> ## Documentation Index
> Fetch the complete documentation index at: https://clickhouse.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Replication + scaling

> By going through this tutorial, you will learn how to set up a simple ClickHouse cluster.

export const Image = ({img, alt, size = "lg"}) => {
  const normalizedSize = ["sm", "md", "lg"].includes(size) ? size : "lg";
  return <div className={`ch-image-${normalizedSize}`}>
      <Frame>
        <img src={img} alt={alt} />
      </Frame>
    </div>;
};

> In this example, you'll learn how to set up a simple ClickHouse cluster which
> both replicates and scales. It consisting of two shards and two replicas with a
> 3-node ClickHouse Keeper cluster for managing coordination and keeping quorum
> in the cluster.

The architecture of the cluster you will be setting up is shown below:

<Image img="https://mintcdn.com/private-7c7dfe99/NvnCM4vX9aZ07JxK/images/deployment-guides/replication-sharding-examples/both.webp?fit=max&auto=format&n=NvnCM4vX9aZ07JxK&q=85&s=444ba6cf62f0cd793a61a70b98468d6c" size="md" alt="Architecture diagram for 2 shards and 1 replica" width="1200" height="800" data-path="images/deployment-guides/replication-sharding-examples/both.webp" />

<Note>
  Although it is possible to run ClickHouse Server and ClickHouse Keeper combined on the same server,
  we strongly recommend using *dedicated* hosts for ClickHouse keeper in production environments,
  which is the approach we will demonstrate in this example.

  Keeper servers can be smaller, and 4GB RAM is generally enough for each Keeper server
  until your ClickHouse Servers grow large.
</Note>

<h2 id="prerequisites">
  Prerequisites
</h2>

* You've set up a [local ClickHouse server](/docs/get-started/setup/install) before
* You're familiar with basic configuration concepts of ClickHouse such as [configuration files](/docs/concepts/features/configuration/server-config/configuration-files)
* You have docker installed on your machine

<Steps>
  <Step title="Set up directory structure and test environment" id="set-up">
    <Tip>
      **Example files**

      The following steps will walk you through setting up the cluster from
      scratch. If you prefer to skip these steps and jump straight to running the
      cluster, you can obtain the example
      files from the examples repository ['docker-compose-recipes' directory](https://github.com/ClickHouse/examples/tree/main/docker-compose-recipes/recipes).
    </Tip>

    In this tutorial, you will use [Docker compose](https://docs.docker.com/compose/) to
    set up the ClickHouse cluster. This setup could be modified to work
    for separate local machines, virtual machines or cloud instances as well.

    Run the following commands to set up the directory structure for this example:

    ```bash theme={null}
    mkdir cluster_2S_2R
    cd cluster_2S_2R

    # Create clickhouse-keeper directories
    for i in {01..03}; do
      mkdir -p fs/volumes/clickhouse-keeper-${i}/etc/clickhouse-keeper
    done

    # Create clickhouse-server directories
    for i in {01..04}; do
      mkdir -p fs/volumes/clickhouse-${i}/etc/clickhouse-server
    done
    ```

    Add the following `docker-compose.yml` file to the `clickhouse-cluster` directory:

