Re: Change log level for notifying hot standby is waiting non-overflowed snapshot

On 2025/03/12 21:57, torikoshia wrote:
> Hi,
>
> After an off-list discussion with Fujii-san, I'm now trying to modify the following message that is output when a client attempts to connect instead of changing the log level as the original proposal:
>
>   $ psql: error: connection to server at "localhost" (::1), port 5433 failed: FATAL:  the database system is not yet accepting connections
>     DETAIL:  Consistent recovery state has not been yet reached.
>
> I have now 2 candidates to do this.

Thanks for the patches!

> The 1st one(v1-0001-Change-log-message-when-hot-standby-is-not-access.patch) is a simple update to the existing log messages, explicitly mentioning that snapshot overflow could be a possible cause.
> The 2nd(v1-0001-Make-it-clear-when-hot-standby-is-inaccessible-du.patch) one introduces new states for pmState and CAC_state (which manages whether connections can be accepted) to represent waiting for a non-overflowed snapshot.
>
> The advantage of the 2nd one is that it makes it clear whether the connection failure is due to not reaching a consistent recovery state or a snapshot overflow. However, I haven't found other significant benefits, and I feel it might be overkill.

I agree that adding a new postmaster signal and state for
this minor issue seems unnecessary.

> Personally, I feel 1st patch may be sufficient, but I would appreciate any feedback.

Agreed.

- errdetail("Consistent recovery state has not been yet reached.")));
+ errdetail("Consistent recovery state has not been yet reached, or snappshot is pending because subtransaction is overflowed."),
+ errhint("In the latter case, find and close the transaction with more than %d subtransactions", PGPROC_MAX_CACHED_SUBXIDS)));

This message might be too detailed. Instead, how about simplifying it
to something like: "Consistent recovery state has not been reached,
or snapshot is not ready for hot standby."

We can then update the documentation to clarify that overflowed subtransactions
may delay snapshot readiness for hot standby and explain how to address it.
For example, the current description - "it will begin accepting connections once
the recovery has brought the system to a consistent state." - should be updated
to reflect this condition.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION