Re: Conflict detection and logging in logical replication

On Wed, Jul 10, 2024 at 3:09 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Wed, Jul 3, 2024 at 8:31 AM Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > On Wednesday, June 26, 2024 10:58 AM Zhijie Hou (Fujitsu) <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
> >
> > Hi,
> >
> > As suggested by Sawada-san in another thread[1].
> >
> > I am attaching the V4 patch set which tracks the delete_differ
> > conflict in logical replication.
> >
> > delete_differ means that the replicated DELETE is deleting a row
> > that was modified by a different origin.
> >

Thanks for the patch. please find few comments for patch002:

1)
Commit msg says: The conflicts will be tracked only when
track_conflict option of the subscription is enabled.

track_conflict --> detect_conflict

2)
monitoring.sgml: Below are my suggestions, please change if you feel apt.

2a) insert_exists_count : Number of times inserting a row that
violates a NOT DEFERRABLE unique constraint while applying changes.
Suggestion: Number of times a row insertion violated a NOT DEFERRABLE
unique constraint while applying changes.

2b) update_differ_count : Number of times updating a row that was
previously modified by another source while applying changes.
Suggestion: Number of times update was performed on a row that was
previously modified by another source while applying changes.

2c) delete_differ_count: Number of times deleting a row that was
previously modified by another source while applying changes.
Suggestion: Number of times delete was performed on a row that was
previously modified by another source while applying changes.

2d) To be consistent, we can change 'is not found' to 'was not found'
in update_missing_count , delete_missing_count cases as well.

3)
create_subscription.sgml has:
When conflict detection is enabled, additional logging is triggered
and the conflict statistics are collected in the following scenarios:

--Can we rephrase a little and link pg_stat_subscription_stats
structure here for reference.

4)
IIUC, conflict_count array (in pgstat.h) maps directly to ConflictType
enum. So if the order of entries ever changes in this enum, without
changing it in pg_stat_subscription_stats and pg_proc, we may get
wrong values under each column when querying
pg_stat_subscription_stats. If so, then perhaps it is good to add a
comment atop ConflictType that if someone changes this order, order in
other files too needs to be changed.

5)
conflict.h:CONFLICT_NUM_TYPES

--Shall the macro be CONFLICT_TYPES_NUM instead?

6)
pgstatsfuncs.c

-----
/* conflict count */
for (int i = 0; i < CONFLICT_NUM_TYPES; i++)
values[3 + i] = Int64GetDatum(subentry->conflict_count[i]);

/* stats_reset */
if (subentry->stat_reset_timestamp == 0)
nulls[8] = true;
else
values[8] = TimestampTzGetDatum(subentry->stat_reset_timestamp);
-----

After setting values for [3+i], we abruptly had [8]. I think it will
be better to use i++ to increment values' index. And at the end, we
can check if it reached 'PG_STAT_GET_SUBSCRIPTION_STATS_COLS'.

thanks
Shveta