Hi,
hi, I'm playing with Synchronous Replication on PostgreSQL 9.2.2.
And I saw a strange behavior.
Unless you left out something, the configuration you described
actually sets up asynchronous replication.
Thank you for the comment.
I was thinking to promote one of them and set
synchronous_standby_names = 'sby' and reload configure file.
Before that, I noticed the issue.
=================================================================
[issues]
two standbys are connected on each other, but
no master is there.
...
=================================================================
I did not see the situation like above on PostgreSQL 9.1.7.
Is this intended change?
In 9.1, this scenario was impossible because you could not connect a
standby to another standby. In 9.2, that's allowed. It's a new feature
called "cascading replication", see
http://www.postgresql.org/docs/9.2/static/warm-standby.html#CASCADING-REPLICATION.
With that feature, it's indeed possible to form a cycle of standby
servers connected to each other. There was just a long discussion on
pgsql-hackers on whether we should try to detect that scenario [1],
but the consensus seems to be that we should not. It would be
difficult to implement such detection, and sometimes it's useful to
have such a cycle, as a transient state at a failover, for example.
Sorry, I had not read the discussion and have reported it again.
And I understand it's hard to detect cyclic situation.
So the bottom line is that this is an intended change, and the admin
will just have to avoid doing that.
OK.
This situation was made by playing, so it's not big problem for now.
This makes me wonder if there should be a GUC to forbid cascading
replication, though. If you don't want to do cascading replication
(which is quite rare, I'd say), you could just disable it to avoid a
situation like this.
I'm not sure but what about adding the parameter("cascade_mode") on
recovery.conf ?
The parameter represents a will to connect to standby server when
starting as standby.
If the parameter is set to on, connect to a server forcely like
PostgreSQL 9.2,
and if the parameter is set to off, connect to the another standby server is
refused like PostgreSQL 9.1.
[1] http://archives.postgresql.org/pgsql-hackers/2012-12/msg01134.php
- Heikki
regards,
--------
NTT Software Corporation
Tomonari Katsumata
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