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
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