transactions waiting for commit will be allowed to proceed after
these standby servers confirm receipt of their data.
The synchronous standbys will be those whose names appear
- earlier in this list, and
+ in this list, and
that are both currently connected and streaming data in real-time
(as shown by a state of streaming in the
pg_stat_replication view).
- Other standby servers appearing later in this list represent potential
- synchronous standbys. If any of the current synchronous
- standbys disconnects for whatever reason,
- it will be replaced immediately with the next-highest-priority standby.
- Specifying more than one standby name can allow very high availability.
+ Specifying more than one standby names can allow very high availability.
This parameter specifies a list of standby servers using
either of the following syntaxes:
-num_sync ( standby_name [, ...] )
+[FIRST] num_sync ( standby_name [, ...] )
+ANY num_sync ( standby_name [, ...] )
standby_name [, ...]
where num_sync is
the number of synchronous standbys that transactions need to
wait for replies from,
and standby_name
- is the name of a standby server. For example, a setting of
- 3 (s1, s2, s3, s4) makes transaction commits wait
- until their WAL records are received by three higher-priority standbys
- chosen from standby servers s1, s2,
- s3 and s4.
-
- The second syntax was used before
PostgreSQL+ is the name of a standby server.
+ FIRST and ANY specify the method to choose
+ synchronous standbys from the listed servers.
+
+ The keyword FIRST, coupled with
+ num_sync, specifies a
+ priority-based synchronous replication and makes transaction commits
+ wait until their WAL records are replicated to
+ num_sync synchronous
+ standbys chosen based on their priorities. For example, a setting of
+ FIRST 3 (s1, s2, s3, s4) will cause each commit to wait for
+ replies from three higher-priority standbys chosen from standby servers
+ s1, s2, s3 and s4.
+ The standbys whose names appear earlier in the list are given higher
+ priority and will be considered as synchronous. Other standby servers
+ appearing later in this list represent potential synchronous standbys.
+ If any of the current synchronous standbys disconnects for whatever
+ reason, it will be replaced immediately with the next-highest-priority
+ standby. The keyword FIRST is optional.
+
+ The keyword ANY, coupled with
+ num_sync, specifies a
+ quorum-based synchronous replication and makes transaction commits
+ wait until their WAL records are replicated to at least
+ num_sync listed standbys.
+ For example, a setting of ANY 3 (s1, s2, s3, s4) will cause
+ each commit to proceed as soon as at least any three standbys of
+ s1, s2, s3 and s4
+ reply.
+
+ FIRST and ANY are case-insensitive. If these
+ keywords are used as the name of a standby server,
+ its standby_name must
+ be double-quoted.
+
+ The third syntax was used before
PostgreSQL version 9.6 and is still supported. It's the same as the first syntax
- with num_sync equal to 1.
- For example, 1 (s1, s2) and
- s1, s2 have the same meaning: either s1
- or s2 is chosen as a synchronous standby.
+ with FIRST and
+ num_sync equal to 1.
+ For example, FIRST 1 (s1, s2) and s1, s2 have
+ the same meaning: either s1 or s2 is chosen
+ as a synchronous standby.
The name of a standby server for this purpose is the
as synchronous confirm receipt of their data. The number of synchronous
standbys that transactions must wait for replies from is specified in
synchronous_standby_names. This parameter also specifies
- a list of standby names, which determines the priority of each standby
- for being chosen as a synchronous standby. The standbys whose names
- appear earlier in the list are given higher priority and will be considered
- as synchronous. Other standby servers appearing later in this list
- represent potential synchronous standbys. If any of the current
- synchronous standbys disconnects for whatever reason, it will be replaced
- immediately with the next-highest-priority standby.
+ a list of standby names and the method (FIRST and
+ ANY) to choose synchronous standbys from the listed ones.
