or only, standby should crash.
   </para>

   <para>
    The best solution for avoiding data loss is to ensure you don't lose
    your last remaining synchronous standby. This can be achieved by naming multiple
    potential synchronous standbys using <varname>synchronous_standby_names</>.
    The first named standby will be used as the synchronous standby. Standbys
    listed after this will take over the role of synchronous standby if the
    first one should fail.
   </para>

   <para>
    When a standby first attaches to the primary, it will not yet be properly
    synchronized. This is described as <literal>catchup</> mode. Once
    the lag between standby and primary reaches zero for the first time
    we move to real-time <literal>streaming</> state.
    The catch-up duration may be long immediately after the standby has
    been created. If the standby is shut down, then the catch-up period
    will increase according to the length of time the standby has been down.
    The standby is only able to become a synchronous standby
    once it has reached <literal>streaming</> state.
   </para>

   <para>
    If primary restarts while commits are waiting for acknowledgement, those
    waiting transactions will be marked fully committed once the primary
    database recovers.
    There is no way to be certain that all standbys have received all
    outstanding WAL data at time of the crash of the primary. Some
    transactions may not show as committed on the standby, even though
    they show as committed on the primary. The guarantee we offer is that
    the application will not receive explicit acknowledgement of the
    successful commit of a transaction until the WAL data is known to be
    safely received by the standby.
   </para>

   <para>
    If you really do lose your last standby server then you should disable
    <varname>synchronous_standby_names</> and reload the configuration file
    on the primary server.
   </para>

   <para>
    If the primary is isolated from remaining standby servers you should
    fail over to the best candidate of those other remaining standby servers.
   </para>

   <para>
    If you need to re-create a standby server while transactions are
    waiting, make sure that the commands pg_start_backup() and
    pg_stop_backup() are run in a session with
    <varname>synchronous_commit</> = <literal>off</>, otherwise those
    requests will wait forever for the standby to appear.
   </para>

   </sect3>
  </sect2>
  </sect1>

  <sect1 id="warm-standby-failover">
   <title>Failover</title>

   <para>
    If the primary server fails then the standby server should begin
    failover procedures.
   </para>

   <para>
    If the standby server fails then no failover need take place. If the
    standby server can be restarted, even some time later, then the recovery
    process can also be restarted immediately, taking advantage of
    restartable recovery. If the standby server cannot be restarted, then a
    full new standby server instance should be created.
   </para>

   <para>
    If the primary server fails and the standby server becomes the
    new primary, and then the old primary restarts, you must have
    a mechanism for informing the old primary that it is no longer the primary. This is
    sometimes known as <acronym>STONITH</> (Shoot The Other Node In The Head), which is
    necessary to avoid situations where both systems think they are the
    primary, which will lead to confusion and ultimately data loss.
   </para>

   <para>
    Many failover systems use just two systems, the primary and the standby,
    connected by some kind of heartbeat mechanism to continually verify the
    connectivity between the two and the viability of the primary. It is
    also possible to use a third system (called a witness server) to prevent
    some cases of inappropriate failover, but the additional complexity
    might not be worthwhile unless it is set up with sufficient care and
    rigorous testing.
   </para>

   <para>
    <productname>PostgreSQL</productname> does not provide the system
    software required to identify a failure on the primary and notify
    the standby database server.  Many such tools exist and are well
    integrated with the operating system facilities required for
    successful failover, such as IP address migration.
   </para>

   <para>
    Once failover to the standby occurs, there is only a
    single server in operation. This is known as a degenerate state.
    The former standby is now the primary, but the former primary is down
    and might stay down.  To return to normal operation, a standby server
    must be recreated,
    either on the former primary system when it comes up, or on a third,
    possibly new, system. Once complete, the primary and standby can be
    considered to have switched roles. Some people choose to use a third
    server to provide backup for the new primary until the new standby
    server is recreated,
    though clearly this complicates the system configuration and
    operational processes.
   </para>

   <para>
    So, switching from primary to standby server can be fast but requires
    some time to re-prepare the failover cluster. Regular switching from
    primary to standby is useful, since it allows regular downtime on
    each system for maintenance. This also serves as a test of the
    failover mechanism to ensure that it will really work when you need it.
    Written administration procedures are advised.
   </para>

   <para>
    To trigger failover of a log-shipping standby server,
    run <command>pg_ctl promote</> or create a trigger
    file with the file name and path specified by the <varname>trigger_file</>
    setting in <filename>recovery.conf</>. If you're planning to use
    <command>pg_ctl promote</> to fail over, <varname>trigger_file</> is
    not required. If you're setting up the reporting servers that are
    only used to offload read-only queries from the primary, not for high
    availability purposes, you don't need to promote it.
   </para>
  </sect1>

  <sect1 id="log-shipping-alternative">
   <title>Alternative Method for Log Shipping</title>

