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Design overview

The Percona Operator for MySQL automates and simplifies deploying and managing open source MySQL clusters on Kubernetes.

Containers deployed with the Operator include the following components:

The design of the Operator is highly bound to Percona Server for MySQL and the high availability implementation based on Orchestrator, which in its turn can be briefly described with the following diagram.

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Being a regular MySQL Server instance, each node contains the same set of data synchronized across nodes. The recommended configuration is to have at least 3 nodes. In a basic setup with this amount of nodes, Percona Server for MySQL provides high availability, continuing to function if you take any of the nodes down.

To provide high availability operator uses node affinity to run Percona Server for MySQL instances on separate worker nodes if possible. If some node fails, the pod in it is automatically re-created on another node.

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To provide data storage for stateful applications, Kubernetes uses Persistent Volumes. A PersistentVolumeClaim (PVC) is used to implement the automatic storage provisioning to pods. If a failure occurs, the Container Storage Interface (CSI) should be able to re-mount storage on a different node. The PVC StorageClass must support this feature (Kubernetes and OpenShift support this in versions 1.9 and 3.9 respectively).

The Operator functionality extends the Kubernetes API with PerconaServerMySQL object, and it is implemented as a golang application. Each PerconaServerMySQL object maps to one separate MySQL setup. The Operator listens to all events on the created objects. When a new PerconaServerMySQL object is created, or an existing one undergoes some changes or deletion, the Operator automatically creates/changes/deletes all needed Kubernetes objects with the appropriate settings to provide a proper Percona Server for MySQL operation.

Replication types and proxy solutions

The process in which data from a primary MySQL instance is copied and sent to other MySQL instances of the database cluster (co-called replicas) is known as MySQL replication.

The Operator provides you a choice between two replication types:

  • Asynchronous replication means that write is completed on the primary and is not influenced by the replicas themselves. After completion of its changes, the primary populates the binary log with the data modification or the actual statement. Then the replica executes each change on its own database and obtains consistent set of data.

  • Group Replication means that read-write transaction can be executed on any instance, and when it happens, MySQL tries to get consensus with the other instances before returning it completed back to the client.

Both replication types have their pros and cons.

Asynchronous replication is faster. Also, you can distribute read and write requests of your application to different instances. From the other hand, it can be less reliable in terms of consistency: replicas may lag behind the primary instance, impacting any applications that depend on real-time data. Also, some transaction committed on the primary instance may not be available on replicas if the primary fails. Finally, asynchronous replication doesn’t allow you scaling write requests horizontally, leaving vertical scaling (primary instance RAM and CPU increase) as the only available option.

Group replication allows horizontal scaling of writes, but is not so fast, because writes require consensus (so-called “certification”). Also, concurrent writes of the same row on different instances are treated as data conflicts : the transaction that was ordered first commits on all servers, and the transaction ordered second aborts. Finally, group replication has some specific limitations . Particularly, the number of MySQL instances in a single replication group can’t exceed 9. Also, extra large transaction can cause noticeable system slowdown, and in some cases even can cause the replication member fault when transaction message cannot be copied between group members over the network within a 5-second window.

Proxy solution used by the Operator for the database cluster depends on the replication type used in the cluster. HAProxy can be used with both replication types, while MySQL Router can be used with Group Replication.

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Last update: 2024-10-29