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, 43 insertions, 1 deletions

diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 052a314d74e..ae624a08154 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.93 2009/06/26 20:29:04 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.94 2009/12/19 01:32:42 sriggs Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -133,7 +133,45 @@ typedef struct XLogRecData
 } XLogRecData;
 
 extern TimeLineID ThisTimeLineID;		/* current TLI */
+
+/*
+ * Prior to 8.4, all activity during recovery was carried out by Startup
+ * process. This local variable continues to be used in many parts of the
+ * code to indicate actions taken by RecoveryManagers. Other processes who
+ * potentially perform work during recovery should check RecoveryInProgress()
+ * see XLogCtl notes in xlog.c
+ */
 extern bool InRecovery;
+
+/*
+ * Like InRecovery, standbyState is only valid in the startup process.
+ *
+ * In DISABLED state, we're performing crash recovery or hot standby was
+ * disabled in recovery.conf.
+ *
+ * In INITIALIZED state, we haven't yet received a RUNNING_XACTS or shutdown
+ * checkpoint record to initialize our master transaction tracking system.
+ *
+ * When the transaction tracking is initialized, we enter the SNAPSHOT_PENDING
+ * state. The tracked information might still be incomplete, so we can't allow
+ * connections yet, but redo functions must update the in-memory state when
+ * appropriate.
+ *
+ * In SNAPSHOT_READY mode, we have full knowledge of transactions that are
+ * (or were) running in the master at the current WAL location. Snapshots
+ * can be taken, and read-only queries can be run.
+ */
+typedef enum
+{
+	STANDBY_DISABLED,
+	STANDBY_INITIALIZED,
+	STANDBY_SNAPSHOT_PENDING,
+	STANDBY_SNAPSHOT_READY
+} HotStandbyState;
+extern HotStandbyState standbyState;
+
+#define InHotStandby (standbyState >= STANDBY_SNAPSHOT_PENDING)
+
 extern XLogRecPtr XactLastRecEnd;
 
 /* these variables are GUC parameters related to XLOG */
@@ -143,9 +181,12 @@ extern bool XLogArchiveMode;
 extern char *XLogArchiveCommand;
 extern int	XLogArchiveTimeout;
 extern bool log_checkpoints;
+extern bool XLogRequestRecoveryConnections;
+extern int MaxStandbyDelay;
 
 #define XLogArchivingActive()	(XLogArchiveMode)
 #define XLogArchiveCommandSet() (XLogArchiveCommand[0] != '\0')
+#define XLogStandbyInfoActive()	(XLogRequestRecoveryConnections && XLogArchiveMode)
 
 #ifdef WAL_DEBUG
 extern bool XLOG_DEBUG;
@@ -203,6 +244,7 @@ extern void xlog_desc(StringInfo buf, uint8 xl_info, char *rec);
 
 extern bool RecoveryInProgress(void);
 extern bool XLogInsertAllowed(void);
+extern TimestampTz GetLatestXLogTime(void);
 
 extern void UpdateControlFile(void);
 extern Size XLOGShmemSize(void);