Thread: invisible commit question for sync replication
When run a cluster with sync replication, if DML is done on primary, but primary is isolated from all slave, then the DML will hang, if cancel it DML, it will say:
WARNING: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby
So the workflow is
1: commit to local.
2: waiting for ACK from remote sync.
When cancel the DML at step 2. the data are arealy on local, that's why it's warning.
But when runs an insert which is waiting for remote ACK, and then query from another session, I didn't find that row. Why this happen? If the insert is already one locally, whey another session can't read it?
Hi,
On Wed, Feb 01, 2023 at 02:52:49PM +0800, qihua wu wrote:
> When run a cluster with sync replication, if DML is done on primary, but
> primary is isolated from all slave, then the DML will hang, if cancel it
> DML, it will say:
> WARNING: canceling wait for synchronous replication due to user request
> DETAIL: The transaction has already committed locally, but might not have
> been replicated to the standby
>
> So the workflow is
> 1: commit to local.
> 2: waiting for ACK from remote sync.
>
> When cancel the DML at step 2. the data are arealy on local, that's why
> it's warning.
>
> But when runs an insert which is waiting for remote ACK, and then query
> from another session, I didn't find that row. Why this happen? If the
> insert is already one locally, whey another session can't read it?
It works as expected for me, are you sure both sessions are actually connected
to the same server and/or querying the same table?
[1456]rjuju@127.0.0.1:14295) rjuju=# select * from tt;
id | val
----+-----
(0 rows)
[1456]rjuju@127.0.0.1:14295) rjuju=# insert into tt select 1;
^CCancel request sent
WARNING: 01000: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
LOCATION: SyncRepWaitForLSN, syncrep.c:287
INSERT 0 1
[1456]rjuju@127.0.0.1:14295) rjuju=# select pg_backend_pid(), * from tt;
pg_backend_pid | id | val
----------------+----+--------
1456 | 1 | <NULL>
(1 row)
and another session:
[3327]rjuju@127.0.0.1:14295) rjuju=# select pg_backend_pid(), * from tt;
pg_backend_pid | id | val
----------------+----+--------
3327 | 1 | <NULL>
(1 row)
On Wednesday, February 1, 2023, Julien Rouhaud <rjuju123@gmail.com> wrote:
Hi,
On Wed, Feb 01, 2023 at 02:52:49PM +0800, qihua wu wrote:
> When run a cluster with sync replication, if DML is done on primary, but
> primary is isolated from all slave, then the DML will hang, if cancel it
> DML, it will say:
> WARNING: canceling wait for synchronous replication due to user request
> DETAIL: The transaction has already committed locally, but might not have
> been replicated to the standby
>
> So the workflow is
> 1: commit to local.
> 2: waiting for ACK from remote sync.
>
> When cancel the DML at step 2. the data are arealy on local, that's why
> it's warning.
>
> But when runs an insert which is waiting for remote ACK, and then query
> from another session, I didn't find that row. Why this happen? If the
> insert is already one locally, whey another session can't read it?
It works as expected for me, are you sure both sessions are actually connected
to the same server and/or querying the same table?
[1456]rjuju@127.0.0.1:14295) rjuju=# select * from tt;
id | val
----+-----
(0 rows)
[1456]rjuju@127.0.0.1:14295) rjuju=# insert into tt select 1;
^CCancel request sent
WARNING: 01000: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
LOCATION: SyncRepWaitForLSN, syncrep.c:287
INSERT 0 1
[1456]rjuju@127.0.0.1:14295) rjuju=# select pg_backend_pid(), * from tt;
pg_backend_pid | id | val
----------------+----+--------
1456 | 1 | <NULL>
(1 row)
and another session:
[3327]rjuju@127.0.0.1:14295) rjuju=# select pg_backend_pid(), * from tt;
pg_backend_pid | id | val
----------------+----+--------
3327 | 1 | <NULL>
(1 row)
This wasn’t the question though. Can the second session see the inserted row before you cancel the insert that is waiting for sync ack?
Supposedly it can (not able to test myself). Basically, the primary waits to make the local transaction visible until either sync ack or until the wait for sync ack is cancelled. It doesn’t make sense to make it visible while waiting for sync ack since that would defeat the very behavior sync ack provides for.
David J.
==》Can the second session see the inserted row before you cancel the insert that is waiting for sync ack?
The second session can NOT see the inserted row if the first session is still waiting for sync ACK.
I checked the source code, it makes sense to me now:
The waiting for sync ACK is called in
EndPrepare(gxact);
and after that it will call
ProcArrayClearTransaction(MyProc);
and only after ProcArrayClearTransaction(MyProc) is called, will the row be visible to others.