    ```yaml title="docker-compose.yml" theme={null}
    version: '3.8'
    services:
      clickhouse-01:
        image: "clickhouse/clickhouse-server:latest"
        user: "101:101"
        container_name: clickhouse-01
        hostname: clickhouse-01
        volumes:
          - ${PWD}/fs/volumes/clickhouse-01/etc/clickhouse-server/config.d/config.xml:/etc/clickhouse-server/config.d/config.xml
          - ${PWD}/fs/volumes/clickhouse-01/etc/clickhouse-server/users.d/users.xml:/etc/clickhouse-server/users.d/users.xml
        ports:
          - "127.0.0.1:8123:8123"
          - "127.0.0.1:9000:9000"
        depends_on:
          - clickhouse-keeper-01
          - clickhouse-keeper-02
          - clickhouse-keeper-03
      clickhouse-02:
        image: "clickhouse/clickhouse-server:latest"
        user: "101:101"
        container_name: clickhouse-02
        hostname: clickhouse-02
        volumes:
          - ${PWD}/fs/volumes/clickhouse-02/etc/clickhouse-server/config.d/config.xml:/etc/clickhouse-server/config.d/config.xml
          - ${PWD}/fs/volumes/clickhouse-02/etc/clickhouse-server/users.d/users.xml:/etc/clickhouse-server/users.d/users.xml
        ports:
          - "127.0.0.1:8124:8123"
          - "127.0.0.1:9001:9000"
        depends_on:
          - clickhouse-keeper-01
          - clickhouse-keeper-02
          - clickhouse-keeper-03
      clickhouse-03:
        image: "clickhouse/clickhouse-server:latest"
        user: "101:101"
        container_name: clickhouse-03
        hostname: clickhouse-03
        volumes:
          - ${PWD}/fs/volumes/clickhouse-03/etc/clickhouse-server/config.d/config.xml:/etc/clickhouse-server/config.d/config.xml
          - ${PWD}/fs/volumes/clickhouse-03/etc/clickhouse-server/users.d/users.xml:/etc/clickhouse-server/users.d/users.xml
        ports:
          - "127.0.0.1:8125:8123"
          - "127.0.0.1:9002:9000"
        depends_on:
          - clickhouse-keeper-01
          - clickhouse-keeper-02
          - clickhouse-keeper-03
      clickhouse-04:
        image: "clickhouse/clickhouse-server:latest"
        user: "101:101"
        container_name: clickhouse-04
        hostname: clickhouse-04
        volumes:
          - ${PWD}/fs/volumes/clickhouse-04/etc/clickhouse-server/config.d/config.xml:/etc/clickhouse-server/config.d/config.xml
          - ${PWD}/fs/volumes/clickhouse-04/etc/clickhouse-server/users.d/users.xml:/etc/clickhouse-server/users.d/users.xml
        ports:
          - "127.0.0.1:8126:8123"
          - "127.0.0.1:9003:9000"
        depends_on:
          - clickhouse-keeper-01
          - clickhouse-keeper-02
          - clickhouse-keeper-03
      clickhouse-keeper-01:
        image: "clickhouse/clickhouse-keeper:latest-alpine"
        user: "101:101"
        container_name: clickhouse-keeper-01
        hostname: clickhouse-keeper-01
        volumes:
          - ${PWD}/fs/volumes/clickhouse-keeper-01/etc/clickhouse-keeper/keeper_config.xml:/etc/clickhouse-keeper/keeper_config.xml
        ports:
          - "127.0.0.1:9181:9181"
      clickhouse-keeper-02:
        image: "clickhouse/clickhouse-keeper:latest-alpine"
        user: "101:101"
        container_name: clickhouse-keeper-02
        hostname: clickhouse-keeper-02
        volumes:
          - ${PWD}/fs/volumes/clickhouse-keeper-02/etc/clickhouse-keeper/keeper_config.xml:/etc/clickhouse-keeper/keeper_config.xml
        ports:
          - "127.0.0.1:9182:9181"
      clickhouse-keeper-03:
        image: "clickhouse/clickhouse-keeper:latest-alpine"
        user: "101:101"
        container_name: clickhouse-keeper-03
        hostname: clickhouse-keeper-03
        volumes:
          - ${PWD}/fs/volumes/clickhouse-keeper-03/etc/clickhouse-keeper/keeper_config.xml:/etc/clickhouse-keeper/keeper_config.xml
        ports:
          - "127.0.0.1:9183:9181"
    ```

    Create the following sub-directories and files:

    ```bash theme={null}
    for i in {01..04}; do
      mkdir -p fs/volumes/clickhouse-${i}/etc/clickhouse-server/config.d
      mkdir -p fs/volumes/clickhouse-${i}/etc/clickhouse-server/users.d
      touch fs/volumes/clickhouse-${i}/etc/clickhouse-server/config.d/config.xml
      touch fs/volumes/clickhouse-${i}/etc/clickhouse-server/users.d/users.xml
    done
    ```

    * The `config.d` directory contains ClickHouse server configuration file `config.xml`,
      in which custom configuration for each ClickHouse node is defined. This
      configuration gets combined with the default `config.xml` ClickHouse configuration
      file that comes with every ClickHouse installation.
    * The `users.d` directory contains user configuration file `users.xml`, in which
      custom configuration for users is defined. This configuration gets combined with
      the default ClickHouse `users.xml` configuration file that comes with every
      ClickHouse installation.

    <Tip>
      **Custom configuration directories**

      It is a best practice to make use of the `config.d` and `users.d` directories when
      writing your own configuration, rather than directly modifying the default configuration
      in `/etc/clickhouse-server/config.xml` and `etc/clickhouse-server/users.xml`.

