Il 28/11/2015 20:58, Bryan Davis ha scritto:
On Sat, Nov 28, 2015 at 12:28 PM, Ricordisamoa
<[email protected]> wrote:
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 :-)
I'll try:
* Each master server has a "heartbeat" table where it updates a row
every 0.5 seconds with a timestamp value.
* Each replica server has a copy of this heartbeat table that only
receives updated timestamps via replication.
* Each replica server also has a view that shows the difference
between the current system time and the heartbeat table's timestamp.
* This difference (current system time - last timestamp seen from
master) is the true replication delta between the master and the
replica.
* The only way that a replica server could see an updated tiemstamp
from the master without also having all other changes locally would be
via explicit configuration that allowed heartbeat updates but excluded
others.
Bryan
Thank you :-)
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