1.9 KiB
Migrating with Statement Based Replication
Even though gh-ost
relies on Row Based Replication (RBR), it does not mean you can't keep your Statement Based Replication (SBR).
gh-ost
is happy to, and actually prefers and suggests to, connect to a replica. On this replica, it is happy to:
- issue the heavyweight
INFORMATION_SCHEMA
queries that make a table structure analysis - issue a
select count(*) from mydb.mytable
, should--exact-rowcount
be provided - connect itself as a fake replica to get the binary log stream
All of the above can be executed on the master, but we're more comfortable that they execute on a replica.
Please note the third item: gh-ost
connects as a fake replica and pulls the binary logs. This is how gh-ost
finds the table's changelog: it looks up entries in the binary log.
The magic is that your master can still produce SBR, but if you have a replica with log-slave-updates
, you can also configure it to have binlog_format='ROW'
. Such a replica accepts SBR statements from its master, and produces RBR statements onto its binary logs.
gh-ost
is happy to modify the binlog_format
on the replica for you:
- If you supply
--switch-to-rbr
,gh-ost
will convert the binlog format for you, and restart replication to make sure this takes effect. - If your replica is an intermediate master, i.e. further serves as a master to other replicas,
gh-ost
will not convert thebinlog_format
. - At any case,
gh-ost
will not convert back toSTATEMENT
(SBR). This is because you may be running multiple migrations concurrently. Being able to run concurrent migrations is one of the design goals of this tool. It's your own responsibility to switch back to SBR once all pending migrations are complete.
Summary
- If you're already using RBR, all is well for you
- If not, convert one of your replicas to
binlog_format='ROW'
, or letgh-ost
do this for you.