ClickHouse® cluster maintenance

Maintenance is a process that helps maintain the health and performance of your cluster.

Maintenance includes:

• Automatic installation of software updates and revisions (including stopped clusters)

• Security and compliance settings deployment and evaluation

• Other maintenance activities.

Maintenance window

A maintenance window helps you track or set a schedule for the maintenance activities on your managed clusters.

You can configure the Maintenance window when creating your cluster or managing its settings.

Maintenance process

In Managed Service for ClickHouse®, the maintenance procedure varies depending on the number of hosts on your cluster:

• In single-host clusters, maintenance is performed on a single ClickHouse® host, which results in your cluster becoming unavailable until the procedures are complete.

• If a cluster consists of multiple ClickHouse® hosts in a single shard, host replicas undergo maintenance consecutively.

  If a host needs restarting during maintenance, it'll be unavailable until the restart is complete. If you access the cluster using the FQDN or IP address of the ClickHouse® host, such a cluster may become unavailable.

• If a cluster consists of multiple shards, our Managed Service for ClickHouse® performs shard maintenance in parallel: first, the maintenance is performed on hosts of one shard, then two shards, four, and so on, up to ten.

  This is what a step-by-step maintenance sequence looks like for a cluster with four shards (A - D), three hosts each (1 - 3):

  1: A1
  2: A2 B1
  3: A3 B2 C1 D1
  4: B3 C2 D2
  5: C3 D3
  

  This sequence makes sure no shard becomes unresponsive during maintenance.

  Host maintenance works the same as in clusters with a single shard. If you access the cluster shard using the FQDN or IP address of the ClickHouse® host, such a shard may become unavailable.