---
title: "ClickHouse® Cluster Fault Tolerance"
sidebar_label: "Cluster fault tolerance"
sidebar_position: 14
description: "Placing nodes in ClickHouse® cluster node groups, how to change the number of nodes in a group"
---

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# ClickHouse® Cluster Fault Tolerance

ClickHouse® cluster fault tolerance is affected by:

* availability of [data replication](#data-replication);
* [node allocation type](#node-allocation-type).

## Data replication \{#data-replication}

Replication is supported only for replicated tables. Read more about replicated tables in the ClickHouse® documentation guide [Replicated\* table engines](https://clickhouse.com/docs/engines/table-engines/mergetree-family/replication).

To ensure data replication and enable the creation of replicated tables, the cluster must consist of:

* at least one [shard](/managed-databases/clickhouse/about-node-groups.mdx#shard) with two or more nodes;
* and node groups [ClickHouse® Keeper](/managed-databases/clickhouse/about-node-groups.mdx#clickhouse-keeper).

You can specify the number of nodes in a shard when [creating a cluster](/managed-databases/clickhouse/create-cluster.mdx). You can also [add](#add-node) or [delete nodes](#delete-node) in a shard of an existing cluster. The maximum number of nodes in a shard is ten.

If you specify more than one node in a shard when creating a cluster, ClickHouse® Keeper will be added automatically.

To add nodes to an existing cluster consisting of one shard with one node, you must first [add ClickHouse® Keeper](/managed-databases/clickhouse/resize-cluster.mdx#add-clickhouse-keeper). When adding ClickHouse® Keeper, existing MergeTree tables are automatically converted to replicated tables (for example, to ReplicatedMergeTree). When adding new nodes to such a cluster, they are automatically synchronized with the existing replicas in the shard.

## Node allocation type \{#node-allocation-type}

The node allocation type in a cluster depends on:

* the number of nodes in [shards](/managed-databases/clickhouse/about-node-groups.mdx#shard);
* the type of [pool](/infrastructure/locations.mdx#pool) where the cluster is located;
* the number of segments in the pool;
* [configuration line](/managed-databases/clickhouse/configurations.mdx#configuration-lines).

You can view the list of pools and segments in the [Selectel Infrastructure](/infrastructure/locations.mdx#selectel-infrastructure) table of the [Locations: countries, regions, availability zones, pools, and data center addresses](/infrastructure/locations.mdx) instruction.

Four node allocation types are available:

* Single-Segment;
* Multi-Segment;
* Single-AZ;
* Multi-AZ.

<Tabs queryString="node-allocation-type">
  <TabItem value="single-segment-allocation" default>
    <TabItemLabel>
      Single-Segment
    </TabItemLabel>

    nodes are placed in the same segment of a [single-AZ pool](/infrastructure/locations.mdx#single-az-pool). Applicable to:

    * clusters consisting of one shard with one node;
    * shards with multiple nodes, if the cluster is located in a pool that consists of a single segment;
    * shards with multiple nodes in [HighFreq](/managed-databases/clickhouse/configurations.mdx#highfreq-line) configuration lines.

    :::note

    For example, you have created a cluster in the ru-2 pool with ClickHouse® Keeper and a shard with two nodes. For both node groups, you used configurations from the HighFreq line. All nodes will be placed in the same pool segment, for example, ru-2c.

    :::
  </TabItem>

  <TabItem value="multi-segment-allocation">
    <TabItemLabel>
      Multi-Segment
    </TabItemLabel>

    nodes are placed in different segments of a [single-AZ pool](/infrastructure/locations.mdx#single-az-pool). Applicable for clusters that consist of shards with multiple nodes if the cluster is located in a multi-segment pool.

    Segments are located in different racks of the same data center. Nodes of each shard are distributed across segments sequentially.

    :::note

    For example, you have created a cluster in the ru-1 pool with one shard, in which there are four nodes. The first three nodes will be placed sequentially in the pool segments ru-1a, ru-1b, and ru-1c. The fourth node will be placed in the ru-1a segment. If you add a fifth node, it will be placed in the ru-1b pool segment.

    :::
  </TabItem>

  <TabItem value="single-az-allocation">
    <TabItemLabel>
      Single-AZ
    </TabItemLabel>

    nodes are placed in the same segment of a [multi-AZ pool](/infrastructure/locations.mdx#multi-az-pool). Applicable for clusters that consist of one shard with one node if the cluster is located in the ru-6 pool.

    If you increase the number of nodes in a shard, the node allocation type will change to Multi-AZ.

    :::note

    For example, you have created a cluster in the ru-6 pool with one shard, in which there is one node. The node will be placed in one pool segment, for example, ru-6a.

    :::
  </TabItem>

  <TabItem value="multi-az-allocation">
    <TabItemLabel>
      Multi-AZ
    </TabItemLabel>

    nodes are placed in different segments of a [multi-AZ pool](/infrastructure/locations.mdx#multi-az-pool). Applicable for clusters that consist of shards with multiple nodes if the cluster is located in the ru-6 pool.

    Each pool segment is located in a separate data center. Nodes of each shard are distributed across segments sequentially.

    :::note

    For example, you have created a cluster in the ru-6 pool with one shard, in which there are four nodes. The first three nodes will be placed sequentially in the pool segments ru-6a, ru-6b, and ru-6c. The fourth node will be placed in the ru-6a segment. If you add a fifth node, it will be placed in the ru-6b pool segment.

    :::
  </TabItem>
</Tabs>

## Add a node \{#add-node}

There can be no more than ten nodes in a shard. During the node addition process, other shard nodes will continue to operate for read and write operations.

1. If the cluster consists of only one shard with one node, [add ClickHouse® Keeper](/managed-databases/clickhouse/resize-cluster.mdx#add-clickhouse-keeper).
2. In the [Dashboard](https://my.selectel.ru/vpc/default/dbaas/), on the top menu, click **Products** and select **Managed Databases**.
3. Open the **Active** tab.
4. Open the cluster page → **Settings** tab.
5. In the  shard menu, select **Change node count**.<MoreVerticalIcon />
6. Open the **Add** tab.
7. Specify the new number of nodes in the shard. If there are no free IP addresses in the subnet to which the cluster is connected, you cannot add the node. Each new node occupies a new IP address in the subnet.
8. Click **Save**.

## Delete a node \{#delete-node}

:::danger

Deleting a node will remove all data located on that node.

:::

During deletion, other shard nodes will continue to work in read and write mode. [Public IP address](/managed-databases/clickhouse/public-ip.mdx) assigned to the node will be deleted.

1. In the [Control panel](https://my.selectel.ru/vpc/default/dbaas/), on the top menu, click **Products** and select **Cloud Databases**.
2. Open the **Active** tab.
3. Open the cluster page → **Settings** tab.
4. In the  shard menu, select **Change node count**.<MoreVerticalIcon />
5. Open the **Delete** tab.
6. Select the node you want to delete. There must be at least one node remaining in the shard. You cannot delete all nodes from a shard.
7. Click **Save**.

ClickHouse® is a registered trademark of ClickHouse, Inc. https://clickhouse.com.

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