- An example of synchronous_standby_names for multiple
- synchronous standbys is:
+ The method FIRST specifies a priority-based synchronous
+ replication and makes transaction commits wait until their WAL records are
+ replicated to the requested number of synchronous standbys chosen based on
+ their priorities. The standbys whose names appear earlier in the list are
+ given higher priority and will be considered as synchronous. Other standby
+ servers appearing later in this list represent potential synchronous
+ standbys. If any of the current synchronous standbys disconnects for
+ whatever reason, it will be replaced immediately with the
+ next-highest-priority standby.
+
+ An example of synchronous_standby_names for
+ a priority-based multiple synchronous standbys is:
-synchronous_standby_names = '2 (s1, s2, s3)'
+synchronous_standby_names = 'FIRST 2 (s1, s2, s3)'
In this example, if four standby servers s1, s2,
s3 and s4 are running, the two standbys
s2 fails. s4 is an asynchronous standby since
its name is not in the list.
+ The method ANY specifies a quorum-based synchronous
+ replication and makes transaction commits wait until their WAL records
+ are replicated to at least the requested number of
+ synchronous standbys in the list.
+
+ An example of synchronous_standby_names for
+ a quorum-based multiple synchronous standbys is:
+ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
+
+ In this example, if four standby servers s1, s2,
+ s3 and s4 are running, transaction commits will
+ wait for replies from at least any two standbys of s1,
+ s2 and s3. s4 is an asynchronous
+ standby since its name is not in the list.
+
The synchronous states of standby servers can be viewed using
the pg_stat_replication view.
* searching the through all waiters each time we receive a reply.
*
* In 9.5 or before only a single standby could be considered as
- * synchronous. In 9.6 we support multiple synchronous standbys.
- * The number of synchronous standbys that transactions must wait for
- * replies from is specified in synchronous_standby_names.
- * This parameter also specifies a list of standby names,
- * which determines the priority of each standby for being chosen as
- * a synchronous standby. The standbys whose names appear earlier
- * in the list are given higher priority and will be considered as
- * synchronous. Other standby servers appearing later in this list
- * represent potential synchronous standbys. If any of the current
- * synchronous standbys disconnects for whatever reason, it will be
- * replaced immediately with the next-highest-priority standby.
+ * synchronous. In 9.6 we support a priority-based multiple synchronous
+ * standbys. In 10.0 a quorum-based multiple synchronous standbys is also
+ * supported. The number of synchronous standbys that transactions
+ * must wait for replies from is specified in synchronous_standby_names.
+ * This parameter also specifies a list of standby names and the method
+ * (FIRST and ANY) to choose synchronous standbys from the listed ones.
+ *
+ * The method FIRST specifies a priority-based synchronous replication
+ * and makes transaction commits wait until their WAL records are
+ * replicated to the requested number of synchronous standbys chosen based
+ * on their priorities. The standbys whose names appear earlier in the list
+ * are given higher priority and will be considered as synchronous.
+ * Other standby servers appearing later in this list represent potential
+ * synchronous standbys. If any of the current synchronous standbys
+ * disconnects for whatever reason, it will be replaced immediately with
+ * the next-highest-priority standby.
+ *
+ * The method ANY specifies a quorum-based synchronous replication
+ * and makes transaction commits wait until their WAL records are
+ * replicated to at least the requested number of synchronous standbys
+ * in the list. All the standbys appearing in the list are considered as
+ * candidates for quorum synchronous standbys.
*
* Before the standbys chosen from synchronous_standby_names can
* become the synchronous standbys they must have caught up with
* the primary; that may take some time. Once caught up,
- * the current higher priority standbys which are considered as
- * synchronous at that moment will release waiters from the queue.
+ * the standbys which are considered as synchronous at that moment
+ * will release waiters from the queue.
