Re: Minimal logical decoding on standbys

On Fri, 14 Dec 2018 at 06:25, Andres Freund <andres@anarazel.de> wrote:
> I've a prototype attached, but let's discuss the details in a separate
> thread. This also needs to be changed for pluggable storage, as we don't
> know about table access methods in the startup process, so we can't call
> can't determine which AM the heap is from during
> btree_xlog_delete_get_latestRemovedXid() (and sibling routines).

Attached is a WIP test patch
0003-WIP-TAP-test-for-logical-decoding-on-standby.patch that has a
modified version of Craig Ringer's test cases
(012_logical_decoding_on_replica.pl) that he had attached in [1].
Here, I have also attached his original file
(Craigs_012_logical_decoding_on_replica.pl).

Also attached are rebased versions of couple of Andres's implementation patches.

I have added a new test scenario :
DROP TABLE from master *before* the logical records of the table
insertions are retrieved from standby. The logical records should be
successfully retrieved.


Regarding the test result failures, I could see that when we drop a
logical replication slot at standby server, then the catalog_xmin of
physical replication slot becomes NULL, whereas the test expects it to
be equal to xmin; and that's the reason a couple of test scenarios are
failing :

ok 33 - slot on standby dropped manually
Waiting for replication conn replica's replay_lsn to pass '0/31273E0' on master
done
not ok 34 - physical catalog_xmin still non-null
not ok 35 - xmin and catalog_xmin equal after slot drop
#   Failed test 'xmin and catalog_xmin equal after slot drop'
#   at t/016_logical_decoding_on_replica.pl line 272.
#          got:
#     expected: 2584



Other than the above, there is this test scenario which I had to remove :

#########################################################
# Conflict with recovery: xmin cancels decoding session
#########################################################
#
# Start a transaction on the replica then perform work that should cause a
# recovery conflict with it. We'll check to make sure the client gets
# terminated with recovery conflict.
#
# Temporarily disable hs feedback so we can test recovery conflicts.
# It's fine to continue using a physical slot, the xmin should be
# cleared. We only check hot_standby_feedback when establishing
# a new decoding session so this approach circumvents the safeguards
# in place and forces a conflict.

This test starts pg_recvlogical, and expects it to be terminated due
to recovery conflict because hs feedback is disabled.
But that does not happen; instead, pg_recvlogical does not return.

But I am not sure why it does not terminate with Andres's patch; it
was expected to terminate with Craig Ringer's patch.

Further, there are subsequent test scenarios that test pg_recvlogical
with hs_feedback disabled, which I have removed because pg_recvlogical
does not return. I am yet to clearly understand why that happens. I
suspect that is only because hs_feedback is disabled.

Also, the testcases verify pg_controldata's oldestCatalogXmin values,
which are now not present with Andres's patch; so I removed tracking
of oldestCatalogXmin.

[1] https://www.postgresql.org/message-id/CAMsr+YEVmBJ=dyLw=+kTihmUnGy5_EW4Mig5T0maieg_Zu=XCg@mail.gmail.com

Thanks
-Amit Khandekar