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Added note about table schema needing to be identical

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Gillian Gunson 2016-10-19 05:29:34 -07:00 committed by GitHub
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doc/the-fine-print.md
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@ -4,24 +4,27 @@ Here are technical considerations you may be interested in. We write here things
# Connecting to replica
`gh-ost` prefers connecting to replica. If your master uses Statement Based Replication, this is a _requirement_.
`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 master
- It auto-detects the master
`gh-ost` reads the RBR binary logs from the replica, and applies events onto the master as tables are being migrated.
`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, if you suspect there's data drift between replica & master, take notice. If your master is RBR, do instead connect `gh-ost` to master, via `--allow-on-master` (see [cheatsheet](cheatsheet.md)).
Our take: we trust replica data; if master dies in production, we promote a replica. Our read serving is based on replica(s).
- 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](cheatsheet.md)).
- 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 at such time when replication does lag.
- 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.
# Network usage
@ -30,12 +33,12 @@ THE FINE PRINT:
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](triggerless-design.md).
Our take: we deal with cross-DC migration traffic and this is working well for us.
- Our take: we deal with cross-DC migration traffic and this is working well for us.
# Impersonating as a replica
`gh-ost` impersonates as a replica: connects to a MySQL server, says "oh hey, I'm a replica, please send me binary logs kthx".
`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 down this strange "replica" that you can't really connect to.
- `SHOW SLAVE HOSTS` or `SHOW PROCESSLIST` will list this strange "replica" that you can't really connect to.