On Mon, Feb 1, 2016 at 11:28 PM, Fujii Masao <masao.fu...@gmail.com> wrote:
> On Mon, Feb 1, 2016 at 5:36 PM, Masahiko Sawada <sawada.m...@gmail.com> wrote:
>> On Sun, Jan 31, 2016 at 8:58 PM, Michael Paquier
>> <michael.paqu...@gmail.com> wrote:
>>> On Sun, Jan 31, 2016 at 5:28 PM, Masahiko Sawada <sawada.m...@gmail.com>
>>> wrote:
>>>> On Sun, Jan 31, 2016 at 5:18 PM, Michael Paquier
>>>> <michael.paqu...@gmail.com> wrote:
>>>>> On Sun, Jan 31, 2016 at 5:08 PM, Masahiko Sawada <sawada.m...@gmail.com>
>>>>> wrote:
>>>>>> On Sun, Jan 31, 2016 at 1:17 PM, Michael Paquier
>>>>>> <michael.paqu...@gmail.com> wrote:
>>>>>>> On Thu, Jan 28, 2016 at 10:10 PM, Masahiko Sawada wrote:
>>>>>>>> By the discussions so far, I'm planning to have several replication
>>>>>>>> methods such as 'quorum', 'complex' in the feature, and the each
>>>>>>>> replication method specifies the syntax of s_s_names.
>>>>>>>> It means that s_s_names could have the number of sync standbys like
>>>>>>>> what current patch does.
>>>>>>>
>>>>>>> What if the application_name of a standby node has the format of an
>>>>>>> integer?
>>>>>>
>>>>>> Even if the standby has an integer as application_name, we can set
>>>>>> s_s_names like '2,1,2,3'.
>>>>>> The leading '2' is always handled as the number of sync standbys when
>>>>>> s_r_method = 'priority'.
>>>>>
>>>>> Hm. I agree with Fujii-san here, having the number of sync standbys
>>>>> defined in a parameter that should have a list of names is a bit
>>>>> confusing. I'd rather have a separate GUC, which brings us back to one
>>>>> of the first patches that I came up with, and a couple of people,
>>>>> including Josh were not happy with that because this did not support
>>>>> real quorum. Perhaps the final answer would be really to get a set of
>>>>> hooks, and a contrib module making use of that.
>>>>
>>>> Yeah, I agree with having set of hooks, and postgres core has simple
>>>> multi sync replication mechanism like you suggested at first version.
>>>
>>> If there are hooks, I don't think that we should really bother about
>>> having in core anything more complicated than what we have now. The
>>> trick will be to come up with a hook design modular enough to support
>>> the kind of configurations mentioned on this thread. Roughly perhaps a
>>> refactoring of the syncrep code so as it is possible to wait for
>>> multiple targets some of them being optional,, one modular way in
>>> pg_stat_get_wal_senders to represent the status of a node to user, and
>>> another hook to return to decide which are the nodes to wait for. Some
>>> of the nodes being waited for may be based on conditions for quorum
>>> support. That's a hard problem to do that in a flexible enough way.
>>
>> Hm, I think not-nested quorum and priority are not complicated, and we
>> should support at least both or either simple method in core of
>> postgres.
>> More complicated method like using json-style, or dedicated language
>> would be supported by external module.
>
> So what about the following plan?
>
> [first version]
> Add only synchronous_standby_num which specifies the number of standbys
> that the master must wait for before marking sync replication as completed.
> This version supports simple use cases like "I want to have two synchronous
> standbys".
>
> [second version]
> Add synchronous_replication_method: 'prioriry' and 'quorum'. This version
> additionally supports simple quorum commit case like "I want to ensure
> that WAL is replicated synchronously to at least two standbys from five
> ones listed in s_s_names".
>
> Or
>
> Add something like quorum_replication_num and quorum_standby_names, i.e.,
> the master must wait for at least q_r_num standbys from ones listed in
> q_s_names before marking sync replication as completed. Also the master
> must wait for sync replication according to s_s_num and s_s_num.
> That is, this approach separates 'priority' and 'quorum' to each parameters.