      The line

      ```xml theme={null}
      <clickhouse replace="true">
      ```

      Ensures that the configuration sections defined in the `config.d` and `users.d`
      directories override the default configuration sections defined in the default
      `config.xml` and `users.xml` files.
    </Tip>
  </Step>

  <Step title="Configure ClickHouse nodes" id="configure-clickhouse-servers">
    <h3 id="server-setup">
      Server setup
    </h3>

    Now modify each empty configuration file `config.xml` located at
    `fs/volumes/clickhouse-{}/etc/clickhouse-server/config.d`. The lines which are
    highlighted below need to be changed to be specific to each node:

    ```xml highlight={9,64-67} theme={null}
    <clickhouse replace="true">
        <logger>
            <level>debug</level>
            <log>/var/log/clickhouse-server/clickhouse-server.log</log>
            <errorlog>/var/log/clickhouse-server/clickhouse-server.err.log</errorlog>
            <size>1000M</size>
            <count>3</count>
        </logger>
        <display_name>cluster_2S_2R node 1</display_name>
        <listen_host>0.0.0.0</listen_host>
        <http_port>8123</http_port>
        <tcp_port>9000</tcp_port>
        <user_directories>
            <users_xml>
                <path>users.xml</path>
            </users_xml>
            <local_directory>
                <path>/var/lib/clickhouse/access/</path>
            </local_directory>
        </user_directories>
        <distributed_ddl>
            <path>/clickhouse/task_queue/ddl</path>
        </distributed_ddl>
        <remote_servers>
            <cluster_2S_2R>
                <shard>
                    <internal_replication>true</internal_replication>
                    <replica>
                        <host>clickhouse-01</host>
                        <port>9000</port>
                    </replica>
                    <replica>
                        <host>clickhouse-03</host>
                        <port>9000</port>
                    </replica>
                </shard>
                <shard>
                    <internal_replication>true</internal_replication>
                    <replica>
                        <host>clickhouse-02</host>
                        <port>9000</port>
                    </replica>
                    <replica>
                        <host>clickhouse-04</host>
                        <port>9000</port>
                    </replica>
                </shard>
            </cluster_2S_2R>
        </remote_servers>
        <zookeeper>
            <node>
                <host>clickhouse-keeper-01</host>
                <port>9181</port>
            </node>
            <node>
                <host>clickhouse-keeper-02</host>
                <port>9181</port>
            </node>
            <node>
                <host>clickhouse-keeper-03</host>
                <port>9181</port>
            </node>
        </zookeeper>
        <macros>
            <shard>01</shard>
            <replica>01</replica>
        </macros>
    </clickhouse>
    ```

    | Directory                                                 | File                                                                                                                                                                             |
    | --------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
    | `fs/volumes/clickhouse-01/etc/clickhouse-server/config.d` | [`config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-01/etc/clickhouse-server/config.d/config.xml) |
    | `fs/volumes/clickhouse-02/etc/clickhouse-server/config.d` | [`config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-02/etc/clickhouse-server/config.d/config.xml) |
    | `fs/volumes/clickhouse-03/etc/clickhouse-server/config.d` | [`config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-03/etc/clickhouse-server/config.d/config.xml) |
    | `fs/volumes/clickhouse-04/etc/clickhouse-server/config.d` | [`config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-04/etc/clickhouse-server/config.d/config.xml) |

    Each section of the above configuration file is explained in more detail below.

    <h4 id="networking">
      Networking and logging
    </h4>

    External communication to the network interface is enabled by activating the listen
    host setting. This ensures that the ClickHouse server host is reachable by other
    hosts:

    ```xml theme={null}
    <listen_host>0.0.0.0</listen_host>
    ```

    The port for the HTTP API is set to `8123`:

    ```xml theme={null}
    <http_port>8123</http_port>
    ```

    The TCP port for interaction by ClickHouse's native protocol between clickhouse-client
    and other native ClickHouse tools, and clickhouse-server and other clickhouse-servers
    is set to `9000`:

    ```xml theme={null}
    <tcp_port>9000</tcp_port>
    ```

    Logging configuration is defined in the `<logger>` block. This example configuration gives
    you a debug log that will roll over at 1000M three times:

    ```xml theme={null}
    <logger>
       <level>debug</level>
       <log>/var/log/clickhouse-server/clickhouse-server.log</log>
       <errorlog>/var/log/clickhouse-server/clickhouse-server.err.log</errorlog>
       <size>1000M</size>
       <count>3</count>
    </logger>
    ```

    For more information on logging configuration, see the comments included in the
    default ClickHouse [configuration file](https://github.com/ClickHouse/ClickHouse/blob/master/programs/server/config.xml).

