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

From: Kartyshov Ivan <i(dot)kartyshov(at)postgrespro(dot)ru>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] make async slave to wait for lsn to be replayed
Date: 2020-03-05 21:24:01
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On 2018-03-06 14:50, Simon Riggs wrote:
> On 6 March 2018 at 11:24, Dmitry Ivanov <d(dot)ivanov(at)postgrespro(dot)ru>
> wrote:
>>> In PG11, I propose the following command, sticking mostly to Ants'
>>> syntax, and allowing to wait for multiple events before it returns.
>>> It
>>> doesn't hold snapshot and will not get cancelled by Hot Standby.
>>> WAIT FOR event [, event ...] options
>>> event is
>>> LSN value
>>> TIMESTAMP value
>>> options
>>> TIMEOUT delay
>>> UNTIL TIMESTAMP timestamp
>>> (we have both, so people don't need to do math, they can use
>>> whichever
>>> they have)
>> I have a (possibly) dumb question: if we have specified several
>> events,
>> should WAIT finish if only one of them triggered? It's not immediately
>> obvious if we're waiting for ALL of them to trigger, or just one will
>> suffice (ANY). IMO the syntax could be extended to something like:
>> WAIT FOR [ANY | ALL] event [, event ...] options,
>> with ANY being the default variant.
> +1

Here I made new patch of feature, discussed above.

WAIT FOR - wait statement to pause beneath statements

WAIT FOR [ANY | SOME | ALL] event [, event ...] options
and event is:
LSN value

and options is:
WAIT FOR - make to wait statements (that are beneath) on sleep until
event happens (Don’t process new queries until an event happens).

How to use it
WAIT FOR LSN ‘LSN’ [, timeout in ms];

#Wait until LSN 0/303EC60 will be replayed, or 10 second passed.
WAIT FOR ANY LSN ‘0/303EC60’, TIMEOUT 10000;

#Or same without timeout.
WAIT FOR LSN ‘0/303EC60’;

#Or wait for some timestamp.
WAIT FOR TIMESTAMP '2020-01-02 17:20:19.028161+03';

#Wait until ALL events occur: LSN to be replayed and timestamp
WAIT FOR ALL LSN ‘0/303EC60’, TIMESTAMP '2020-01-28 11:10:39.021341+03';

Notice: WAIT FOR will release on PostmasterDeath or Interruption events
if they come earlier then LSN or timeout.

Testing the implementation
The implementation was tested with src/test/recovery/t/

Ivan Kartyshov
Postgres Professional:
The Russian Postgres Company

Attachment Content-Type Size
wait_for_v1.patch text/x-diff 28.3 KB


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