Re: make async slave to wait for lsn to be replayed

From: Ivan Kartyshov <i(dot)kartyshov(at)postgrespro(dot)ru>
To: David Steele <david(at)pgmasters(dot)net>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Thom Brown <thom(at)linux(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: make async slave to wait for lsn to be replayed
Date: 2017-03-07 11:48:21
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Rebase done.

Meanwhile I made some more changes.

1) WAITLSN is now implemented as an extension called "pg_waitlsn"

2) Call new hook "lsn_updated_hook" right after xact_redo_commit (xlog.c)

3) Corresponding functions:
pg_waitlsn('0/693FF800', 10000) - wait 10 seconds
pg_waitlsn_infinite('0/693FF800') - for infinite wait
pg_waitlsn_no_wait('0/693FF800') - once check if LSN was replayed or not.

4) Add two GUCs which help tuning influence on StartupXLOG:
count_waitlsn (denominator to check not each LSN)
int count_waitlsn = 10;

interval_waitlsn (Interval in milliseconds to additional LSN check)
int interval_waitlsn = 100;

5) Optimize loop that set latches.

How to use it
1) Make "wal_level = replica"
2) Add shared_preload_libraries = 'pg_waitlsn'
hot_standby = on (in postgresql.conf)
3) Create extension pg_waitlsn;
4) And in hot_standby you can wait for LSN (pgsleep), when LSN will
replayed on slave pg_waitlsn will release

select pg_waitlsn(‘LSN’ [, timeout in ms]);
select pg_waitlsn_infinite(‘LSN’);
select pg_waitlsn_no_wait(‘LSN’);

#Wait until LSN 0/303EC60 will be replayed, or 10 second passed.
select pg_waitlsn(‘0/303EC60’, 10000);

#Or same without timeout.
select pg_waitlsn(‘0/303EC60’);
select pg_waitlsn_infinite('0/693FF800');

#To check if LSN is replayed can be used.
select pg_waitlsn_no_wait('0/693FF800');

Notice: select pg_waitlsn will release on PostmasterDeath or
Interruption events if they come earlier then target LSN or timeout.

Ivan Kartyshov
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
pg_waitlsn10_v4.patch text/x-patch 10.4 KB

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