[Labs-l] Lag reporting on lab db replicas

[Ricordisamoa]

Il 26/11/2015 09:19, Jaime Crespo ha scritto:
> > So even if the replicas don't get updated the heartbeat will report 
> them as up to date?
>
> Not sure exactly what you mean with that. The masters will be updated 
> continuously every 0.5 seconds (all slaves are read only- no writes 
> are done there). If replication works, and slaves get updated, that 
> will mean that they will receive the heartbeat with the same 
> replication channel than the rest of the updates. If replication 
> doesn't work, and replicas do not get updated, they will not receive 
> the heartbeat either, as it comes from replication in order. If 
> replication stops/fails, heartbeat update will stop (from the slave 
> perspective), and lag will start to increase from your perspective 
> (difference between last timestamp written and current time).
>
> This measures the replication lag (aka difference with the master), 
> not the last time an edit was done by a user, which was what the first 
> link I sent measured. In other words, if jaimewiki receives only user 
> edits every hour, heartbeat will still do a write to its master every 
> half a seconds, thus proving that it is up to date with that 
> resolution. You can still check the last user edit by checking 
> recentchanges.
>
> The only reason this could fail (heartbeat updated but wiki not) is if 
> there was a specific filter denying replication but allowing hearbeat, 
> only done for specific tables and private wikis. Also the production 
> master could have a problem, but that would affect the wikis itselves, 
> not only labs.
>
> To give you an idea of the accuracy of this method, we (will) use it 
> on production to decide if a slave is usable or not to return 
> up-to-date data.
>
> For more information on how this works, check 
> <https://www.percona.com/doc/percona-toolkit/2.1/pt-heartbeat.html#description>
>

I don't understand, please explain to a 5 years old :-)