Re: Synchronous Replication: Where is data visible first?

On Wednesday, January 10, 2024, P C <puravc(at)gmail(dot)com> wrote:

> Hello,
>
> While discussing with the development team, an interesting question came
> up: in a synchronous streaming replication scenario, with
> synchronous_commit = remote_apply, will the change be first visible on
> Standby (replica)? Primary will wait till the change is applied and
> committed on standby, and hence logically this looks to be correct. But I
> couldn't find this mentioned explicitly anywhere and hence seeking comments
> from the community.
>

IIUC, no, the standbys can never reflect a newer state than what would be
seen on the primary. The state of a given transaction, including most
importantly the locking surrounding it, exists first on the primary and
then is replicated to the secondary. Either the locking will prevent a
dirty read or the dirty read will see the unconfirmed but committed data on
the primary. The locking will be removed on the primary strictly before
the standbys.

David J.