The Fine Print: What are You Not Telling Me?

Here are technical considerations you may be interested in. We write here things that are not an obvious Requirements & Limitations

Connecting to replica

gh-ost prefers connecting to a replica. If your master uses Statement Based Replication, this is a requirement.

What does "connect to replica" mean?

gh-ost connects to the replica as a normal client
It additionally connects as a replica to the replica (pretends to be a MySQL replica itself)
It auto-detects the master

gh-ost reads the RBR binary logs from the replica, and applies events onto the master as part of the table migration.

THE FINE PRINT:

You trust the replica's binary logs to represent events applied on master.
- If you don't trust the replica, or if you suspect there's data drift between replica & master, take notice.
- If the table on the replica has a different schema than the master, gh-ost likely won't work correctly.
- Our take: we trust replica data; if master dies in production, we promote a replica. Our read serving is based on replica(s).
If your master is RBR, do instead connect gh-ost to master, via --allow-on-master (see cheatsheet).
Replication needs to run.
- This is an obvious, but worth stating. You cannot perform a migration with "connect to replica" if your replica lags. gh-ost will actually do all it can so that replication does not lag, and avoid critical operations if replication is lagging.

gh-ost reads binary logs and then applies them onto the migrated server.

THE FINE PRINT:

gh-ost delivers more network traffic than other online-schema-change tools, that let MySQL handle all data transfer internally. This is part of the triggerless design.
- Our take: we deal with cross-DC migration traffic and this is working well for us.

gh-ost impersonates as a replica: it connects to a MySQL server, says "oh hey, I'm a replica, please send me binary logs kthx".

THE FINE PRINT:

SHOW SLAVE HOSTS or SHOW PROCESSLIST will list this strange "replica" that you can't really connect to.