Re: Question: Is it possible to get the new xlog position after query execution?

On 2021-11-01 00:36:16 -0400, Oleg Serov wrote:
> On Sun, Oct 31, 2021 at 4:29 PM Peter J. Holzer <hjp-pgsql@hjp.at> wrote:
>
>     On 2021-10-29 13:22:56 -0400, Oleg Serov wrote:
>     > We are using a master/slave replication system where we perform
>     > writes on master and use replication to offload reads.
>     >
>     > However, sometimes we have a replication lag of a few seconds
>     > and as a result, after the update, the change is not yet
>     > available on the replica. 
>     >
>     > Is there a way to get XLOG position to which specific update
>     > query will be written? That way we can check if our replica
>     > caught up with changes and it is safe to read it from. Can it be
>     > done using SQL functions? Can I get that information from query
>     > protocol?
>
>     I think I would prefer a more direct approach:
>
>     If you know what you've written, can't you just check whether the
>     replica has the new value(s)?
>
> The simplest answer: One thread on a single process on a server knows about it.
> Now another thread on another process/other server does not know about it.

So why would that other thread know the relevant XLOG position?

>     If not, an alternative could be a table which contains a simple counter
>     or timestamp:
>
>         begin;
>         (lots of updates ...)
>         commit;
>         begin;
>         update counter set c = c + 1 returning c; -- save this as c_current
>         commit;
>
>         Select c from counter on the replica in a loop until c >= c_current.
>
> Why invent something totally new when XLOG position is already used for
> replication by postgres? What are the benefits of it?

Because you had to ask. That shows that it isn't obvious. So your
application relies on some non-obvious (and possibly version-dependent)
implementation details of the database to ensure ordering. Using
something that makes sense from the application perspective (a timestamp
or a counter are just examples - your application may already have some
information which can use be used for that purpose) makes it more
obvious for the application programmer. (I'm generally a big fan of
end-to-end checks and testing what you are really interested in. If want
X but argue that X is true if Y is true and Y is true if Z is true, and
then go on to test for Z, that usually makes code hard to understand. It
is sometimes useful or even necessary (e.g. if X cannot be tested
directly), but it should IMHO be restricted to those cases.)

        hp

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