$PostgreSQL: pgsql/src/backend/access/transam/README,v 1.3 2005/05/19 21:35:45 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, or after detecting an error:
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;
/ StartTransaction;
1) < ProcessUtility; << BEGIN
\ BeginTransactionBlock;
\ CommitTransactionCommand;
/ StartTransactionCommand;
2) / ProcessQuery; << SELECT ...
\ CommitTransactionCommand;
\ CommandCounterIncrement;
/ StartTransactionCommand;
3) / ProcessQuery; << INSERT ...
\ CommitTransactionCommand;
\ CommandCounterIncrement;
/ StartTransactionCommand;
/ ProcessUtility; << COMMIT
4) < EndTransactionBlock;
\ CommitTransactionCommand;
\ CommitTransaction;
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_ABORT_END
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
completely. 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. Also a COMMIT or ROLLBACK
command must be able to close out the entire stack. We handle this by having
the utility command subroutine mark all the state stack entries as commit-
pending or abort-pending, and then when the main loop reaches
CommitTransactionCommand, the real work is done. The main point of doing
things this way is that if we get an error while popping state stack entries,
the remaining stack entries still show what we need to do to finish up.
In the case of ROLLBACK TO <savepoint>, we abort all the subtransactions up
through the one identified by the savepoint name, and then re-create that
subtransaction level with the same name. So it's a completely new
subtransaction as far as the internals are concerned.
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 we execute the complete state transition immediately in each
subroutine, rather than deferring some work until CommitTransactionCommand.
Another difference is that BeginInternalSubtransaction is allowed when no
explicit transaction block has been established, while DefineSavepoint is not.
Subtransaction numbering
------------------------
A top-level transaction is always given a TransactionId (XID) as soon as it is
created. This is necessary for a number of reasons, notably XMIN bookkeeping
for VACUUM. However, a subtransaction doesn't need its own XID unless it
(or one of its child subxacts) writes tuples into the database. Therefore,
we postpone assigning XIDs to subxacts until and unless they call
GetCurrentTransactionId. The subsidiary actions of obtaining a lock on the
XID and and entering it into pg_subtrans and PG_PROC are done at the same time.
Internally, a backend needs a way to identify subtransactions whether or not
they have XIDs; but this need only lasts as long as the parent top transaction
endures. Therefore, we have SubTransactionId, which is somewhat like
CommandId in that it's generated from a counter that we reset at the start of
each top transaction. The top-level transaction itself has SubTransactionId 1,
and subtransactions have IDs 2 and up. (Zero is reserved for
InvalidSubTransactionId.)
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 that has been assigned
an XID. 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; 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 with an XID, the TransactionId of its parent transaction. This
information is stored as soon as the subtransaction is assigned an XID.
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/procarray.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.
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