Allow read only connections during recovery, known as Hot Standby.

Enabled by recovery_connections = on (default) and forcing archive recovery using a recovery.conf. Recovery processing now emulates the original transactions as they are replayed, providing full locking and MVCC behaviour for read only queries. Recovery must enter consistent state before connections are allowed, so there is a delay, typically short, before connections succeed. Replay of recovering transactions can conflict and in some cases deadlock with queries during recovery; these result in query cancellation after max_standby_delay seconds have expired. Infrastructure changes have minor effects on normal running, though introduce four new types of WAL record.

New test mode "make standbycheck" allows regression tests of static command behaviour on a standby server while in recovery. Typical and extreme dynamic behaviours have been checked via code inspection and manual testing. Few port specific behaviours have been utilised, though primary testing has been on Linux only so far.

This commit is the basic patch. Additional changes will follow in this release to enhance some aspects of behaviour, notably improved handling of conflicts, deadlock detection and query cancellation. Changes to VACUUM FULL are also required.

Simon Riggs, with significant and lengthy review by Heikki Linnakangas, including streamlined redesign of snapshot creation and two-phase commit.

Important contributions from Florian Pflug, Mark Kirkwood, Merlin Moncure, Greg Stark, Gianni Ciolli, Gabriele Bartolini, Hannu Krosing, Robert Haas, Tatsuo Ishii, Hiroyuki Yamada plus support and feedback from many other community members.

1 files changed, 11 insertions, 9 deletions

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index b616eaca135..21fc83ab4b8 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.596 2009/09/08 17:08:36 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/postmaster.c,v 1.597 2009/12/19 01:32:34 sriggs Exp $
  *
  * NOTES
  *
@@ -245,8 +245,9 @@ static bool RecoveryError = false;		/* T if WAL recovery failed */
  * When archive recovery is finished, the startup process exits with exit
  * code 0 and we switch to PM_RUN state.
  *
- * Normal child backends can only be launched when we are in PM_RUN state.
- * (We also allow it in PM_WAIT_BACKUP state, but only for superusers.)
+ * Normal child backends can only be launched when we are in PM_RUN or
+ * PM_RECOVERY_CONSISTENT state.  (We also allow launch of normal
+ * child backends in PM_WAIT_BACKUP state, but only for superusers.)
  * In other states we handle connection requests by launching "dead_end"
  * child processes, which will simply send the client an error message and
  * quit.  (We track these in the BackendList so that we can know when they
@@ -1868,7 +1869,7 @@ static enum CAC_state
 canAcceptConnections(void)
 {
 	/*
-	 * Can't start backends when in startup/shutdown/recovery state.
+	 * Can't start backends when in startup/shutdown/inconsistent recovery state.
 	 *
 	 * In state PM_WAIT_BACKUP only superusers can connect (this must be
 	 * allowed so that a superuser can end online backup mode); we return
@@ -1882,9 +1883,11 @@ canAcceptConnections(void)
 			return CAC_SHUTDOWN;	/* shutdown is pending */
 		if (!FatalError &&
 			(pmState == PM_STARTUP ||
-			 pmState == PM_RECOVERY ||
-			 pmState == PM_RECOVERY_CONSISTENT))
+			 pmState == PM_RECOVERY))
 			return CAC_STARTUP; /* normal startup */
+		if (!FatalError &&
+			 pmState == PM_RECOVERY_CONSISTENT)
+			return CAC_OK; /* connection OK during recovery */
 		return CAC_RECOVERY;	/* else must be crash recovery */
 	}
 
@@ -4003,9 +4006,8 @@ sigusr1_handler(SIGNAL_ARGS)
 		Assert(PgStatPID == 0);
 		PgStatPID = pgstat_start();
 
-		/* XXX at this point we could accept read-only connections */
-		ereport(DEBUG1,
-				(errmsg("database system is in consistent recovery mode")));
+		ereport(LOG,
+				 (errmsg("database system is ready to accept read only connections")));
 
 		pmState = PM_RECOVERY_CONSISTENT;
 	}