# Requirements and limitations ### Requirements - You will need to have one server serving Row Based Replication (RBR) format binary logs. Right now `FULL` row image is supported. `MINIMAL` to be supported in the near future. `gh-ost` prefers to work with replicas. You may [still have your master configured with Statement Based Replication](migrating-with-sbr.md) (SBR). - If you are using a replica, the table must have an identical schema between the master and replica. - `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 `*.*` - either: - `SUPER, REPLICATION SLAVE` on `*.*`, or: - `REPLICATION CLIENT, REPLICATION SLAVE` on `*.*` The `SUPER` privilege is required for `STOP SLAVE`, `START SLAVE` operations. These are used on: - Switching your `binlog_format` to `ROW`, in the case where it is _not_ `ROW` and you explicitly specified `--switch-to-rbr` - If your replication is already in RBR (`binlog_format=ROW`) you can specify `--assume-rbr` to avoid the `STOP SLAVE/START SLAVE` operations, hence no need for `SUPER`. - Running `--test-on-replica`: before the cut-over phase, `gh-ost` stops replication so that you can compare the two tables and satisfy that the migration is sound. ### Limitations - Foreign key constraints are not supported. They may be supported in the future, to some extent. - Triggers are not supported. They may be supported in the future. - MySQL 5.7 `JSON` columns are supported but not as part of `PRIMARY KEY` - The two _before_ & _after_ tables must share a `PRIMARY KEY` or other `UNIQUE KEY`. This key will be used by `gh-ost` to iterate through the table rows when copying. [Read more](shared-key.md) - The migration key must not include columns with NULL values. This means either: 1. The columns are `NOT NULL`, or 2. The columns are nullable but don't contain any NULL values. - by default, `gh-ost` will not run if the only `UNIQUE KEY` includes nullable columns. - You may override this via `--allow-nullable-unique-key` but make sure there are no actual `NULL` values in those columns. Existing NULL values can't guarantee data integrity on the migrated table. - 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 works, but has its own [limitations](rds.md). - Google Cloud SQL works, `--gcp` flag required. - Aliyun RDS works, `--aliyun-rds` flag required. - Multisource is not supported when migrating via replica. It _should_ work (but never tested) when connecting directly to master (`--allow-on-master`) - Master-master setup is only supported in active-passive setup. Active-active (where table is being written to on both masters concurrently) is unsupported. It may be supported in the future. - If you have an `enum` field as part of your migration key (typically the `PRIMARY KEY`), migration performance will be degraded and potentially bad. [Read more](https://github.com/github/gh-ost/pull/277#issuecomment-254811520) - Migrating a `FEDERATED` table is unsupported and is irrelevant to the problem `gh-ost` tackles. - [Encrypted binary logs](https://www.percona.com/blog/2018/03/08/binlog-encryption-percona-server-mysql/) are not supported. - `ALTER TABLE ... RENAME TO some_other_name` is not supported (and you shouldn't use `gh-ost` for such a trivial operation).