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<chapter id="failover">
 <title>Failover, Replication, Load Balancing, and Clustering Options</title>

 <indexterm><primary>failover</></>
 <indexterm><primary>replication</></>
 <indexterm><primary>load balancing</></>
 <indexterm><primary>clustering</></>

 <para>
  Database servers can work together to allow a backup server to
  quickly take over if the primary server fails (failover), 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 or "master" server.  Servers with read-only
  data are called backup or "slave" servers.  As you will see below,
  these terms cover a variety of implementations.  Some servers
  are masters of some data sets, and slave of others.  Some slaves
  cannot be accessed until they are changed to master servers,
  while other slaves can reply to read-only queries while they are
  slaves.
 </para>

 <para>
  Some failover and load balancing 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 with no propagation delay. Asynchronous updating has
  a small delay between the time of 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 failover or load balancing
  choice.  There is usually a tradeoff between functionality and
  performance.  For example, a full 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.
 </para>

 <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 backup server
    is able to mount and start the database as though it was recovering from
    a database crash.  This allows rapid failover with no data loss.
   </para>

   <para>
    Shared hardware functionality is common in network storage devices.  One
    significant limitation of this method is that if the shared disk array
    fails or becomes corrupt, the primary and backup servers are both
    nonfunctional.
   </para>
  </listitem>
 </varlistentry>

 <varlistentry>
  <term>Warm Standby Using Point-In-Time Recovery</term>
  <listitem>

   <para>
    A warm standby server (see <xref linkend="warm-standby">) can
    be kept current by reading a stream of write-ahead log (WAL)
    records.  If the main server fails, the warm standby contains
    almost all of the data of the main server, and can be quickly
    made the new master database server.  This is asynchronous and
    can only be done for the entire database server.
   </para>
  </listitem>
 </varlistentry>

 <varlistentry>
  <term>Continuously Running Replication Server</term>
  <listitem>

   <para>
    A continuously running replication server allows the backup server to
    answer read-only queries while the master server is running.  It
    receives a continuous stream of write activity from the master server.
    Because the backup server can be used for read-only database requests,
    it is ideal for data warehouse queries.
   </para>

   <para>
    Slony-I is an example of this type of replication, with per-table
    granularity.  It updates the backup server in batches, so the replication
    is asynchronous and might lose data during a fail over.
   </para>
  </listitem>
 </varlistentry>

 <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.  While London
    and Paris servers have all data records, only London can modify
    London records, and Paris can only modify Paris records.  This
    is similar to the "Continuously Running Replication Server"
    item above, except that instead of having a read/write server
    and a read-only server, each server has a read/write data set
    and a read-only data set.
   </para>

   <para>
    Such partitioning provides both failover and load balancing.  Failover
    is achieved because the data resides on both servers, and this is an
    ideal way to enable failover if the servers share a slow communication
    channel. Load balancing is possible because read requests can go to any
    of the servers, and write requests are split among the servers.  Of
    course, the communication to keep all the servers up-to-date adds
    overhead, so ideally the write load should be low, or localized as in
    the London/Paris example above.
   </para>

    <para>
    Data partitioning is usually handled by application code, though rules
    and triggers can be used to keep the read-only data sets current.  Slony-I
    can also be used in such a setup.  While Slony-I replicates only entire
    tables, London and Paris can be placed in separate tables, and
    inheritance can be used to access both tables using a single table name.
   </para>
  </listitem>
 </varlistentry>

 <varlistentry>
  <term>Query Broadcast Load Balancing</term>
  <listitem>

   <para>
    Query broadcast load balancing is accomplished by having a
    program intercept every SQL query and send it to all servers.
    This is unique because most replication solutions have the write
    server propagate its changes to the other servers.  With query
    broadcasting, each server operates independently.  Read-only
    queries can be sent to a single server because there is no need
    for all servers to process it.
   </para>

   <para>
    One limitation of this solution is that 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, applications must query such values from a
    single server and then use those values in write queries.
    Also, care must be taken that all transactions either commit
    or abort on all servers, perhaps using two-phase commit (<xref
    linkend="sql-prepare-transaction"
    endterm="sql-prepare-transaction-title"> and <xref
    linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">.
    Pgpool is an example of this type of replication.
   </para>
  </listitem>
 </varlistentry>

 <varlistentry>
  <term>Clustering For Load Balancing</term>
  <listitem>

    <para>
    In clustering, 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.  Clustering
    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 read/write and read-only servers.
   </para>

   <para>
    Clustering is implemented by <productname>Oracle</> in their
    <productname><acronym>RAC</></> product.  <productname>PostgreSQL</>
    does not offer this type of load balancing, though
    <productname>PostgreSQL</> two-phase commit (<xref
    linkend="sql-prepare-transaction"
    endterm="sql-prepare-transaction-title"> and <xref
    linkend="sql-commit-prepared" endterm="sql-commit-prepared-title">)
    can be used to implement this in application code or middleware.
   </para>
  </listitem>
 </varlistentry>

 <varlistentry>
  <term>Clustering For Parallel Query Execution</term>
  <listitem>

   <para>
    This allows multiple servers to work concurrently on a single
    query.  One possible way this could work is for the data to be
    split among servers and for each server to execute its part of
    the query and results sent to a central server to be combined
    and returned to the user.  There currently is no
    <productname>PostgreSQL</> open source solution for this.
   </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>

</chapter>