On Wed, Feb 1, 2023 at 3:21 PM David G. Johnston <david.g.johnston@gmail.com> wrote:
On Wednesday, February 1, 2023, Julien Rouhaud <rjuju123@gmail.com> wrote:Hi,
On Wed, Feb 01, 2023 at 02:52:49PM +0800, qihua wu wrote:
> When run a cluster with sync replication, if DML is done on primary, but
> primary is isolated from all slave, then the DML will hang, if cancel it
> DML, it will say:
> WARNING: canceling wait for synchronous replication due to user request
> DETAIL: The transaction has already committed locally, but might not have
> been replicated to the standby
>
> So the workflow is
> 1: commit to local.
> 2: waiting for ACK from remote sync.
>
> When cancel the DML at step 2. the data are arealy on local, that's why
> it's warning.
>
> But when runs an insert which is waiting for remote ACK, and then query
> from another session, I didn't find that row. Why this happen? If the
> insert is already one locally, whey another session can't read it?
It works as expected for me, are you sure both sessions are actually connected
to the same server and/or querying the same table?
[1456]rjuju@127.0.0.1:14295) rjuju=# select * from tt;
id | val
----+-----
(0 rows)
[1456]rjuju@127.0.0.1:14295) rjuju=# insert into tt select 1;
^CCancel request sent
WARNING: 01000: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
LOCATION: SyncRepWaitForLSN, syncrep.c:287
INSERT 0 1
[1456]rjuju@127.0.0.1:14295) rjuju=# select pg_backend_pid(), * from tt;
pg_backend_pid | id | val
----------------+----+--------
1456 | 1 | <NULL>
(1 row)
and another session:
[3327]rjuju@127.0.0.1:14295) rjuju=# select pg_backend_pid(), * from tt;
pg_backend_pid | id | val
----------------+----+--------
3327 | 1 | <NULL>
(1 row)This wasn’t the question though. Can the second session see the inserted row before you cancel the insert that is waiting for sync ack?Supposedly it can (not able to test myself). Basically, the primary waits to make the local transaction visible until either sync ack or until the wait for sync ack is cancelled. It doesn’t make sense to make it visible while waiting for sync ack since that would defeat the very behavior sync ack provides for.David J.
On Wed, 2023-02-01 at 14:52 +0800, qihua wu wrote: > When run a cluster with sync replication, if DML is done on primary, but primary is > isolated from all slave, then the DML will hang, if cancel it DML, it will say: > WARNING: canceling wait for synchronous replication due to user request > DETAIL: The transaction has already committed locally, but might not have been replicated to the standby > > So the workflow is > 1: commit to local. > 2: waiting for ACK from remote sync. > > When cancel the DML at step 2. the data are arealy on local, that's why it's warning. > > But when runs an insert which is waiting for remote ACK, and then query from another > session, I didn't find that row. Why this happen? If the insert is already one locally, > whey another session can't read it? COMMIT is not as atomic as it appears. When the backend is waiting for the standby, it has already committed the transaction on disk, but that fact is not advertised to the other backends yet. Yours, Laurenz Albe
In the code it will write a warning to postgresql log. Why not also write the detailed sql? with the exact sql, DBA might do something to fix the issue.
if (ProcDiePending)
{
ereport(WARNING,
(errcode(ERRCODE_ADMIN_SHUTDOWN),
errmsg("canceling the wait for synchronous replication and terminating connection due to administrator command"),
errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
whereToSendOutput = DestNone;
SyncRepCancelWait();
break;
}
{
ereport(WARNING,
(errcode(ERRCODE_ADMIN_SHUTDOWN),
errmsg("canceling the wait for synchronous replication and terminating connection due to administrator command"),
errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
whereToSendOutput = DestNone;
SyncRepCancelWait();
break;
}
On Wed, Feb 1, 2023 at 4:38 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:
On Wed, 2023-02-01 at 14:52 +0800, qihua wu wrote:
> When run a cluster with sync replication, if DML is done on primary, but primary is
> isolated from all slave, then the DML will hang, if cancel it DML, it will say:
> WARNING: canceling wait for synchronous replication due to user request
> DETAIL: The transaction has already committed locally, but might not have been replicated to the standby
>
> So the workflow is
> 1: commit to local.
> 2: waiting for ACK from remote sync.
>
> When cancel the DML at step 2. the data are arealy on local, that's why it's warning.
>
> But when runs an insert which is waiting for remote ACK, and then query from another
> session, I didn't find that row. Why this happen? If the insert is already one locally,
> whey another session can't read it?
COMMIT is not as atomic as it appears. When the backend is waiting for the standby,
it has already committed the transaction on disk, but that fact is not advertised to
the other backends yet.
Yours,
Laurenz Albe