Re: [HACKERS] make async slave to wait for lsn to be replayed

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From: Andy Fan <zhihuifan1213@163.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
Cc: Euler Taveira <euler@eulerto.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Peter Eisentraut <peter@eisentraut.org>
Cc: Amit Kapila <amit.kapila16@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Peter Smith <smithpb2250@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: [HACKERS] make async slave to wait for lsn to be replayed
Date: Tue, 02 Apr 2024 16:14:49 +0800
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Hi Alexander!

>> +        </indexterm>
>> +        <function>pg_wal_replay_wait</function> (
>> +          <parameter>target_lsn</parameter> <type>pg_lsn</type>,
>> +          <parameter>timeout</parameter> <type>bigint</type> <literal>DEFAULT</literal> <literal>0</literal>)
>> +        <returnvalue>void</returnvalue>
>> +       </para>
>>
>> Should we return the millseconds of waiting time?  I think this
>> information may be useful for customer if they want to know how long
>> time it waits for for minitor purpose.
>
> Please, check it more carefully.  In v17 timeout is in integer milliseconds.

I guess one of us misunderstand the other:( and I do didn't check the
code very carefully. 

Acutally I meant the "return value" rather than function argument. IIUC
the current return value is void per below statement.

>> +        <returnvalue>void</returnvalue>

If so, when users call pg_wal_replay_wait, they can get informed when
the wal is replaied to the target_lsn, but they can't know how long time
it waits unless they check it in application side, I think such
information will be useful for monitor purpose sometimes. 

select pg_wal_replay_wait(lsn, 1000);  may just take 1ms in fact, in
this case, I want this function return 1. 

-- 
Best Regards
Andy Fan

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