   <para>
    An alternative to the built-in standby mode described in the previous
    sections is to use a <varname>restore_command</> that polls the archive location.
    This was the only option available in versions 8.4 and below. In this
    setup, set <varname>standby_mode</> off, because you are implementing
    the polling required for standby operation yourself. See the
    <xref linkend="pgstandby"> module for a reference
    implementation of this.
   </para>

   <para>
    Note that in this mode, the server will apply WAL one file at a
    time, so if you use the standby server for queries (see Hot Standby),
    there is a delay between an action in the master and when the
    action becomes visible in the standby, corresponding the time it takes
    to fill up the WAL file. <varname>archive_timeout</> can be used to make that delay
    shorter. Also note that you can't combine streaming replication with
    this method.
   </para>

   <para>
    The operations that occur on both primary and standby servers are
    normal continuous archiving and recovery tasks. The only point of
    contact between the two database servers is the archive of WAL files
    that both share: primary writing to the archive, standby reading from
    the archive. Care must be taken to ensure that WAL archives from separate
    primary servers do not become mixed together or confused. The archive
    need not be large if it is only required for standby operation.
   </para>

   <para>
    The magic that makes the two loosely coupled servers work together is
    simply a <varname>restore_command</> used on the standby that,
    when asked for the next WAL file, waits for it to become available from
    the primary. The <varname>restore_command</> is specified in the
    <filename>recovery.conf</> file on the standby server. Normal recovery
    processing would request a file from the WAL archive, reporting failure
    if the file was unavailable.  For standby processing it is normal for
    the next WAL file to be unavailable, so the standby must wait for
    it to appear. For files ending in <literal>.backup</> or
    <literal>.history</> there is no need to wait, and a non-zero return
    code must be returned. A waiting <varname>restore_command</> can be
    written as a custom script that loops after polling for the existence of
    the next WAL file. There must also be some way to trigger failover, which
    should interrupt the <varname>restore_command</>, break the loop and
    return a file-not-found error to the standby server. This ends recovery
    and the standby will then come up as a normal server.
   </para>

   <para>
    Pseudocode for a suitable <varname>restore_command</> is:
<programlisting>
triggered = false;
while (!NextWALFileReady() &amp;&amp; !triggered)
{
    sleep(100000L);         /* wait for ~0.1 sec */
    if (CheckForExternalTrigger())
        triggered = true;
}
if (!triggered)
        CopyWALFileForRecovery();
</programlisting>
   </para>

   <para>
    A working example of a waiting <varname>restore_command</> is provided
    in the <xref linkend="pgstandby"> module. It
    should be used as a reference on how to correctly implement the logic
    described above. It can also be extended as needed to support specific
    configurations and environments.
   </para>

   <para>
    The method for triggering failover is an important part of planning
    and design. One potential option is the <varname>restore_command</>
    command.  It is executed once for each WAL file, but the process
    running the <varname>restore_command</> is created and dies for
    each file, so there is no daemon or server process, and
    signals or a signal handler cannot be used. Therefore, the
    <varname>restore_command</> is not suitable to trigger failover.
    It is possible to use a simple timeout facility, especially if
    used in conjunction with a known <varname>archive_timeout</>
    setting on the primary. However, this is somewhat error prone
    since a network problem or busy primary server might be sufficient
    to initiate failover. A notification mechanism such as the explicit
    creation of a trigger file is ideal, if this can be arranged.
   </para>

  <sect2 id="warm-standby-config">
   <title>Implementation</title>

   <para>
    The short procedure for configuring a standby server using this alternative
    method is as follows. For
    full details of each step, refer to previous sections as noted.
    <orderedlist>
     <listitem>
      <para>
       Set up primary and standby systems as nearly identical as
       possible, including two identical copies of
       <productname>PostgreSQL</> at the same release level.
      </para>
     </listitem>
     <listitem>
      <para>
       Set up continuous archiving from the primary to a WAL archive
       directory on the standby server. Ensure that
       <xref linkend="guc-archive-mode">,
       <xref linkend="guc-archive-command"> and
       <xref linkend="guc-archive-timeout">
       are set appropriately on the primary
       (see <xref linkend="backup-archiving-wal">).
      </para>
     </listitem>
     <listitem>
      <para>
       Make a base backup of the primary server (see <xref
       linkend="backup-base-backup">), and load this data onto the standby.
      </para>
     </listitem>
     <listitem>
      <para>
       Begin recovery on the standby server from the local WAL
       archive, using a <filename>recovery.conf</> that specifies a
       <varname>restore_command</> that waits as described
       previously (see <xref linkend="backup-pitr-recovery">).
      </para>
     </listitem>
    </orderedlist>
   </para>

   <para>
    Recovery treats the WAL archive as read-only, so once a WAL file has
    been copied to the standby system it can be copied to tape at the same
    time as it is being read by the standby database server.
    Thus, running a standby server for high availability can be performed at
    the same time as files are stored for longer term disaster recovery
    purposes.
   </para>

