- You wish to be notified when `gh-ost` postpones cut-over (at your demand), thus ready to complete (at your leisure)
- RDS users who wish to `--test-on-replica`, but who cannot have `gh-ost` issue a `STOP SLAVE`, would use a hook to command RDS to stop replication
- Send a status message to your chatops every hour
- Perform cleanup on the _ghost_ table (drop/rename/nibble) once migration completes
- etc.
`gh-ost` defines certain points of interest (event types), and executes hooks at those points.
Notes:
- You may have more than one hook per event type.
-`gh-ost` will invoke relevant hooks _sequentially_ and _synchronously_
- thus, you would generally like the hooks to execute as fast as possible, or otherwise issue tasks in the background
- A hook returning with error code will propagate the error in `gh-ost`. Thus, you are able to force `gh-ost` to fail migration on your conditions.
- Make sure to only return an error code when you do indeed wish to fail the rest of the migration
### Creating hooks
All hooks are expected to reside in a single directory. This directory is indicated by `--hooks-path`. When not provided, no hooks are executed.
`gh-ost` will dynamically search for hooks in said directory. You may add and remove hooks to/from this directory as `gh-ost` makes progress (though likely you don't want to). Hook files are expected to be executable processes.
In an effort to simplify code and to standardize usage, `gh-ost` expects hooks in explicit naming conventions. As an example, the `onStartup` hook expects processes named `gh-ost-on-startup*`. It will match and accept files named:
-`gh-ost-on-startup`
-`gh-ost-on-startup--send-notification-mail`
-`gh-ost-on-startup12345`
- etc.
The full list of supported hooks is best found in code: [hooks.go](https://github.com/github/gh-ost/blob/master/go/logic/hooks.go). Documentation will always be a bit behind. At this time, though, the following are recognized:
`gh-ost` will set environment variables per hook invocation. Hooks are then able to read those variables, indicating schema name, table name, `alter` statement, migrated host name etc. Some variables are available on all hooks, and some are available on relevant hooks.
The following variables are available on all hooks:
