Maintenance in Managed Service for ClickHouse®
In Managed Service for ClickHouse®, maintenance means:
- Automatic installation of DBMS updates and revisions for hosts (including disabled clusters).
- Changes to the host class and storage size.
- Other maintenance activities.
Changing a major DBMS version is not part of maintenance. For more information about major version changes, see ClickHouse® version upgrade.
Maintenance window
You can set the preferred maintenance time when creating a cluster or updating its settings:
- The arbitrary option (default) allows performing maintenance at any time.
- The by schedule option allows setting the preferred maintenance start day and time (UTC). For example, you can choose a time when the cluster is least loaded.
Maintenance procedure
The maintenance procedure for Managed Service for ClickHouse® clusters depends on the number of shards and hosts in the shards:
- In single-host clusters, maintenance is performed on a single ClickHouse® host. This means, if a cluster needs to be restarted during maintenance, it will become unavailable.
- If a cluster consists of multiple ClickHouse® hosts in a single shard, host replicas undergo maintenance one by one. The hosts are queued randomly. If a host needs to be restarted during maintenance, it becomes unavailable while being restarted. If you access a cluster using the FQDN of the ClickHouse® host, the cluster may become unavailable. To make your application continuously available, access the cluster using a special FQDN always pointing to the available host.
- If a cluster consists of multiple shards, maintenance is performed shard by shard in ascending order by shard number. Maintenance is first performed on hosts of one shard, then two shards, four, and so on, up to ten. Host maintenance is the same as in single-shard clusters. If you access a cluster shard using the FQDN of the ClickHouse® host, the shard may become unavailable. To make your application continuously available, access the shard using a special FQDN, always pointing to the available host in a shard.
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