$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.1 2004/08/01 20:57:59 tgl Exp $

The Transaction System
----------------------

PostgreSQL's transaction system is a three-layer system.  The bottom layer
implements low-level transactions and subtransactions, on top of which rests
the mainloop's control code, which in turn implements user-visible
transactions and savepoints.

The middle layer of code is called by postgres.c before and after the
processing of each query:

		StartTransactionCommand
		CommitTransactionCommand
		AbortCurrentTransaction

Meanwhile, the user can alter the system's state by issuing the SQL commands
BEGIN, COMMIT, ROLLBACK, SAVEPOINT, ROLLBACK TO or RELEASE.  The traffic cop
redirects these calls to the toplevel routines

		BeginTransactionBlock
		EndTransactionBlock
		UserAbortTransactionBlock
		DefineSavepoint
		RollbackToSavepoint
		ReleaseSavepoint

respectively.  Depending on the current state of the system, these functions
call low level functions to activate the real transaction system:

		StartTransaction
		CommitTransaction
		AbortTransaction
		CleanupTransaction
		StartSubTransaction
		CommitSubTransaction
		AbortSubTransaction
		CleanupSubTransaction

Additionally, within a transaction, CommandCounterIncrement is called to
increment the command counter, which allows future commands to "see" the
effects of previous commands within the same transaction.  Note that this is
done automatically by CommitTransactionCommand after each query inside a
transaction block, but some utility functions also do it internally to allow
some operations (usually in the system catalogs) to be seen by future
operations in the same utility command (for example, in DefineRelation it is
done after creating the heap so the pg_class row is visible, to be able to
lock it).


For example, consider the following sequence of user commands:

1)		BEGIN
2)		SELECT * FROM foo
3)		INSERT INTO foo VALUES (...)
4)		COMMIT

In the main processing loop, this results in the following function call
sequence:

	 /	StartTransactionCommand;
	/	ProcessUtility;				<< BEGIN
1) <			BeginTransactionBlock;
	\	CommitTransactionCommand;
	 \		StartTransaction;

	/	StartTransactionCommand;
2) /		ProcessQuery;				<< SELECT * FROM foo
   \		CommitTransactionCommand;
	\		CommandCounterIncrement;

	/	StartTransactionCommand;
3) /		ProcessQuery;				<< INSERT INTO foo VALUES (...)
   \		CommitTransactionCommand;
	\		CommandCounterIncrement;

	 /	StartTransactionCommand;
	/	ProcessUtility;				<< COMMIT
4) <			EndTransactionBlock;
	\			CommitTransaction;
	 \	CommitTransactionCommand;

The point of this example is to demonstrate the need for
StartTransactionCommand and CommitTransactionCommand to be state smart -- they
should call CommandCounterIncrement between the calls to BeginTransactionBlock
and EndTransactionBlock and outside these calls they need to do normal start,
commit or abort processing.

Furthermore, suppose the "SELECT * FROM foo" caused an abort condition.	In
this case AbortCurrentTransaction is called, and the transaction is put in
aborted state.  In this state, any user input is ignored except for
transaction-termination statements, or ROLLBACK TO <savepoint> commands.

Transaction aborts can occur in two ways:

1)	system dies from some internal cause  (syntax error, etc)
2)	user types ROLLBACK

The reason we have to distinguish them is illustrated by the following two
situations:

	case 1					case 2
	------					------
1) user types BEGIN			1) user types BEGIN
2) user does something			2) user does something
3) user does not like what		3) system aborts for some reason
   she sees and types ABORT		   (syntax error, etc)

In case 1, we want to abort the transaction and return to the default state.
In case 2, there may be more commands coming our way which are part of the
same transaction block; we have to ignore these commands until we see a COMMIT
or ROLLBACK.

Internal aborts are handled by AbortCurrentTransaction, while user aborts are
handled by UserAbortTransactionBlock.  Both of them rely on AbortTransaction
to do all the real work.  The only difference is what state we enter after
AbortTransaction does its work:

* AbortCurrentTransaction leaves us in TBLOCK_ABORT,
* UserAbortTransactionBlock leaves us in TBLOCK_ENDABORT

Low-level transaction abort handling is divided in two phases:
* AbortTransaction executes as soon as we realize the transaction has
  failed.  It should release all shared resources (locks etc) so that we do
  not delay other backends unnecessarily.
* CleanupTransaction executes when we finally see a user COMMIT
  or ROLLBACK command; it cleans things up and gets us out of the transaction
  internally.  In particular, we mustn't destroy TopTransactionContext until
  this point.