> This increases the number of GUC parameters, but ISTM less confusing, and
> it supports a bit complicated case like "there is one local standby and three
> remote standbys, then I want to ensure that WAL is replicated synchronously
> to the local standby and at least two remote one", e.g.,
>
> s_s_num = 1, s_s_names = 'local'
> q_s_num = 2, q_s_names = 'remote1, remote2, remote3'
>
> [third version]
> Add the hooks for more complicated sync replication cases.
>
> I'm thinking that the realistic target for 9.6 might be the first one.
>
Thank you for suggestion.
I agree with first version, and attached the updated patch which are
modified so that it supports simple multiple sync replication you
suggested.
(but test cases are not included yet.)
Regards,
--
Masahiko Sawada
diff --git a/src/backend/replication/syncrep.c b/src/backend/replication/syncrep.c
index 7f85b88..9a2f7e7 100644
--- a/src/backend/replication/syncrep.c
+++ b/src/backend/replication/syncrep.c
@@ -29,10 +29,10 @@
* single ordered queue of waiting backends, so that we can avoid
* searching the through all waiters each time we receive a reply.
*
- * In 9.1 we support only a single synchronous standby, chosen from a
- * priority list of synchronous_standby_names. Before it can become the
- * synchronous standby it must have caught up with the primary; that may
- * take some time. Once caught up, the current highest priority standby
+ * In 9.6 we support multiple synchronous standbys, chosen from a priority
+ * list of synchronous_standby_names. Before any standby can become a
+ * synchronous standbys it must have caught up with the primary; that may
+ * take some time. Once caught up, the current highest priority standbys
* will release waiters from the queue.
*
* Portions Copyright (c) 2010-2016, PostgreSQL Global Development Group
@@ -59,9 +59,12 @@
/* User-settable parameters for sync rep */
char *SyncRepStandbyNames;
+int synchronous_standby_num;
#define SyncStandbysDefined() \
- (SyncRepStandbyNames != NULL && SyncRepStandbyNames[0] != '\0')
+ (SyncRepStandbyNames != NULL && \
+ SyncRepStandbyNames[0] != '\0' && \
+ synchronous_standby_num > 0)
static bool announce_next_takeover = true;
@@ -349,57 +352,168 @@ SyncRepInitConfig(void)
}
/*
- * Find the WAL sender servicing the synchronous standby with the lowest
- * priority value, or NULL if no synchronous standby is connected. If there
- * are multiple standbys with the same lowest priority value, the first one
- * found is selected. The caller must hold SyncRepLock.
+ * Is this wal sender managing a standby that is streaming and
+ * listed as a synchronous standby?
*/
-WalSnd *
-SyncRepGetSynchronousStandby(void)
+bool
+SyncRepActiveListedWalSender(int num)
{
- WalSnd *result = NULL;
- int result_priority = 0;
- int i;
+ volatile WalSnd *walsnd = &WalSndCtl->walsnds[num];
+
+ /* Must be active */
+ if (walsnd->pid == 0)
+ return false;
+
+ /* Must be streaming */
+ if (walsnd->state != WALSNDSTATE_STREAMING)
+ return false;
+
+ /* Must be synchronous */
+ if (walsnd->sync_standby_priority == 0)
+ return false;
+
+ /* Must have a valid flush position */
+ if (XLogRecPtrIsInvalid(walsnd->flush))
+ return false;
+
+ return true;
+}
+
+/*
+ * Get both LSNs: write and flush, and confirm whether we have advanced
+ * to LSN or not.
+ */
+bool
+SyncRepSyncedLsnAdvancedTo(XLogRecPtr *write_pos, XLogRecPtr *flush_pos)
+{
+ XLogRecPtr tmp_write_pos;
+ XLogRecPtr tmp_flush_pos;
+ bool ret = false;
+
+ ret = SyncRepGetSyncLsns(&tmp_write_pos, &tmp_flush_pos);
+
+ /* Have we advanced LSN? */
+ if (ret)
+ {
+ if (MyWalSnd->write >= tmp_write_pos)
+ *write_pos = tmp_write_pos;
+ if (MyWalSnd->flush >= tmp_flush_pos)
+ *flush_pos = tmp_flush_pos;
+
+ return true;
+ }
+
+ return false;
+}
+
+/*
+ * Populate a caller-supplied buffer with the walsnds indexes of the
+ * highest priority active synchronous standbys, up to the a limit of
+ * 'synchronous_standby_num'. The order of the results is undefined.
