Meaning of pg_stats_subscription.latest_end_time - Mailing list pgsql-hackers

Hi,

having just spoken with Harald Armin Massa of 2ndQuadrant, he encouraged me to ask my question here. This is about
publisher/subscriberreplication and was originally posted in german to pgsql-de-allgemein@lists.postgresql.org. 

According to documentation, the field pg_stats_subscription.latest_end_time contains the "Time of last write-ahead log
locationreported to origin WAL sender" [1]. 
The view is built from the internal function pg_stat_get_subscription() [2, line 1097], defined in launcher.c, where we
cansee that the field is populated from a struct LogicalRepWorker. Write access to the relevant struct happens in
worker.cfrom UpdateWorkerStats() but only if reply=true [3, line 1109], which in turn only happens whenever
LogicalRepApplyLoop()deals with a message beginning with 'k' [3, line 1227]. This message is constructed by the
publisherin walsender.c in WalSndKeepalive() [4, line 3435], where the corresponding value is always
GetCurrentTimestamp().

As far as I understand, the WAL replication process has the publisher send messages with WAL slices (type 'w') and
keepalivemessages (type 'k'), to which the subscriber answers with a heartbeat. Other message types are not relevant
here.If the documentation is correct, shouldn't latest_end_time use the creation time of the publisher's most recent
WALslice? On the other hand, wouldn't "time of last communication from WAL sender" be a more appropriate description of
theactual behaviour? And in that case, why only consider 'k'-messages instead of 'w'-messages that contain data? 

Either way, I'm confused as to the meaning of pg_stats_subscription.latest_end_time. The documentation as is also lead
todiscussions in the office, because "reported to ... sender" is ambiguous as to who reports: It might mean some kind
offeedback from the subscriber. 


Please enlighten me, and please excuse the footer, which I can't turn off.

[1] https://www.postgresql.org/docs/12/monitoring-stats.html#PG-STAT-SUBSCRIPTION
[2]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/logical/launcher.c;h=186057bd9327383b7dfdc2d2dda878290caa7967;hb=5060275aa8a1ead56e0a41308d7a43049a6cbe43
[3]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/logical/worker.c;h=790a28fc77e47c7024b8605448311aaa4325fb5b;hb=5060275aa8a1ead56e0a41308d7a43049a6cbe43
[4]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/walsender.c;h=89fbb6fd92788fca66eeb93aaabed7f63dd940c2;hb=5060275aa8a1ead56e0a41308d7a43049a6cbe43


Im Auftrag
Simon Schneider

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