Re: Synchronous Log Shipping Replication

On Tue, Sep 9, 2008 at 8:38 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> There's one thing I haven't figured out in this discussion. Does the write
> to the disk happen before or after the write to the slave? Can you guarantee
> that if a transaction is committed in the master, it's also committed in the
> slave, or vice versa?

We can guarantee that a transaction is committed in both the master and
the slave if we can wait for that one fsyncs WAL to disk and the other holds
it to memory or disk. Even if one fails, the other can continue service.
Even if both fail, the node which wrote WAL can continue service. A transaction
is lost in neither of the cases.

> Agreed. But what happens if you hit that timeout?

The stand-alone master continues service when it it that timeout. On the other
hand, the slave waits for the order by the sysadmin or the clustering software,
then it exits or becomes master.

> Should we enforce that
> timeout within the server, or should we leave that to the external heartbeat
> system?

Within the server. All users do not use such an external system. It's not simple
for the external system to leave the master stand-alone.

regards

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center