Re: synchronous streaming replication

On Sat, 2022-10-22 at 17:28 +0200, Joseph Kennedy wrote:
> Hi,I have a question. Its stays at the intersection of software engineering and PostgreSQL.
>
> I have configured streaming synchronous replication and whit setting "synchronous_commit=remote_apply"
> to make sure that the slave will always respond the same as the MASTER (this is a developers'
> requirement that the MASTER always responds the same as SLAVE). I set "hot_standby_feedback=on"
> and "max_standby_streaming_delay=-1",
> max_standby_streaming_delay set to -1 to make MASTER wait indefinitely before SELECT conflicts on the SLAVE will end.
>  
> Here's where the problem arises, because not long after the replication has been started some serious
> delays occur in the form of "replay_lag" - which rather indicates the appearance of conflicts;
> the replication stops working properly.
>  
> From the server logs it appears that UPDATE (select for update) has occurred on the MASTER, and SELECT
> queries are in progress on SLAVE causing replication conflicts, with setting "max_standby_streaming_delay=-1"
> they never ends and there are huge lags.
>  
> From the findings with the developers it emerged that they do not want me to set max_standby_streaming_delay
> to a value after which the queries conflicted with replication will be canceled.
>  * So I'm wondering if, in this configuration, it can work properly at all without setting, for example,
> "max_standby_streaming_delay=30" ?
>  * On the other hand I wonder if the application should not be developed in such a way to support replication
> of PostgreSQL configured as a streaming synchronous replication cluster with "synchronous_commit=remote_apply" ?
>  * Or perhaps "synchronous streaming replication" is a bad choice, maybe logical replication would be better ?
>  * What are the best practices?
>  * Perhaps you just need to force/teach applications to work with synchronous replication in such a way that
> when the SELECT causes conflicts with replication such queries are canceled and the application should resend/repeat query ?
>  * I also think that after setting, for example, "max_standby_streaming_delay=30" queries (addressed) to the
> database should be very well optimized, so that too long queries are not canceled too frequently?
>  * Do you know any books focused on applications adapted to work in postgresql synchronous streaming
> replication environment i.e. High Availability?

This can never work properly. If you have synchronous replication with "synchronous_commit = remote_apply",
COMMIT on the primary will wait until the information has been replayed on the standby. If you set
"max_standby_streaming_delay = -1", replication can be delayed indefinitely long in the event of a replication
conflict, so COMMIT can take arbitrarily long.

You can reduce replication conflicts (by setting "hot_standby_feedback = on" and by altering all tables to
set "vacuum_truncate = off"), but you will never get rid of them completely.

You will either have to accept stale ready on the standby (by setting "synchronous_commit" to something lower)
or you have to accept canceled queries on the standby (by lowering "max_standby_streaming_delay").

Yours,
Laurenz Albe