+ * Return the number of results actually written.
+ */
+int
+SyncRepGetSyncStandbys(int *sync_standbys)
+{
+ int priority = 0;
+ int num_sync = 0;
+ int i;
for (i = 0; i < max_wal_senders; i++)
{
/* Use volatile pointer to prevent code rearrangement */
volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
- int this_priority;
+ int j;
- /* Must be active */
- if (walsnd->pid == 0)
+ /* Is this wal sender considerable one? */
+ if (!SyncRepActiveListedWalSender(i))
continue;
- /* Must be streaming */
- if (walsnd->state != WALSNDSTATE_STREAMING)
- continue;
+ if (num_sync == synchronous_standby_num)
+ {
+ if (walsnd->sync_standby_priority > priority)
+ continue;
+
+ for (j = 0; j < num_sync; j++)
+ {
+ volatile WalSnd *walsndloc = &WalSndCtl->walsnds[sync_standbys[j]];
+
+ /* Found lowest priority standby, so replace it */
+ if (walsndloc->sync_standby_priority == priority &&
+ walsnd->sync_standby_priority < priority)
+ sync_standbys[j] = i;
+
+ /* Update highest priority standby */
+ if (priority < walsndloc->sync_standby_priority)
+ priority = walsndloc->sync_standby_priority;
+ }
+ }
+ else
+ {
+ sync_standbys[num_sync] = i;
+ num_sync++;
- /* Must be synchronous */
- this_priority = walsnd->sync_standby_priority;
- if (this_priority == 0)
- continue;
+ /* Keep track highest priority standby */
+ if (priority < walsnd->sync_standby_priority)
+ priority = walsnd->sync_standby_priority;
+ }
+ }
- /* Must have a lower priority value than any previous ones */
- if (result != NULL && result_priority <= this_priority)
- continue;
+ return num_sync;
+}
- /* Must have a valid flush position */
- if (XLogRecPtrIsInvalid(walsnd->flush))
+/*
+ * Obtain currently synced LSN location: write and flush.
+ */
+bool
+SyncRepGetSyncLsns(XLogRecPtr *write_pos, XLogRecPtr *flush_pos)
+{
+ int sync_standbys[SYNC_REP_MAX_SYNC_STANDBY_NUM];
+ int num_sync;
+ int i;
+ XLogRecPtr synced_write = InvalidXLogRecPtr;
+ XLogRecPtr synced_flush = InvalidXLogRecPtr;
+
+ num_sync = SyncRepGetSyncStandbys(sync_standbys);
+
+ for (i = 0; i < num_sync; i++)
+ {
+ elog(WARNING, "sync_standbys[%d] = %d", i, sync_standbys[i]);
+ }
+ elog(WARNING, "num_sync = %d, s_s_num = %d", num_sync, synchronous_standby_num);
+
+ /* Just return, if sync standby is not enough */
+ if (num_sync < synchronous_standby_num)
+ return false;
+
+ for (i = 0; i < num_sync; i++)
+ {
+ volatile WalSnd *walsndloc = &WalSndCtl->walsnds[sync_standbys[i]];
+
+ SpinLockAcquire(&walsndloc->mutex);
+
+ /* Store first candidate */
+ if (XLogRecPtrIsInvalid(synced_write) && XLogRecPtrIsInvalid(synced_flush))
+ {
+ synced_write = walsndloc->write;
+ synced_flush = walsndloc->flush;
+ SpinLockRelease(&walsndloc->mutex);
continue;
+ }
- result = (WalSnd *) walsnd;
- result_priority = this_priority;
+ /* Keep/Collect the earliest write and flush LSNs among prioritized standbys */
+ if (synced_write > walsndloc->write)
+ synced_write = walsndloc->write;
+ if (synced_flush > walsndloc->flush)
+ synced_flush = walsndloc->flush;
- /*
- * If priority is equal to 1, there cannot be any other WAL senders
- * with a lower priority, so we're done.