   <para>
    For testing purposes, it is possible to run both primary and standby
    servers on the same system. This does not provide any worthwhile
    improvement in server robustness, nor would it be described as HA.
   </para>
  </sect2>

  <sect2 id="warm-standby-record">
   <title>Record-based Log Shipping</title>

   <para>
    It is also possible to implement record-based log shipping using this
    alternative method, though this requires custom development, and changes
    will still only become visible to hot standby queries after a full WAL
    file has been shipped.
   </para>

   <para>
    An external program can call the <function>pg_xlogfile_name_offset()</>
    function (see <xref linkend="functions-admin">)
    to find out the file name and the exact byte offset within it of
    the current end of WAL.  It can then access the WAL file directly
    and copy the data from the last known end of WAL through the current end
    over to the standby servers.  With this approach, the window for data
    loss is the polling cycle time of the copying program, which can be very
    small, and there is no wasted bandwidth from forcing partially-used
    segment files to be archived.  Note that the standby servers'
    <varname>restore_command</> scripts can only deal with whole WAL files,
    so the incrementally copied data is not ordinarily made available to
    the standby servers.  It is of use only when the primary dies &mdash;
    then the last partial WAL file is fed to the standby before allowing
    it to come up.  The correct implementation of this process requires
    cooperation of the <varname>restore_command</> script with the data
    copying program.
   </para>

   <para>
    Starting with <productname>PostgreSQL</> version 9.0, you can use
    streaming replication (see <xref linkend="streaming-replication">) to
    achieve the same benefits with less effort.
   </para>
  </sect2>
 </sect1>

 <sect1 id="hot-standby">
  <title>Hot Standby</title>

  <indexterm zone="high-availability">
   <primary>Hot Standby</primary>
  </indexterm>

   <para>
    Hot Standby is the term used to describe the ability to connect to
    the server and run read-only queries while the server is in archive
    recovery or standby mode. This
    is useful both for replication purposes and for restoring a backup
    to a desired state with great precision.
    The term Hot Standby also refers to the ability of the server to move
    from recovery through to normal operation while users continue running
    queries and/or keep their connections open.
   </para>

   <para>
    Running queries in hot standby mode is similar to normal query operation,
    though there are several usage and administrative differences
    explained below.
   </para>

  <sect2 id="hot-standby-users">
   <title>User's Overview</title>

   <para>
    When the <xref linkend="guc-hot-standby"> parameter is set to true on a
    standby server, it will begin accepting connections once the recovery has
    brought the system to a consistent state.  All such connections are
    strictly read-only; not even temporary tables may be written.
   </para>

   <para>
    The data on the standby takes some time to arrive from the primary server
    so there will be a measurable delay between primary and standby. Running the
    same query nearly simultaneously on both primary and standby might therefore
    return differing results. We say that data on the standby is
    <firstterm>eventually consistent</firstterm> with the primary.  Once the
    commit record for a transaction is replayed on the standby, the changes
    made by that transaction will be visible to any new snapshots taken on
    the standby.  Snapshots may be taken at the start of each query or at the
    start of each transaction, depending on the current transaction isolation
    level.  For more details, see <xref linkend="transaction-iso">.
   </para>

   <para>
    Transactions started during hot standby may issue the following commands:

    <itemizedlist>
     <listitem>
      <para>
       Query access - <command>SELECT</>, <command>COPY TO</>
      </para>
     </listitem>
     <listitem>
      <para>
       Cursor commands - <command>DECLARE</>, <command>FETCH</>, <command>CLOSE</>
      </para>
     </listitem>
     <listitem>
      <para>
       Parameters - <command>SHOW</>, <command>SET</>, <command>RESET</>
      </para>
     </listitem>
     <listitem>
      <para>
       Transaction management commands
        <itemizedlist>
         <listitem>
          <para>
           <command>BEGIN</>, <command>END</>, <command>ABORT</>, <command>START TRANSACTION</>
          </para>
         </listitem>
         <listitem>
          <para>
           <command>SAVEPOINT</>, <command>RELEASE</>, <command>ROLLBACK TO SAVEPOINT</>
          </para>
         </listitem>
         <listitem>
          <para>
           <command>EXCEPTION</> blocks and other internal subtransactions
          </para>
         </listitem>
        </itemizedlist>
      </para>
     </listitem>
     <listitem>
      <para>
       <command>LOCK TABLE</>, though only when explicitly in one of these modes:
       <literal>ACCESS SHARE</>, <literal>ROW SHARE</> or <literal>ROW EXCLUSIVE</>.
      </para>
     </listitem>
     <listitem>
      <para>
       Plans and resources - <command>PREPARE</>, <command>EXECUTE</>,
       <command>DEALLOCATE</>, <command>DISCARD</>
      </para>
     </listitem>
     <listitem>
      <para>
       Plugins and extensions - <command>LOAD</>
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    Transactions started during hot standby will never be assigned a
    transaction ID and cannot write to the system write-ahead log.
    Therefore, the following actions will produce error messages:

