# Disaster recovery in Managed ClickHouse®


Managed ClickHouse® prevents and mitigates emergencies or crises with multiple disaster recovery methods to keep your data safe and sound.

Disaster recovery is a process of coping with emergencies or crises using dedicated methods
for protecting resources and/or reestablishing their desired status. In the context of data
infrastructure, well-established disaster recovery methods are of a particular importance
for preventing data loss or corruption. Software failure, loss of a node,
or datacenter outage are only a few examples of emergencies when disaster recovery comes in.

## High availability

High availability (HA) is an entity's ability to continuously maintain
a certain level of operational performance for a desired period of time.
HA is typically achieved by redundancy - securing replicas of databases
or services to be highly available. To support disaster recovery
technologies, a database service needs to stay highly available, for
example, by operating on a few nodes holding the same data.

With Exoscale, HA for your service is supported in Business and Premium
plans. See [Plan
comparison](https://www.exoscale.com/pricing/#dbaas)
for details.

## Backup and restore

### Service backup

Backups of Managed ClickHouse services happen automatically on a daily
basis.

Backups cover the following:

-   Access entities (for example, users, roles, passwords, or secrets)
    stored in ZooKeeper
-   Database definitions
-   Table schemas
-   Table content (`part files`)
-   Dictionaries

You can
[restore your service from a selected backup](/product/dbaas/service-specific/clickhouse/how-to/restore-backup/).

> [!NOTE]
> **Part files**
> With the ClickHouse's ReplicatedMergeTree table engine, each INSERT
> query results in creating a new file, so-called part, written only once
> and not modifiable.
>
> Using part files allows incremental backups in Managed ClickHouse:
> only changed parts are backed up and files already available in the
> object storage are left out from the backup.

For more information on backups in Exoscale, see
[Backups at Exoscale](/product/dbaas/how-to/backups-restore/).

### Service recovery

You can create a copy of your Managed ClickHouse service and
[restore the data from a selected service backup](/product/dbaas/service-specific/clickhouse/how-to/restore-backup/).
For this purpose, you create a fork from the original service. This spins up a new service
that hosts the data recovered from the selected backup.

## Sharding

Essentially, sharding is a technique of splitting database rows across
multiple database nodes, which usually significantly increases
performance. Each shard is in turn replicated across several nodes, so
the rows it holds survive the loss of any single node. Replication at
the shard level is what provides high availability and helps to achieve
disaster recovery.

With Managed ClickHouse [Business and Premium
plans](https://www.exoscale.com/pricing/#dbaas),
each shard is replicated across three nodes running on separate hosts in
the zone. The service and the data stay available even if a node is
lost.

> [!NOTE]
> Although sharding with replicated nodes can reduce failures, it still
> cannot save a service from the loss of an entire zone.

For information on how to work with shards in Managed ClickHouse, see
[Enable reading and writing data across shards](/product/dbaas/service-specific/clickhouse/how-to/use-shards-with-distributed-table/).

## Limitations

Managed ClickHouse has a few restrictions on the disaster recovery
capability.

-   No backup to another zone
-   No point in time recovery (PITR)

For all the restrictions and limits for Managed ClickHouse, see
[Managed ClickHouse limits and limitations](/product/dbaas/service-specific/clickhouse/service-boundaries/limits-and-quotas/).

## Related pages

- [Fork and restore from Managed ClickHouse® backups](/product/dbaas/service-specific/clickhouse/how-to/restore-backup/)
- [Managed ClickHouse® service architecture](/product/dbaas/service-specific/clickhouse/overview/service-architecture/)