- */
- if (this_priority == 1)
- return result;
+ SpinLockRelease(&walsndloc->mutex);
}
- return result;
+ *write_pos = synced_write;
+ *flush_pos = synced_flush;
+
+ return true;
}
/*
@@ -413,9 +527,9 @@ void
SyncRepReleaseWaiters(void)
{
volatile WalSndCtlData *walsndctl = WalSndCtl;
- WalSnd *syncWalSnd;
- int numwrite = 0;
- int numflush = 0;
+ XLogRecPtr write_pos = InvalidXLogRecPtr;
+ XLogRecPtr flush_pos = InvalidXLogRecPtr;
+ int numwrite, numflush;
/*
* If this WALSender is serving a standby that is not on the list of
@@ -428,23 +542,12 @@ SyncRepReleaseWaiters(void)
XLogRecPtrIsInvalid(MyWalSnd->flush))
return;
- /*
- * We're a potential sync standby. Release waiters if we are the highest
- * priority standby.
- */
LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
- syncWalSnd = SyncRepGetSynchronousStandby();
- /* We should have found ourselves at least */
- Assert(syncWalSnd != NULL);
-
- /*
- * If we aren't managing the highest priority standby then just leave.
- */
- if (syncWalSnd != MyWalSnd)
+ /* Get currently synced LSNs according */
+ if (!(SyncRepSyncedLsnAdvancedTo(&write_pos, &flush_pos)))
{
LWLockRelease(SyncRepLock);
- announce_next_takeover = true;
return;
}
@@ -452,14 +555,14 @@ SyncRepReleaseWaiters(void)
* Set the lsn first so that when we wake backends they will release up to
* this location.
*/
- if (walsndctl->lsn[SYNC_REP_WAIT_WRITE] < MyWalSnd->write)
+ if (walsndctl->lsn[SYNC_REP_WAIT_WRITE] < write_pos)
{
- walsndctl->lsn[SYNC_REP_WAIT_WRITE] = MyWalSnd->write;
+ walsndctl->lsn[SYNC_REP_WAIT_WRITE] = write_pos;
numwrite = SyncRepWakeQueue(false, SYNC_REP_WAIT_WRITE);
}
- if (walsndctl->lsn[SYNC_REP_WAIT_FLUSH] < MyWalSnd->flush)
+ if (walsndctl->lsn[SYNC_REP_WAIT_FLUSH] < flush_pos)
{
- walsndctl->lsn[SYNC_REP_WAIT_FLUSH] = MyWalSnd->flush;
+ walsndctl->lsn[SYNC_REP_WAIT_FLUSH] = flush_pos;
numflush = SyncRepWakeQueue(false, SYNC_REP_WAIT_FLUSH);
}
@@ -506,6 +609,10 @@ SyncRepGetStandbyPriority(void)
if (am_cascading_walsender)
return 0;
+ /* If no synchronous standby allowed, no cake for this WAL sender */
+ if (!SyncStandbysDefined())
+ return 0;
+
/* Need a modifiable copy of string */
rawstring = pstrdup(SyncRepStandbyNames);
@@ -521,8 +628,9 @@ SyncRepGetStandbyPriority(void)
foreach(l, elemlist)
{
- char *standby_name = (char *) lfirst(l);
+ char *standby_name;
+ standby_name = (char *) lfirst(l);
priority++;
if (pg_strcasecmp(standby_name, application_name) == 0 ||
@@ -683,7 +791,6 @@ SyncRepQueueIsOrderedByLSN(int mode)
* Synchronous Replication functions executed by any process
* ===========================================================
*/
-
bool
check_synchronous_standby_names(char **newval, void **extra, GucSource source)
{
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index c03e045..8586af4 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -2749,9 +2749,11 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
Tuplestorestate *tupstore;
MemoryContext per_query_ctx;
MemoryContext oldcontext;
- WalSnd *sync_standby;
+ int *sync_standbys;
+ int num_sync;
int i;
+
/* check to see if caller supports us returning a tuplestore */
if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
ereport(ERROR,
@@ -2777,11 +2779,13 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
MemoryContextSwitchTo(oldcontext);
+ sync_standbys = (int *) palloc(sizeof(int) * synchronous_standby_num);
+
/*
- * Get the currently active synchronous standby.
