Re: Shared WAL archive between master and standby: WALs not always identical

On Mon, Feb 27, 2017 at 5:40 PM, Sasa Vilic <sasavilic(at)gmail(dot)com> wrote:

> Aren't WALs from master and standby supposed to be identical?
>

​This would seem unwise to assume on its face and at least one piece of
documentation directly mentions that it is false:

https://www.postgresql.org/docs/9.6/static/warm-standby.html#CONTINUOUS-ARCHIVING-IN-STANDBY

"""
When continuous WAL archiving is used in a standby, there are two different
scenarios: the WAL archive can be shared between the primary and the
standby, or the standby can have its own WAL archive. When the standby has
its own WAL archive, set archive_mode to always, and the standby will call
the archive command for every WAL segment it receives, whether it's by
restoring from the archive or by streaming replication. *The shared archive
can be handled similarly, but the archive_command must test if the file
being archived exists already, and if the existing file has identical
contents*. This requires more care in the archive_command, as it must be
careful to not overwrite an existing file with different contents, but
return success if the exactly same file is archived twice. And all that
must be done free of race conditions, if two servers attempt to archive the
same file at the same time.
"""

​The contents of both must match with respect to the data files but there
are likely things that go into the master WAL stream solely for the purpose
of communicating with a standby - ​and possibly some standby concepts that
would be unique to the standby's WAL - that would cause them to differ.
Not familiar enough to quickly list examples of what those might be. But
IIUC the system seems designed around master->slave replication and doesn't
support slave daisy-chains.

David J.