    <itemizedlist>
     <listitem>
      <para>
       Data Manipulation Language (DML) - <command>INSERT</>,
       <command>UPDATE</>, <command>DELETE</>, <command>COPY FROM</>,
       <command>TRUNCATE</>.
       Note that there are no allowed actions that result in a trigger
       being executed during recovery.  This restriction applies even to
       temporary tables, because table rows cannot be read or written without
       assigning a transaction ID, which is currently not possible in a
       Hot Standby environment.
      </para>
     </listitem>
     <listitem>
      <para>
       Data Definition Language (DDL) - <command>CREATE</>,
       <command>DROP</>, <command>ALTER</>, <command>COMMENT</>.
       This restriction applies even to temporary tables, because carrying
       out these operations would require updating the system catalog tables.
      </para>
     </listitem>
     <listitem>
      <para>
       <command>SELECT ... FOR SHARE | UPDATE</>, because row locks cannot be
       taken without updating the underlying data files.
      </para>
     </listitem>
     <listitem>
      <para>
       Rules on <command>SELECT</> statements that generate DML commands.
      </para>
     </listitem>
     <listitem>
      <para>
       <command>LOCK</> that explicitly requests a mode higher than <literal>ROW EXCLUSIVE MODE</>.
      </para>
     </listitem>
     <listitem>
      <para>
       <command>LOCK</> in short default form, since it requests <literal>ACCESS EXCLUSIVE MODE</>.
      </para>
     </listitem>
     <listitem>
      <para>
       Transaction management commands that explicitly set non-read-only state:
        <itemizedlist>
         <listitem>
          <para>
            <command>BEGIN READ WRITE</>,
            <command>START TRANSACTION READ WRITE</>
          </para>
         </listitem>
         <listitem>
          <para>
            <command>SET TRANSACTION READ WRITE</>,
            <command>SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE</>
          </para>
         </listitem>
         <listitem>
          <para>
           <command>SET transaction_read_only = off</>
          </para>
         </listitem>
        </itemizedlist>
      </para>
     </listitem>
     <listitem>
      <para>
       Two-phase commit commands - <command>PREPARE TRANSACTION</>,
       <command>COMMIT PREPARED</>, <command>ROLLBACK PREPARED</>
       because even read-only transactions need to write WAL in the
       prepare phase (the first phase of two phase commit).
      </para>
     </listitem>
     <listitem>
      <para>
       Sequence updates - <function>nextval()</>, <function>setval()</>
      </para>
     </listitem>
     <listitem>
      <para>
       <command>LISTEN</>, <command>UNLISTEN</>, <command>NOTIFY</>
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    In normal operation, <quote>read-only</> transactions are allowed to
    update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
    <command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
    restrictions than ordinary read-only sessions.  It is possible that some
    of these restrictions might be loosened in a future release.
   </para>

   <para>
    During hot standby, the parameter <varname>transaction_read_only</> is always
    true and may not be changed.  But as long as no attempt is made to modify
    the database, connections during hot standby will act much like any other
    database connection.  If failover or switchover occurs, the database will
    switch to normal processing mode.  Sessions will remain connected while the
    server changes mode.  Once hot standby finishes, it will be possible to
    initiate read-write transactions (even from a session begun during
    hot standby).
   </para>

   <para>
    Users will be able to tell whether their session is read-only by
    issuing <command>SHOW transaction_read_only</>.  In addition, a set of
    functions (<xref linkend="functions-recovery-info-table">) allow users to
    access information about the standby server. These allow you to write
    programs that are aware of the current state of the database. These
    can be used to monitor the progress of recovery, or to allow you to
    write complex programs that restore the database to particular states.
   </para>
  </sect2>

  <sect2 id="hot-standby-conflict">
   <title>Handling Query Conflicts</title>

   <para>
    The primary and standby servers are in many ways loosely connected. Actions
    on the primary will have an effect on the standby. As a result, there is
    potential for negative interactions or conflicts between them. The easiest
    conflict to understand is performance: if a huge data load is taking place
    on the primary then this will generate a similar stream of WAL records on the
    standby, so standby queries may contend for system resources, such as I/O.
   </para>

   <para>
    There are also additional types of conflict that can occur with Hot Standby.
    These conflicts are <emphasis>hard conflicts</> in the sense that queries
    might need to be canceled and, in some cases, sessions disconnected to resolve them.
    The user is provided with several ways to handle these
    conflicts. Conflict cases include:

      <itemizedlist>
       <listitem>
        <para>
         Access Exclusive locks taken on the primary server, including both
         explicit <command>LOCK</> commands and various <acronym>DDL</>
         actions, conflict with table accesses in standby queries.
        </para>
       </listitem>
       <listitem>
        <para>
         Dropping a tablespace on the primary conflicts with standby queries
         using that tablespace for temporary work files.
        </para>
       </listitem>
       <listitem>
        <para>
         Dropping a database on the primary conflicts with sessions connected
         to that database on the standby.
        </para>
       </listitem>
       <listitem>
        <para>
         Application of a vacuum cleanup record from WAL conflicts with
         standby transactions whose snapshots can still <quote>see</> any of
         the rows to be removed.
        </para>
       </listitem>
       <listitem>
        <para>
         Application of a vacuum cleanup record from WAL conflicts with
         queries accessing the target page on the standby, whether or not
         the data to be removed is visible.
        </para>
       </listitem>
      </itemizedlist>
   </para>

   <para>
    On the primary server, these cases simply result in waiting; and the
    user might choose to cancel either of the conflicting actions.  However,
    on the standby there is no choice: the WAL-logged action already occurred
    on the primary so the standby must not fail to apply it.  Furthermore,
    allowing WAL application to wait indefinitely may be very undesirable,
    because the standby's state will become increasingly far behind the
    primary's.  Therefore, a mechanism is provided to forcibly cancel standby
    queries that conflict with to-be-applied WAL records.
   </para>

   <para>
    An example of the problem situation is an administrator on the primary
    server running <command>DROP TABLE</> on a table that is currently being
    queried on the standby server.  Clearly the standby query cannot continue
    if the <command>DROP TABLE</> is applied on the standby. If this situation
    occurred on the primary, the <command>DROP TABLE</> would wait until the
    other query had finished. But when <command>DROP TABLE</> is run on the
    primary, the primary doesn't have information about what queries are
    running on the standby, so it will not wait for any such standby
    queries. The WAL change records come through to the standby while the
    standby query is still running, causing a conflict.  The standby server
    must either delay application of the WAL records (and everything after
    them, too) or else cancel the conflicting query so that the <command>DROP
    TABLE</> can be applied.
   </para>

   <para>
    When a conflicting query is short, it's typically desirable to allow it to
    complete by delaying WAL application for a little bit; but a long delay in
    WAL application is usually not desirable.  So the cancel mechanism has
    parameters, <xref linkend="guc-max-standby-archive-delay"> and <xref
    linkend="guc-max-standby-streaming-delay">, that define the maximum
    allowed delay in WAL application.  Conflicting queries will be canceled
    once it has taken longer than the relevant delay setting to apply any
    newly-received WAL data.  There are two parameters so that different delay
    values can be specified for the case of reading WAL data from an archive
    (i.e., initial recovery from a base backup or <quote>catching up</> a
    standby server that has fallen far behind) versus reading WAL data via
    streaming replication.
   </para>

   <para>
    In a standby server that exists primarily for high availability, it's
    best to set the delay parameters relatively short, so that the server
    cannot fall far behind the primary due to delays caused by standby
    queries.  However, if the standby server is meant for executing
    long-running queries, then a high or even infinite delay value may be
    preferable.  Keep in mind however that a long-running query could
    cause other sessions on the standby server to not see recent changes
    on the primary, if it delays application of WAL records.
   </para>

   <para>
    Once the delay specified by <varname>max_standby_archive_delay</> or
    <varname>max_standby_streaming_delay</> has been exceeded, conflicting
    queries will be canceled.  This usually results just in a cancellation
    error, although in the case of replaying a <command>DROP DATABASE</>
    the entire conflicting session will be terminated.  Also, if the conflict
    is over a lock held by an idle transaction, the conflicting session is
    terminated (this behavior might change in the future).
   </para>

   <para>
    Canceled queries may be retried immediately (after beginning a new
    transaction, of course).  Since query cancellation depends on
    the nature of the WAL records being replayed, a query that was
    canceled may well succeed if it is executed again.
   </para>

   <para>
    Keep in mind that the delay parameters are compared to the elapsed time
    since the WAL data was received by the standby server.  Thus, the grace
    period allowed to any one query on the standby is never more than the
    delay parameter, and could be considerably less if the standby has already
    fallen behind as a result of waiting for previous queries to complete, or
    as a result of being unable to keep up with a heavy update load.
   </para>

   <para>
    The most common reason for conflict between standby queries and WAL replay
    is <quote>early cleanup</>.  Normally, <productname>PostgreSQL</> allows
    cleanup of old row versions when there are no transactions that need to
    see them to ensure correct visibility of data according to MVCC rules.
    However, this rule can only be applied for transactions executing on the
    master.  So it is possible that cleanup on the master will remove row
    versions that are still visible to a transaction on the standby.
   </para>

