# Requirements and limitations

### Requirements

- You will need to have one server serving Row Based Replication (RBR) format binary logs. `gh-ost` prefers to work with replicas. You may [still have your master configured with Statement Based Replication](migrating-with-sbr) (SBR).

- `gh-ost` requires an account with these privileges:

  - `ALTER, CREATE, DELETE, DROP, INDEX, INSERT, LOCK TABLES, SELECT, TRIGGER, UPDATE` on the database (schema) where your migrated table is, or of course on `*.*`
  - `SUPER, REPLICATION SLAVE` on `*.*`

### Limitations

- Foreign keys not supported. They may be supported in the future, to some extent.
- Triggers are not supported. they may be supported in the future.
- The two _before_ & _after_ tables must share some `UNIQUE KEY`. Such key would be used by `gh-ost` to iterate the table.
  - As an example, if your table has a single `UNIQUE KEY` and no `PRIMARY KEY`, and you wish to replace it with a `PRIMARY KEY`, you will need two migrations: one to add the `PRIMARY KEY` (this migration will use the existing `UNIQUE KEY`), another to drop the now redundant `UNIQUE KEY` (this migration will use the `PRIMARY KEY`).
- The chosen migration key must not include columns with `NULL` values.
  - `gh-ost` will do its best to pick a migration key with non-nullable columns. It will by default refuse a migration where the only possible `UNIQUE KEY` includes nullable-columns. You may override this refusal via `--allow-nullable-unique-key` but **you must** be sure there are no actual `NULL` values in those columns. Such `NULL` values would cause a data integrity problem and potentially a corrupted migration.
- It is not allowed to migrate a table where another table exists with same name and different upper/lower case.
  - For example, you may not migrate `MyTable` if another table called `MYtable` exists in the same schema.
- Amazon `RDS` is probably not supported (due to `SUPER` requirement)
- Multisource is not supported when migrating via replica. It _should_ work (but never tested) when connecting directly to master (`--allow-on-master`)