*
* Portions Copyright (c) 2010-2016, PostgreSQL Global Development Group
*
static bool announce_next_takeover = true;
-static SyncRepConfigData *SyncRepConfig = NULL;
+SyncRepConfigData *SyncRepConfig = NULL;
static int SyncRepWaitMode = SYNC_REP_NO_WAIT;
static void SyncRepQueueInsert(int mode);
static void SyncRepCancelWait(void);
static int SyncRepWakeQueue(bool all, int mode);
-static bool SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr,
- XLogRecPtr *flushPtr,
- XLogRecPtr *applyPtr,
- bool *am_sync);
+static bool SyncRepGetSyncRecPtr(XLogRecPtr *writePtr,
+ XLogRecPtr *flushPtr,
+ XLogRecPtr *applyPtr,
+ bool *am_sync);
+static void SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr,
+ XLogRecPtr *flushPtr,
+ XLogRecPtr *applyPtr,
+ List *sync_standbys);
+static void SyncRepGetNthLatestSyncRecPtr(XLogRecPtr *writePtr,
+ XLogRecPtr *flushPtr,
+ XLogRecPtr *applyPtr,
+ List *sync_standbys, uint8 nth);
static int SyncRepGetStandbyPriority(void);
+static List *SyncRepGetSyncStandbysPriority(bool *am_sync);
+static List *SyncRepGetSyncStandbysQuorum(bool *am_sync);
+static int cmp_lsn(const void *a, const void *b);
#ifdef USE_ASSERT_CHECKING
static bool SyncRepQueueIsOrderedByLSN(int mode);
XLogRecPtr writePtr;
XLogRecPtr flushPtr;
XLogRecPtr applyPtr;
- bool got_oldest;
+ bool got_recptr;
bool am_sync;
int numwrite = 0;
int numflush = 0;
LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
/*
- * Check whether we are a sync standby or not, and calculate the oldest
+ * Check whether we are a sync standby or not, and calculate the synced
* positions among all sync standbys.
*/
- got_oldest = SyncRepGetOldestSyncRecPtr(&writePtr, &flushPtr,
- &applyPtr, &am_sync);
+ got_recptr = SyncRepGetSyncRecPtr(&writePtr, &flushPtr, &applyPtr, &am_sync);
/*
* If we are managing a sync standby, though we weren't prior to this,
if (announce_next_takeover && am_sync)
{
announce_next_takeover = false;
- ereport(LOG,
- (errmsg("standby \"%s\" is now a synchronous standby with priority %u",
- application_name, MyWalSnd->sync_standby_priority)));
+
+ if (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY)
+ ereport(LOG,
+ (errmsg("standby \"%s\" is now a synchronous standby with priority %u",
+ application_name, MyWalSnd->sync_standby_priority)));
+ else
+ ereport(LOG,
+ (errmsg("standby \"%s\" is now a candidate for quorum synchronous standby",
+ application_name)));
}
/*
* If the number of sync standbys is less than requested or we aren't
* managing a sync standby then just leave.
*/
- if (!got_oldest || !am_sync)
+ if (!got_recptr || !am_sync)
{
LWLockRelease(SyncRepLock);
announce_next_takeover = !am_sync;
}
/*
- * Calculate the oldest Write, Flush and Apply positions among sync standbys.
+ * Calculate the synced Write, Flush and Apply positions among sync standbys.
*
* Return false if the number of sync standbys is less than
* synchronous_standby_names specifies. Otherwise return true and
- * store the oldest positions into *writePtr, *flushPtr and *applyPtr.
+ * store the positions into *writePtr, *flushPtr and *applyPtr.
*
* On return, *am_sync is set to true if this walsender is connecting to
* sync standby. Otherwise it's set to false.
*/
static bool
-SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
+SyncRepGetSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
XLogRecPtr *applyPtr, bool *am_sync)
{
List *sync_standbys;
- ListCell *cell;
*writePtr = InvalidXLogRecPtr;
*flushPtr = InvalidXLogRecPtr;
}
/*
- * Scan through all sync standbys and calculate the oldest Write, Flush
- * and Apply positions.
