MySQL Shell 8.0  /  MySQL InnoDB ReplicaSet

Chapter 9 MySQL InnoDB ReplicaSet

The AdminAPI includes support for InnoDB ReplicaSet, that enables you to administer a set of MySQL instances running asynchronous GTID-based replication in a similar way to InnoDB Cluster. An InnoDB ReplicaSet consists of a single primary and multiple secondaries (traditionally referred to as the MySQL replication source and replicas). You administer your ReplicaSets using a ReplicaSet object and the AdminAPI operations, for example to check the status of the InnoDB ReplicaSet, and manually failover to a new primary in the event of a failure.

Similar to InnoDB Cluster, MySQL Router supports bootstrapping against InnoDB ReplicaSet, which means you can automatically configure MySQL Router to use your InnoDB ReplicaSet without having to manually configure it. This makes InnoDB ReplicaSet a quick and easy way to get MySQL replication and MySQL Router up and running, making it well suited to scaling out reads, and provides manual failover capabilities in use cases that do not require the high availability offered by InnoDB Cluster.

In addition to deploying an InnoDB ReplicaSet using AdminAPI, you can adopt an existing replication setup. AdminAPI configures the InnoDB ReplicaSet based on the topology of the replication setup. Once the replication setup has been adopted, you administer it in the same way as an InnoDB ReplicaSet deployed from scratch. This enables you to take advantage of AdminAPI and MySQL Router without the need to create a new ReplicaSet. For more information see Section 9.3, “Adopting an Existing Replication Set Up”.

InnoDB ReplicaSet may be used over a wide area network with no impact on write performance, because the server instances are connected by asynchronous replication channels and do not need consensus on transactions. However, replication lag will be greater over a wide area network, causing the secondary servers in the InnoDB ReplicaSet to be further behind the primary server.

InnoDB ReplicaSet Limitations.  An InnoDB ReplicaSet has several limitations compared to an InnoDB Cluster, so it is recommended that you deploy InnoDB Cluster wherever possible. Generally, an InnoDB ReplicaSet on its own does not provide high availability. Among the limitations of InnoDB ReplicaSet are:

  • No automatic failover. In events where the primary becomes unavailable, a failover needs to be triggered manually using AdminAPI before any changes are possible again. However, secondary instances remain available for reads.

  • No protection from partial data loss due to an unexpected halt or unavailability. Transactions that have not yet been applied by the time of the halt could become lost.

  • No protection against inconsistencies after an unexpected exit or unavailability. If a failover promotes a secondary while the former primary is still available (for example due to a network partition), inconsistencies could be introduced because of the split-brain.

  • InnoDB ReplicaSet does not support a multi-primary mode. Data consistency cannot be guaranteed with classic replication topologies that allow writes in all members.

  • Read scale-out is limited because InnoDB ReplicaSet is based on asynchronous replication and therefore there is no possible tuning of flow control as there is with Group Replication.

  • All secondary members replicate from a single source. For some particular scenarios or use-cases, this might have an impact on the source. For example, lots of very small updates going on.