+ * Get the currently active synchronous standbys.
*/
LWLockAcquire(SyncRepLock, LW_SHARED);
- sync_standby = SyncRepGetSynchronousStandby();
+ num_sync = SyncRepGetSyncStandbys(sync_standbys);
LWLockRelease(SyncRepLock);
for (i = 0; i < max_wal_senders; i++)
@@ -2854,18 +2858,34 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
*/
if (priority == 0)
values[7] = CStringGetTextDatum("async");
- else if (walsnd == sync_standby)
- values[7] = CStringGetTextDatum("sync");
else
- values[7] = CStringGetTextDatum("potential");
+ {
+ int j;
+ bool found = false;
+
+ for (j = 0; j < num_sync; j++)
+ {
+ /* Found sync standby */
+ if (i == sync_standbys[j])
+ {
+ values[7] = CStringGetTextDatum("sync");
+ found = true;
+ break;
+ }
+ }
+ if (!found)
+ values[7] = CStringGetTextDatum("potential");
+ }
}
tuplestore_putvalues(tupstore, tupdesc, values, nulls);
}
/* clean up and return the tuplestore */
+ pfree(sync_standbys);
tuplestore_donestoring(tupstore);
+
return (Datum) 0;
}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 38ba82f..3638c0d 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2683,6 +2683,16 @@ static struct config_int ConfigureNamesInt[] =
NULL, NULL, NULL
},
+ {
+ {"synchronous_standby_num", PGC_SIGHUP, REPLICATION_MASTER,
+ gettext_noop("Number fo sync standbys."),
+ NULL,
+ },
+ &synchronous_standby_num,
+ 1, 0, SYNC_REP_MAX_SYNC_STANDBY_NUM,
+ NULL, NULL, NULL
+ },
+
/* End-of-list marker */
{
{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 029114f..06f1a51 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -241,6 +241,7 @@
#synchronous_standby_names = '' # standby servers that provide sync rep
# comma-separated list of application_name
# from standby(s); '*' = all
+#synchronous_standby_num = 0 # number of standby servers using sync rep
#vacuum_defer_cleanup_age = 0 # number of xacts by which cleanup is delayed
# - Standby Servers -
diff --git a/src/include/replication/syncrep.h b/src/include/replication/syncrep.h
index 96e059b..0fb7fc6 100644
--- a/src/include/replication/syncrep.h
+++ b/src/include/replication/syncrep.h
@@ -31,8 +31,13 @@
#define SYNC_REP_WAITING 1
#define SYNC_REP_WAIT_COMPLETE 2
+/* Limit of the number of synchronous standbys */
+#define SYNC_REP_MAX_SYNC_STANDBY_NUM 256
+
/* user-settable parameters for synchronous replication */
extern char *SyncRepStandbyNames;
+extern int synchronous_replication_method;
+extern int synchronous_standby_num;
/* called by user backend */
extern void SyncRepWaitForLSN(XLogRecPtr XactCommitLSN);
@@ -49,7 +54,12 @@ extern void SyncRepUpdateSyncStandbysDefined(void);
/* forward declaration to avoid pulling in walsender_private.h */
struct WalSnd;
-extern struct WalSnd *SyncRepGetSynchronousStandby(void);
+
+extern int SyncRepGetSyncStandbys(int *sync_standbys);
+extern bool SyncRepSyncedLsnAdvancedTo(XLogRecPtr *write_pos, XLogRecPtr *flush_pos);
+extern bool SyncRepActiveListedWalSender(int num);
+extern int SyncRepGetSyncStandbys(int *sync_standbys);
+extern bool SyncRepGetSyncLsns(XLogRecPtr *write_pos, XLogRecPtr *flush_pos);
extern bool check_synchronous_standby_names(char **newval, void **extra, GucSource source);
extern void assign_synchronous_commit(int newval, void *extra);
--
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