+ * In a priority-based sync replication, the synced positions are the
+ * oldest ones among sync standbys. In a quorum-based, they are the Nth
+ * latest ones.
+ *
+ * SyncRepGetNthLatestSyncRecPtr() also can calculate the oldest positions.
+ * But we use SyncRepGetOldestSyncRecPtr() for that calculation because
+ * it's a bit more efficient.
+ *
+ * XXX If the numbers of current and requested sync standbys are the same,
+ * we can use SyncRepGetOldestSyncRecPtr() to calculate the synced
+ * positions even in a quorum-based sync replication.
+ */
+ if (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY)
+ {
+ SyncRepGetOldestSyncRecPtr(writePtr, flushPtr, applyPtr,
+ sync_standbys);
+ }
+ else
+ {
+ SyncRepGetNthLatestSyncRecPtr(writePtr, flushPtr, applyPtr,
+ sync_standbys, SyncRepConfig->num_sync);
+ }
+
+ list_free(sync_standbys);
+ return true;
+}
+
+/*
+ * Calculate the oldest Write, Flush and Apply positions among sync standbys.
+ */
+static void
+SyncRepGetOldestSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
+ XLogRecPtr *applyPtr, List *sync_standbys)
+{
+ ListCell *cell;
+
+ /*
+ * Scan through all sync standbys and calculate the oldest
+ * Write, Flush and Apply positions.
*/
- foreach(cell, sync_standbys)
+ foreach (cell, sync_standbys)
{
- WalSnd *walsnd = &WalSndCtl->walsnds[lfirst_int(cell)];
+ WalSnd *walsnd = &WalSndCtl->walsnds[lfirst_int(cell)];
XLogRecPtr write;
XLogRecPtr flush;
XLogRecPtr apply;
if (XLogRecPtrIsInvalid(*applyPtr) || *applyPtr > apply)
*applyPtr = apply;
}
+}
- list_free(sync_standbys);
- return true;
+/*
+ * Calculate the Nth latest Write, Flush and Apply positions among sync
+ * standbys.
+ */
+static void
+SyncRepGetNthLatestSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
+ XLogRecPtr *applyPtr, List *sync_standbys, uint8 nth)
+{
+ ListCell *cell;
+ XLogRecPtr *write_array;
+ XLogRecPtr *flush_array;
+ XLogRecPtr *apply_array;
+ int len;
+ int i = 0;
+
+ len = list_length(sync_standbys);
+ write_array = (XLogRecPtr *) palloc(sizeof(XLogRecPtr) * len);
+ flush_array = (XLogRecPtr *) palloc(sizeof(XLogRecPtr) * len);
+ apply_array = (XLogRecPtr *) palloc(sizeof(XLogRecPtr) * len);
+
+ foreach (cell, sync_standbys)
+ {
+ WalSnd *walsnd = &WalSndCtl->walsnds[lfirst_int(cell)];
+
+ SpinLockAcquire(&walsnd->mutex);
+ write_array[i] = walsnd->write;
+ flush_array[i] = walsnd->flush;
+ apply_array[i] = walsnd->apply;
+ SpinLockRelease(&walsnd->mutex);
+
+ i++;
+ }
+
+ qsort(write_array, len, sizeof(XLogRecPtr), cmp_lsn);
+ qsort(flush_array, len, sizeof(XLogRecPtr), cmp_lsn);
+ qsort(apply_array, len, sizeof(XLogRecPtr), cmp_lsn);
+
+ /* Get Nth latest Write, Flush, Apply positions */
+ *writePtr = write_array[nth - 1];
+ *flushPtr = flush_array[nth - 1];
+ *applyPtr = apply_array[nth - 1];
+
+ pfree(write_array);
+ pfree(flush_array);
+ pfree(apply_array);
+}
+
+/*
+ * Compare lsn in order to sort array in descending order.
