gh-ost

# gh-ost

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#### GitHub's online schema migration for MySQL <img src="doc/images/gh-ost-logo-light-160.png" align="right">

 `gh-ost` is a triggerless online schema migration solution for MySQL. It is testable and provides pausability, dynamic control/reconfiguration, auditing, and many operational perks.

`gh-ost` produces a light workload on the master throughout the migration, decoupled from the existing workload on the migrated table.

It has been designed based on years of experience with existing solutions, and changes the paradigm of table migrations.



## How?

All existing online-schema-change tools operate in similar manner: they create a _ghost_ table in the likeness of your original table, migrate that table while empty, slowly and incrementally copy data from your original table to the _ghost_ table, meanwhile propagating ongoing changes (any `INSERT`, `DELETE`, `UPDATE` applied to your table) to the _ghost_ table. Finally, at the right time, they replace your original table with the _ghost_ table.

`gh-ost` uses the same pattern. However it differs from all existing tools by not using triggers. We have recognized the triggers to be the source of [many limitations and risks](doc/why-triggerless.md).

Instead, `gh-ost` [uses the binary log stream](doc/triggerless-design.md) to capture table changes, and asynchronously applies them onto the _ghost_ table. `gh-ost` takes upon itself some tasks that other tools leave for the database to perform. As result, `gh-ost` has greater control over the migration process; can truly suspend it; can truly decouple the migration's write load from the master's workload.

In addition, it offers many [operational perks](doc/perks.md) that make it safer, trustworthy and fun to use.

![gh-ost general flow](doc/images/gh-ost-general-flow.png)

## Highlights

- Build your trust in `gh-ost` by testing it on replicas. `gh-ost` will issue same flow as it would have on the master, to migrate a table on a replica, without actually replacing the original table, leaving the replica with two tables you can then compare and satisfy yourself that the tool operates correctly. This is how we continuously test `gh-ost` in production.
- True pause: when `gh-ost` [throttles](doc/throttle.md), it truly ceases writes on master: no row copies and no ongoing events processing. By throttling, you return your master to its original workload
- Dynamic control: you can [interactively](doc/interactiv