Meaning of pg_stats_subscription.latest_end_time

Hi,

having just spoken with Harald Armin Massa of 2ndQuadrant, he encouraged me to ask my question here. This is about publisher/subscriber replication 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 location reported to origin WAL sender" [1]https://www.postgresql.org/docs/12/monitoring-stats.html#PG-STAT-SUBSCRIPTION.
The view is built from the internal function pg_stat_get_subscription() [2, line 1097], defined in launcher.c, where we can see that the field is populated from a struct LogicalRepWorker. Write access to the relevant struct happens in worker.c from 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 publisher in 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 keepalive messages (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 WAL slice? On the other hand, wouldn't "time of last communication from WAL sender" be a more appropriate description of the actual 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 to discussions in the office, because "reported to ... sender" is ambiguous as to who reports: It might mean some kind of feedback 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

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