On 2015-02-05 9:36 PM, Angus Lees wrote:
On Fri Feb 06 2015 at 12:59:13 PM Gregory Haynes <g...@greghaynes.net
<mailto:g...@greghaynes.net>> wrote:
Along those lines and in an effort to be a bit less doom-and-gloom, I
spent my lunch break trying to find non-marketing documentation on the
Galera replication protocol and how it is exposed. (It was surprisingly
difficult to find such information *)
It's easy to get the transaction ID of the last commit
(wsrep_last_committed), but I can't find a way to wait until at least a
particular transaction ID has been synced. If we can find that latter
functionality, then we can expose that sequencer all the way through
(HTTP header?) and then any follow-on commands can mention the sequencer
of the previous write command that they really need to see the effects of.
In practice, this should lead to zero additional wait time, since the
Galera replication has almost certainly already caught up by the time
the second command comes in - and we can just read from the local server
with no additional delay.
See the various *Index variables in the etcd API, for how the same idea
gets used there.
I don't use Galera but managed to understand that you don't need all
this complex system, it's already built-in within Galera. Matthew Booth
already mentioned it in his first post.
The wsrep_sync_wait [1][2][3] variable can be scoped to the session and
force a synchronous/committed read if you *really* need it but will
result in larger read latencies.
[1]
http://galeracluster.com/documentation-webpages/mysqlwsrepoptions.html#wsrep-sync-wait
[2]
http://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-system-index.html#wsrep_sync_wait
[3]
https://mariadb.com/kb/en/mariadb/galera-cluster-system-variables/#wsrep_sync_wait
--
Mathieu
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