+ */
+static int
+cmp_lsn(const void *a, const void *b)
+{
+ XLogRecPtr lsn1 = *((const XLogRecPtr *) a);
+ XLogRecPtr lsn2 = *((const XLogRecPtr *) b);
+
+ if (lsn1 > lsn2)
+ return -1;
+ else if (lsn1 == lsn2)
+ return 0;
+ else
+ return 1;
}
/*
* Return the list of sync standbys, or NIL if no sync standby is connected.
*
- * If there are multiple standbys with the same priority,
- * the first one found is selected preferentially.
* The caller must hold SyncRepLock.
*
* On return, *am_sync is set to true if this walsender is connecting to
* sync standby. Otherwise it's set to false.
*/
List *
-SyncRepGetSyncStandbys(bool *am_sync)
+SyncRepGetSyncStandbys(bool *am_sync)
+{
+ /* Set default result */
+ if (am_sync != NULL)
+ *am_sync = false;
+
+ /* Quick exit if sync replication is not requested */
+ if (SyncRepConfig == NULL)
+ return NIL;
+
+ return (SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY) ?
+ SyncRepGetSyncStandbysPriority(am_sync) :
+ SyncRepGetSyncStandbysQuorum(am_sync);
+}
+
+/*
+ * Return the list of all the candidates for quorum sync standbys,
+ * or NIL if no such standby is connected.
+ *
+ * The caller must hold SyncRepLock. This function must be called only in
+ * a quorum-based sync replication.
+ *
+ * On return, *am_sync is set to true if this walsender is connecting to
+ * sync standby. Otherwise it's set to false.
+ */
+static List *
+SyncRepGetSyncStandbysQuorum(bool *am_sync)
+{
+ List *result = NIL;
+ int i;
+ volatile WalSnd *walsnd; /* Use volatile pointer to prevent code
+ * rearrangement */
+
+ Assert(SyncRepConfig->syncrep_method == SYNC_REP_QUORUM);
+
+ for (i = 0; i < max_wal_senders; i++)
+ {
+ walsnd = &WalSndCtl->walsnds[i];
+
+ /* Must be active */
+ if (walsnd->pid == 0)
+ continue;
+
+ /* Must be streaming */
+ if (walsnd->state != WALSNDSTATE_STREAMING)
+ continue;
+
+ /* Must be synchronous */
+ if (walsnd->sync_standby_priority == 0)
+ continue;
+
+ /* Must have a valid flush position */
+ if (XLogRecPtrIsInvalid(walsnd->flush))
+ continue;
+
+ /*
+ * Consider this standby as a candidate for quorum sync standbys
+ * and append it to the result.
+ */
+ result = lappend_int(result, i);
+ if (am_sync != NULL && walsnd == MyWalSnd)
+ *am_sync = true;
+ }
+
+ return result;
+}
+
+/*
+ * Return the list of sync standbys chosen based on their priorities,
+ * or NIL if no sync standby is connected.
+ *
+ * If there are multiple standbys with the same priority,
+ * the first one found is selected preferentially.
+ *
+ * The caller must hold SyncRepLock. This function must be called only in
+ * a priority-based sync replication.
+ *
+ * On return, *am_sync is set to true if this walsender is connecting to
+ * sync standby. Otherwise it's set to false.
+ */
+static List *
+SyncRepGetSyncStandbysPriority(bool *am_sync)
{
List *result = NIL;
List *pending = NIL;
volatile WalSnd *walsnd; /* Use volatile pointer to prevent code
* rearrangement */
- /* Set default result */
- if (am_sync != NULL)
- *am_sync = false;
-
- /* Quick exit if sync replication is not requested */
- if (SyncRepConfig == NULL)
- return NIL;
+ Assert(SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY);
lowest_priority = SyncRepConfig->nmembers;
next_highest_priority = lowest_priority + 1;
projects / postgresql.git / commitdiff