Re: Upcoming hot standby replication question

Ivan Voras wrote:
> I'd like to ask about the asynchronous nature of upcoming replication
> implementation in 9.0 - what guarantees does it give with regards to
> delays and latency? E.g. do COMMITs "finish" and return to the caller
> before or after the data is sent to the slave? (being asynchronous, they
> probably don't wait for the other side's confirmation, right?).
>

Exactly--synchronous replication, the only way to enforce that data is
on the slave before completing the COMMIT, was postponed from this
release.  It should make it into 9.1 as an option, but it will always be
expensive to turn on.

What is in 9.0 is eventual consistency.  If your slave is keeping up
with traffic being sent by the master, it should receive each
incremental commit shortly after it's made.  In practice, slaves should
only lag some number of seconds behind the master.  But there are zero
guarantees that will be the case, or that latency will be bounded at
all.  Recommended practice is to carefully monitor how much latency lag
there is on the standby and trigger alerts if it exceed your expectations.

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us