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gh-ost/doc/hooks.md

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Hooks

gh-ost supports hooks: external processes which gh-ost executes at particular points of interest.

Use cases include:

  • You wish to be notified ny mail when a migration completes/fails
  • 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. Documentation will always be a bit behind. At this time, though, the following are recognized:

  • gh-ost-on-startup
  • gh-ost-on-validated
  • gh-ost-on-rowcount-complete
  • gh-ost-on-before-row-copy
  • gh-ost-on-status
  • gh-ost-on-interactive-command
  • gh-ost-on-row-copy-complete
  • gh-ost-on-stop-replication
  • gh-ost-on-begin-postponed
  • gh-ost-on-before-cut-over
  • gh-ost-on-success
  • gh-ost-on-failure

Context

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:

  • GH_OST_DATABASE_NAME
  • GH_OST_TABLE_NAME
  • GH_OST_GHOST_TABLE_NAME
  • GH_OST_OLD_TABLE_NAME
  • GH_OST_DDL
  • GH_OST_ELAPSED_SECONDS
  • GH_OST_MIGRATED_HOST
  • GH_OST_INSPECTED_HOST
  • GH_OST_EXECUTING_HOST
  • GH_OST_HOOKS_HINT

The following variable are available on particular hooks:

  • GH_OST_COMMAND is only available in gh-ost-on-interactive-command
  • GH_OST_STATUS and GH_OST_ELAPSED_SECONDS are only available in gh-ost-on-status

Examples

See sample hooks, as bash implementation samples.