Re: Core team statement on replication in PostgreSQL

Tom Lane wrote:
> In practice, simple asynchronous single-master-multiple-slave
> replication covers a respectable fraction of use cases, so we have
> concluded that we should allow such a feature to be included in the core
> project. We emphasize that this is not meant to prevent continued
> development of add-on replication projects that cover more complex use
> cases.

IMHO, this will help PostgreSQL adoption, mindshare and even boost interest in
development for the other replication use cases.

> We believe that the most appropriate base technology for this is
> probably real-time WAL log shipping, as was demoed by NTT OSS at PGCon.

The slides are up at http://www.pgcon.org/2008/schedule/events/76.en.html
From what I gather from those slides it seems to me that the NTT solution is
synchronous not asynchronous. In my opinion it's even better, but I do
understand that others might prefer asynchronous. I'm going to speculate, but I
would think it should be possible (without a substancial rewrite) to support
both modes (or even some intermediate modes, like DRBD on Linux).

> We hope that such a feature can be completed for 8.4. Ideally this
> would be coupled with the ability to execute read-only queries on the
> slave servers, but we see technical difficulties that might prevent that
> from being completed before 8.5 or even further out. (The big problem
> is that long-running slave-side queries might still need tuples that are
> vacuumable on the master, and so replication of vacuuming actions would
> cause the slave's queries to deliver wrong answers.)

From the 8.4dev documentation, another problem for read-only slaves would be :
« Operations on hash indexes are not presently WAL-logged, so replay will not
update these indexes. The recommended workaround is to manually REINDEX each
such index after completing a recovery operation. ».

Sincerely,
--
Mathias Brossard