   <para>
    Experienced users should note that both row version cleanup and row version
    freezing will potentially conflict with standby queries. Running a manual
    <command>VACUUM FREEZE</> is likely to cause conflicts even on tables with
    no updated or deleted rows.
   </para>

   <para>
    Users should be clear that tables that are regularly and heavily updated
    on the primary server will quickly cause cancellation of longer running
    queries on the standby. In such cases the setting of a finite value for
    <varname>max_standby_archive_delay</> or
    <varname>max_standby_streaming_delay</> can be considered similar to
    setting <varname>statement_timeout</>.
   </para>

   <para>
    Remedial possibilities exist if the number of standby-query cancellations
    is found to be unacceptable.  The first option is to set the parameter
    <varname>hot_standby_feedback</>, which prevents <command>VACUUM</> from
    removing recently-dead rows and so cleanup conflicts do not occur.
    If you do this, you
    should note that this will delay cleanup of dead rows on the primary,
    which may result in undesirable table bloat. However, the cleanup
    situation will be no worse than if the standby queries were running
    directly on the primary server, and you are still getting the benefit of
    off-loading execution onto the standby.
    <varname>max_standby_archive_delay</> must be kept large in this case,
    because delayed WAL files might already contain entries that conflict with
    the desired standby queries.
   </para>

   <para>
    Another option is to increase <xref linkend="guc-vacuum-defer-cleanup-age">
    on the primary server, so that dead rows will not be cleaned up as quickly
    as they normally would be.  This will allow more time for queries to
    execute before they are canceled on the standby, without having to set
    a high <varname>max_standby_streaming_delay</>.  However it is
    difficult to guarantee any specific execution-time window with this
    approach, since <varname>vacuum_defer_cleanup_age</> is measured in
    transactions executed on the primary server.
   </para>

   <para>
    The number of query cancels and the reason for them can be viewed using
    the <structname>pg_stat_database_conflicts</> system view on the standby
    server. The <structname>pg_stat_database</> system view also contains
    summary information.
   </para>
  </sect2>

  <sect2 id="hot-standby-admin">
   <title>Administrator's Overview</title>

   <para>
    If <varname>hot_standby</> is turned <literal>on</> in
    <filename>postgresql.conf</> and there is a <filename>recovery.conf</>
    file present, the server will run in Hot Standby mode.
    However, it may take some time for Hot Standby connections to be allowed,
    because the server will not accept connections until it has completed
    sufficient recovery to provide a consistent state against which queries
    can run.  During this period,
    clients that attempt to connect will be refused with an error message.
    To confirm the server has come up, either loop trying to connect from
    the application, or look for these messages in the server logs:

<programlisting>
LOG:  entering standby mode

... then some time later ...

LOG:  consistent recovery state reached
LOG:  database system is ready to accept read only connections
</programlisting>

    Consistency information is recorded once per checkpoint on the primary.
    It is not possible to enable hot standby when reading WAL
    written during a period when <varname>wal_level</> was not set to
    <literal>hot_standby</> or <literal>logical</> on the primary.  Reaching
    a consistent state can also be delayed in the presence of both of these
    conditions:

      <itemizedlist>
       <listitem>
        <para>
         A write transaction has more than 64 subtransactions
        </para>
       </listitem>
       <listitem>
        <para>
         Very long-lived write transactions
        </para>
       </listitem>
      </itemizedlist>

    If you are running file-based log shipping ("warm standby"), you might need
    to wait until the next WAL file arrives, which could be as long as the
    <varname>archive_timeout</> setting on the primary.
   </para>

   <para>
    The setting of some parameters on the standby will need reconfiguration
    if they have been changed on the primary. For these parameters,
    the value on the standby must
    be equal to or greater than the value on the primary. If these parameters
    are not set high enough then the standby will refuse to start.
    Higher values can then be supplied and the server
    restarted to begin recovery again.  These parameters are:

      <itemizedlist>
       <listitem>
        <para>
         <varname>max_connections</>
        </para>
       </listitem>
       <listitem>
        <para>
         <varname>max_prepared_transactions</>
        </para>
       </listitem>
       <listitem>
        <para>
         <varname>max_locks_per_transaction</>
        </para>
       </listitem>
      </itemizedlist>
   </para>

   <para>
    It is important that the administrator select appropriate settings for
    <xref linkend="guc-max-standby-archive-delay"> and <xref
    linkend="guc-max-standby-streaming-delay">.  The best choices vary
    depending on business priorities.  For example if the server is primarily
    tasked as a High Availability server, then you will want low delay
    settings, perhaps even zero, though that is a very aggressive setting. If
    the standby server is tasked as an additional server for decision support
    queries then it might be acceptable to set the maximum delay values to
    many hours, or even -1 which means wait forever for queries to complete.
   </para>