Also, note that when a transaction is committed, we don't close it right away.
Rather it's put in TBLOCK_END state, which means that when
CommitTransactionCommand is called after the query has finished processing,
the transaction has to be closed.  The distinction is subtle but important,
because it means that control will leave the xact.c code with the transaction
open, and the main loop will be able to keep processing inside the same
transaction.  So, in a sense, transaction commit is also handled in two
phases, the first at EndTransactionBlock and the second at
CommitTransactionCommand (which is where CommitTransaction is actually
called).

The rest of the code in xact.c are routines to support the creation and
finishing of transactions and subtransactions.  For example, AtStart_Memory
takes care of initializing the memory subsystem at main transaction start.


Subtransaction handling
-----------------------

Subtransactions are implemented using a stack of TransactionState structures,
each of which has a pointer to its parent transaction's struct.  When a new
subtransaction is to be opened, PushTransaction is called, which creates a new
TransactionState, with its parent link pointing to the current transaction.
StartSubTransaction is in charge of initializing the new TransactionState to
sane values, and properly initializing other subsystems (AtSubStart routines).

When closing a subtransaction, either CommitSubTransaction has to be called
(if the subtransaction is committing), or AbortSubTransaction and
CleanupSubTransaction (if it's aborting).  In either case, PopTransaction is
called so the system returns to the parent transaction.

One important point regarding subtransaction handling is that several may need
to be closed in response to a single user command.  That's because savepoints
have names, and we allow to commit or rollback a savepoint by name, which is
not necessarily the one that was last opened.  In the case of subtransaction
commit this is not a problem, and we close all the involved subtransactions
right away by calling CommitTransactionToLevel, which in turn calls
CommitSubTransaction and PopTransaction as many times as needed.

In the case of subtransaction abort (when the user issues ROLLBACK TO
<savepoint>), things are not so easy.  We have to keep the subtransactions
open and return control to the main loop.  So what RollbackToSavepoint does is
abort the innermost subtransaction and put it in TBLOCK_SUBENDABORT state, and
put the rest in TBLOCK_SUBABORT_PENDING state.  Then we return control to the
main loop, which will in turn return control to us by calling
CommitTransactionCommand.  At this point we can close all subtransactions that
are marked with the "abort pending" state.  When that's done, the outermost
subtransaction is created again, to conform to SQL's definition of ROLLBACK TO.

Other subsystems are allowed to start "internal" subtransactions, which are
handled by BeginInternalSubtransaction.  This is to allow implementing
exception handling, e.g. in PL/pgSQL.  ReleaseCurrentSubTransaction and
RollbackAndReleaseCurrentSubTransaction allows the subsystem to close said
subtransactions.  The main difference between this and the savepoint/release
path is that BeginInternalSubtransaction is allowed when no explicit
transaction block has been established, while DefineSavepoint is not.


pg_clog and pg_subtrans
-----------------------

pg_clog and pg_subtrans are permanent (on-disk) storage of transaction related
information.  There is a limited number of pages of each kept in memory, so
in many cases there is no need to actually read from disk.  However, if
there's a long running transaction or a backend sitting idle with an open
transaction, it may be necessary to be able to read and write this information
from disk.  They also allow information to be permanent across server restarts.

pg_clog records the commit status for each transaction.  A transaction can be
in progress, committed, aborted, or "sub-committed".  This last state means
that it's a subtransaction that's no longer running, but its parent has not
updated its state yet (either it is still running, or the backend crashed
without updating its status).  A sub-committed transaction's status will be
updated again to the final value as soon as the parent commits or aborts, or
when the parent is detected to be aborted.

Savepoints are implemented using subtransactions.  A subtransaction is a
transaction inside a transaction; it gets its own TransactionId, but its
commit or abort status is not only dependent on whether it committed itself,
but also whether its parent transaction committed.  To implement multiple
savepoints in a transaction we allow unlimited transaction nesting depth, so
any particular subtransaction's commit state is dependent on the commit status
of each and every ancestor transaction.

The "subtransaction parent" (pg_subtrans) mechanism records, for each
transaction, the TransactionId of its parent transaction.  This information is
stored as soon as the subtransaction is created.  Top-level transactions do
not have a parent, so they leave their pg_subtrans entries set to the default
value of zero (InvalidTransactionId).

pg_subtrans is used to check whether the transaction in question is still
running --- the main Xid of a transaction is recorded in the PGPROC struct,
but since we allow arbitrary nesting of subtransactions, we can't fit all Xids
in shared memory, so we have to store them on disk.  Note, however, that for
each transaction we keep a "cache" of Xids that are known to be part of the
transaction tree, so we can skip looking at pg_subtrans unless we know the
cache has been overflowed.  See storage/ipc/sinval.c for the gory details.

slru.c is the supporting mechanism for both pg_clog and pg_subtrans.  It
implements the LRU policy for in-memory buffer pages.  The high-level routines
for pg_clog are implemented in transam.c, while the low-level functions are in
clog.c.  pg_subtrans is contained completely in subtrans.c.
