Maintenance in Managed Service for ClickHouse®

Maintenance in Managed Service for ClickHouse® includes:

• Automatic installation of DBMS updates and patches for hosts (including for stopped clusters).
• Scheduled automatic storage expansion.
• Other maintenance activities.

A major DBMS version update is not part of maintenance. For more information about upgrading between major versions, see ClickHouse® version upgrade.

Maintenance windowMaintenance window

You can set the maintenance window when creating a cluster or updating its settings:

• The At any time option (default) allows performing maintenance at any time.
• The By schedule option allows you to select the day of the week and UTC time interval for the maintenance. For example, you can choose a time when the cluster is least loaded.

Both active and stopped clusters are subject to maintenance. Maintenance operations may include DBMS updates, applying patches, etc.

In the management console, you select the maintenance start time as an hour interval. In other interfaces, you specify this interval by its sequence number, from 1 to 24.

For example, to start maintenance in the interval from 00:00 to 01:00, put 1; from 04:00 to 05:00, 5.

Maintenance workflowMaintenance workflow

The maintenance workflow for Managed Service for ClickHouse® clusters depends on the number of shards and hosts in them:

• In single-host clusters, maintenance is performed on a single ClickHouse® host. This means, if you restart such a cluster during maintenance, it will become unavailable.
• If a cluster consists of multiple ClickHouse® hosts in a single shard, replica hosts undergo maintenance one by one. Such hosts are queued randomly. If you restart a host during maintenance, it will become unavailable for this time period. 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 the hosts of one shard, then four shards, and further on, in batches of 16 shards. 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 the shard.

ClickHouse® is a registered trademark of ClickHouse, Inc.

How maintenance impacts a clusterHow maintenance impacts a cluster

Depending on its type, maintenance can impact your cluster as follows:

• Show little to no impact on database users.
• Break database connections in place, forcing clients to re-establish their connections.
• Make your cluster read-only for a while.
• Restart the master and trigger master failover to one of the replicas.
• Cause master host failover in the cluster, making the database read-only for a while.

Maintenance tasks have an estimated duration and completion date. These time frames are calculated based on historical data from similar operations. The actual completion times may vary depending on your cluster.