   <para>
    Transaction status "hint bits" written on the primary are not WAL-logged,
    so data on the standby will likely re-write the hints again on the standby.
    Thus, the standby server will still perform disk writes even though
    all users are read-only; no changes occur to the data values
    themselves.  Users will still write large sort temporary files and
    re-generate relcache info files, so no part of the database
    is truly read-only during hot standby mode.
    Note also that writes to remote databases using
    <application>dblink</application> module, and other operations outside the
    database using PL functions will still be possible, even though the
    transaction is read-only locally.
   </para>

   <para>
    The following types of administration commands are not accepted
    during recovery mode:

      <itemizedlist>
       <listitem>
        <para>
         Data Definition Language (DDL) - e.g. <command>CREATE INDEX</>
        </para>
       </listitem>
       <listitem>
        <para>
         Privilege and Ownership - <command>GRANT</>, <command>REVOKE</>,
         <command>REASSIGN</>
        </para>
       </listitem>
       <listitem>
        <para>
         Maintenance commands - <command>ANALYZE</>, <command>VACUUM</>,
         <command>CLUSTER</>, <command>REINDEX</>
        </para>
       </listitem>
      </itemizedlist>
   </para>

   <para>
    Again, note that some of these commands are actually allowed during
    "read only" mode transactions on the primary.
   </para>

   <para>
    As a result, you cannot create additional indexes that exist solely
    on the standby, nor statistics that exist solely on the standby.
    If these administration commands are needed, they should be executed
    on the primary, and eventually those changes will propagate to the
    standby.
   </para>

   <para>
    <function>pg_cancel_backend()</>
    and <function>pg_terminate_backend()</> will work on user backends,
    but not the Startup process, which performs
    recovery. <structname>pg_stat_activity</structname> does not show an
    entry for the Startup process, nor do recovering transactions show
    as active. As a result, <structname>pg_prepared_xacts</structname>
    is always empty during recovery. If you wish to resolve in-doubt
    prepared transactions, view <literal>pg_prepared_xacts</> on the
    primary and issue commands to resolve transactions there.
   </para>

   <para>
    <structname>pg_locks</structname> will show locks held by backends,
    as normal. <structname>pg_locks</structname> also shows
    a virtual transaction managed by the Startup process that owns all
    <literal>AccessExclusiveLocks</> held by transactions being replayed by recovery.
    Note that the Startup process does not acquire locks to
    make database changes, and thus locks other than <literal>AccessExclusiveLocks</>
    do not show in <structname>pg_locks</structname> for the Startup
    process; they are just presumed to exist.
   </para>

   <para>
    The <productname>Nagios</> plugin <productname>check_pgsql</> will
    work, because the simple information it checks for exists.
    The <productname>check_postgres</> monitoring script will also work,
    though some reported values could give different or confusing results.
    For example, last vacuum time will not be maintained, since no
    vacuum occurs on the standby.  Vacuums running on the primary
    do still send their changes to the standby.
   </para>

   <para>
    WAL file control commands will not work during recovery,
    e.g. <function>pg_start_backup</>, <function>pg_switch_xlog</> etc.
   </para>

   <para>
    Dynamically loadable modules work, including <structname>pg_stat_statements</>.
   </para>

   <para>
    Advisory locks work normally in recovery, including deadlock detection.
    Note that advisory locks are never WAL logged, so it is impossible for
    an advisory lock on either the primary or the standby to conflict with WAL
    replay. Nor is it possible to acquire an advisory lock on the primary
    and have it initiate a similar advisory lock on the standby. Advisory
    locks relate only to the server on which they are acquired.
   </para>

   <para>
    Trigger-based replication systems such as <productname>Slony</>,
    <productname>Londiste</> and <productname>Bucardo</> won't run on the
    standby at all, though they will run happily on the primary server as
    long as the changes are not sent to standby servers to be applied.
    WAL replay is not trigger-based so you cannot relay from the
    standby to any system that requires additional database writes or
    relies on the use of triggers.
   </para>

   <para>
    New OIDs cannot be assigned, though some <acronym>UUID</> generators may still
    work as long as they do not rely on writing new status to the database.
   </para>

   <para>
    Currently, temporary table creation is not allowed during read only
    transactions, so in some cases existing scripts will not run correctly.
    This restriction might be relaxed in a later release. This is
    both a SQL Standard compliance issue and a technical issue.
   </para>

   <para>
    <command>DROP TABLESPACE</> can only succeed if the tablespace is empty.
    Some standby users may be actively using the tablespace via their
    <varname>temp_tablespaces</> parameter. If there are temporary files in the
    tablespace, all active queries are canceled to ensure that temporary
    files are removed, so the tablespace can be removed and WAL replay
    can continue.
   </para>

