Re: Synchronization levels in SR

On Tue, Sep 7, 2010 at 5:17 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> We can *not* allow the slave to replay WAL ahead of what is known
> committed to disk on the master.  The only way to make that safe
> is the compare-notes-and-ship-WAL-back approach that Robert mentioned.
>
> If you feel that decoupling WAL application is absolutely essential
> to have a credible feature, then you'd better bite the bullet and
> start working on the ship-WAL-back code.
>

In the mode where it is not required that the WAL is applied (only
sent to the slave / synced to slave disk) one alternative is to have a
separate pointer to the last WAL record that can be safely applied on
the slave. Then You can send the un-synced WAL to the slave (while
concurrently syncing it on the master). When both the slave an the
master sync complete, one can give the client a commit notification,
increase the pointer, and send it to the slave (it would be a separate
WAL record type I guess).

In case of master failure, the slave can discard the un-applied WAL
after the pointer.

Greetings
marcin