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Subject: Re: pg_stat_wal_receiver and flushedUpto/writtenUpto
To: Michael Paquier <michael@paquier.xyz>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Postgres hackers <pgsql-hackers@lists.postgresql.org>,
 Thomas Munro <thomas.munro@gmail.com>
References: <20200515230539.GC212736@paquier.xyz>
 <20200515233446.GA334@alvherre.pgsql> <20200516011547.GD212736@paquier.xyz>
 <20200517010857.GI212736@paquier.xyz>
 <c049ffcf-d2fe-90f7-c8ba-0741035aa6a7@oss.nttdata.com>
 <20200519233129.GA2355@paquier.xyz>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <00fb7551-24e5-375b-2dd4-5ea3441458cd@oss.nttdata.com>
Date: Wed, 20 May 2020 10:19:56 +0900
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On 2020/05/20 8:31, Michael Paquier wrote:
> On Tue, May 19, 2020 at 11:38:52PM +0900, Fujii Masao wrote:
>> I found that "received_lsn" is still used in high-availability.sgml.
>> We should apply the following change in high-availability?
>>
>> -     view's <literal>received_lsn</literal> indicates that WAL is being
>> +     view's <literal>flushed_lsn</literal> indicates that WAL is being
> 
> Oops, thanks.  Will fix.

Thanks for the fix!

> 
>> BTW, we have pg_last_wal_receive_lsn() that returns the same lsn as
>> pg_stat_wal_receiver.flushed_lsn. Previously both used the term "receive"
>> in their names, but currently not. IMO it's better to use the same term in
>> those names for the consistency, but it's not good idea to rename
>> pg_last_wal_receive_lsn() to something like pg_last_wal_receive_lsn().
>> I have no better idea for now. So I'm ok with the current names.
> 
> I think you mean renaming pg_last_wal_receive_lsn() to something like
> pg_last_wal_flushed_lsn(), no?

No, that's not good idea, as I told upthread.

> This name may become confusing because
> we lose the "receive" idea in the function, that we have with the
> "receiver" part of pg_stat_wal_receiver.  Maybe something like that,
> though that's long:
> - pg_last_wal_receive_flushed_lsn()
> - pg_last_wal_receive_written_lsn()

Yes, that's long.

> Anyway, a rename of this function does not strike me as strongly
> necessary, as that's less tied with the shared memory structure, and
> we document that pg_last_wal_receive_lsn() tracks the current LSN
> received and flushed.  I am actually wondering if in the future it may
> not be better to remove this function, but it has no maintenance
> cost either so I would just let it as-is.

Yeah, agreed.

Regards,

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