   <para>
    Running <command>DROP DATABASE</> or <command>ALTER DATABASE ... SET
    TABLESPACE</> on the primary
    will generate a WAL entry that will cause all users connected to that
    database on the standby to be forcibly disconnected. This action occurs
    immediately, whatever the setting of
    <varname>max_standby_streaming_delay</>. Note that
    <command>ALTER DATABASE ... RENAME</> does not disconnect users, which
    in most cases will go unnoticed, though might in some cases cause a
    program confusion if it depends in some way upon database name.
   </para>

   <para>
    In normal (non-recovery) mode, if you issue <command>DROP USER</> or <command>DROP ROLE</>
    for a role with login capability while that user is still connected then
    nothing happens to the connected user - they remain connected. The user cannot
    reconnect however. This behavior applies in recovery also, so a
    <command>DROP USER</> on the primary does not disconnect that user on the standby.
   </para>

   <para>
    The statistics collector is active during recovery. All scans, reads, blocks,
    index usage, etc., will be recorded normally on the standby. Replayed
    actions will not duplicate their effects on primary, so replaying an
    insert will not increment the Inserts column of pg_stat_user_tables.
    The stats file is deleted at the start of recovery, so stats from primary
    and standby will differ; this is considered a feature, not a bug.
   </para>

   <para>
    Autovacuum is not active during recovery.  It will start normally at the
    end of recovery.
   </para>

   <para>
    The background writer is active during recovery and will perform
    restartpoints (similar to checkpoints on the primary) and normal block
    cleaning activities. This can include updates of the hint bit
    information stored on the standby server.
    The <command>CHECKPOINT</> command is accepted during recovery,
    though it performs a restartpoint rather than a new checkpoint.
   </para>
  </sect2>

  <sect2 id="hot-standby-parameters">
   <title>Hot Standby Parameter Reference</title>

   <para>
    Various parameters have been mentioned above in
    <xref linkend="hot-standby-conflict"> and
    <xref linkend="hot-standby-admin">.
   </para>

   <para>
    On the primary, parameters <xref linkend="guc-wal-level"> and
    <xref linkend="guc-vacuum-defer-cleanup-age"> can be used.
    <xref linkend="guc-max-standby-archive-delay"> and
    <xref linkend="guc-max-standby-streaming-delay"> have no effect if set on
    the primary.
   </para>

   <para>
    On the standby, parameters <xref linkend="guc-hot-standby">,
    <xref linkend="guc-max-standby-archive-delay"> and
    <xref linkend="guc-max-standby-streaming-delay"> can be used.
    <xref linkend="guc-vacuum-defer-cleanup-age"> has no effect
    as long as the server remains in standby mode, though it will
    become relevant if the standby becomes primary.
   </para>
  </sect2>

  <sect2 id="hot-standby-caveats">
   <title>Caveats</title>

   <para>
    There are several limitations of Hot Standby.
    These can and probably will be fixed in future releases:

  <itemizedlist>
   <listitem>
    <para>
     Operations on hash indexes are not presently WAL-logged, so
     replay will not update these indexes.
    </para>
   </listitem>
   <listitem>
    <para>
     Full knowledge of running transactions is required before snapshots
     can be taken. Transactions that use large numbers of subtransactions
     (currently greater than 64) will delay the start of read only
     connections until the completion of the longest running write transaction.
     If this situation occurs, explanatory messages will be sent to the server log.
    </para>
   </listitem>
   <listitem>
    <para>
     Valid starting points for standby queries are generated at each
     checkpoint on the master. If the standby is shut down while the master
     is in a shutdown state, it might not be possible to re-enter Hot Standby
     until the primary is started up, so that it generates further starting
     points in the WAL logs.  This situation isn't a problem in the most
     common situations where it might happen. Generally, if the primary is
     shut down and not available anymore, that's likely due to a serious
     failure that requires the standby being converted to operate as
     the new primary anyway.  And in situations where the primary is
     being intentionally taken down, coordinating to make sure the standby
     becomes the new primary smoothly is also standard procedure.
    </para>
   </listitem>
   <listitem>
    <para>
     At the end of recovery, <literal>AccessExclusiveLocks</> held by prepared transactions
     will require twice the normal number of lock table entries. If you plan
     on running either a large number of concurrent prepared transactions
     that normally take <literal>AccessExclusiveLocks</>, or you plan on having one
     large transaction that takes many <literal>AccessExclusiveLocks</>, you are
     advised to select a larger value of <varname>max_locks_per_transaction</>,
     perhaps as much as twice the value of the parameter on
     the primary server. You need not consider this at all if
     your setting of <varname>max_prepared_transactions</> is 0.
    </para>
   </listitem>
   <listitem>
    <para>
     The Serializable transaction isolation level is not yet available in hot
     standby.  (See <xref linkend="xact-serializable"> and
     <xref linkend="serializable-consistency"> for details.)
     An attempt to set a transaction to the serializable isolation level in
     hot standby mode will generate an error.
    </para>
   </listitem>
  </itemizedlist>

   </para>
  </sect2>

 </sect1>

</chapter>
