After my presentation at FOSDEM I got a few questions regarding our Galera implementation and why we did things the way we want.
First of all, the slides:
Second of all, the questions I got:
Q: Why first copy all the data to a new MySQL server using innobackupex and then perform the mysqldump?
This is a question regarding the consolidation of multiple existing asynchronous replicated clusters to a new Galera cluster.
In the slides I showed we use an active-inactive Master-Master setup where one of the MySQL masters is receiving all write-traffic while the inactive master is receiving read-traffic. If we would perform the mysqldump on the inactive master we either have to drain the inactive master from read-traffic and stop replication or it will lock the tables and we would not have a frozen snapshot.
A related question was asked why we do not use innobackupex to feed the backup to the Galera cluster and then create the cluster from this as a starting point. That could be done for the first node, however we wish to consolidate multiple clusters into one Galera cluster we have to ensure the data gets replicated into the new cluster online. Therefore mysqldump is the only viable solution here.
Q: Why are you using MMM?
This is a choice we made five years ago. It worked fine enough for us and we stuck to it till today. We do know it is flawed (some say be design) and we know it has a lot of drawbacks and it is actually one of the drivers to start using Galera.
Q: Why don’t you expect clashes when writing the same data twice at the same time?
In our sharded environment (the Spil Storage Platform) will never write the same data twice as every piece of data that is sharded by user, function and location will have its own owner process in this platform. This means there will never be a second process writing the same piece of data. In other words: our environment allows us to isolate writes and we never expect clashes.
In our other (current) environments the number of writes is low, so the chance of a clash will be low.
If you have any other question, don’t hesitate to reach out to us or place a comment below.