    <h4 id="cluster-config">
      Cluster configuration
    </h4>

    Configuration for the cluster is set up in the `<remote_servers>` block.
    Here the cluster name `cluster_2S_2R` is defined.

    The `<cluster_2S_2R></cluster_2S_2R>` block defines the layout of the cluster,
    using the `<shard></shard>` and `<replica></replica>` settings, and acts as a
    template for distributed DDL queries, which are queries that execute across the
    cluster using the `ON CLUSTER` clause. By default, distributed DDL queries
    are allowed, but can also be turned off with setting `allow_distributed_ddl_queries`.

    `internal_replication` is set to true so that data is written to just one of the replicas.

    ```xml theme={null}
    <remote_servers>
       <!-- cluster name (should not contain dots) -->
      <cluster_2S_2R>
          <!-- <allow_distributed_ddl_queries>false</allow_distributed_ddl_queries> -->
          <shard>
              <!-- Optional. Whether to write data to just one of the replicas. Default: false (write data to all replicas). -->
              <internal_replication>true</internal_replication>
              <replica>
                  <host>clickhouse-01</host>
                  <port>9000</port>
              </replica>
              <replica>
                  <host>clickhouse-03</host>
                  <port>9000</port>
              </replica>
          </shard>
          <shard>
              <internal_replication>true</internal_replication>
              <replica>
                  <host>clickhouse-02</host>
                  <port>9000</port>
              </replica>
              <replica>
                  <host>clickhouse-04</host>
                  <port>9000</port>
              </replica>
          </shard>
      </cluster_2S_2R>
    </remote_servers>
    ```

    The `<cluster_2S_2R></cluster_2S_2R>` section defines the layout of the cluster,
    and acts as a template for distributed DDL queries, which are queries that execute
    across the cluster using the `ON CLUSTER` clause.

    <h4 id="keeper-config-explanation">
      Keeper configuration
    </h4>

    The `<ZooKeeper>` section tells ClickHouse where ClickHouse Keeper (or ZooKeeper) is running.
    As we're using a ClickHouse Keeper cluster, each `<node>` of the cluster needs to be specified,
    along with its hostname and port number using the `<host>` and `<port>` tags respectively.

    Set up of ClickHouse Keeper is explained in the next step of the tutorial.

    ```xml theme={null}
    <zookeeper>
        <node>
            <host>clickhouse-keeper-01</host>
            <port>9181</port>
        </node>
        <node>
            <host>clickhouse-keeper-02</host>
            <port>9181</port>
        </node>
        <node>
            <host>clickhouse-keeper-03</host>
            <port>9181</port>
        </node>
    </zookeeper>
    ```

    <Note>
      Although it is possible to run ClickHouse Keeper on the same server as ClickHouse Server,
      in production environments we strongly recommend that ClickHouse Keeper runs on dedicated hosts.
    </Note>

    <h4 id="macros-config-explanation">
      Macros configuration
    </h4>

    Additionally, the `<macros>` section is used to define parameter substitutions for
    replicated tables. These are listed in `system.macros` and allow using substitutions
    like `{shard}` and `{replica}` in queries.

    ```xml theme={null}
    <macros>
       <shard>01</shard>
       <replica>01</replica>
    </macros>
    ```

    <h3 id="cluster-configuration">
      User configuration
    </h3>

    Now modify each empty configuration file `users.xml` located at
    `fs/volumes/clickhouse-{}/etc/clickhouse-server/users.d` with the following:

    ```xml title="/users.d/users.xml" theme={null}
    <?xml version="1.0"?>
    <clickhouse replace="true">
        <profiles>
            <default>
                <max_memory_usage>10000000000</max_memory_usage>
                <use_uncompressed_cache>0</use_uncompressed_cache>
                <load_balancing>in_order</load_balancing>
                <log_queries>1</log_queries>
            </default>
        </profiles>
        <users>
            <default>
                <access_management>1</access_management>
                <profile>default</profile>
                <networks>
                    <ip>::/0</ip>
                </networks>
                <quota>default</quota>
                <access_management>1</access_management>
                <named_collection_control>1</named_collection_control>
                <show_named_collections>1</show_named_collections>
                <show_named_collections_secrets>1</show_named_collections_secrets>
            </default>
        </users>
        <quotas>
            <default>
                <interval>
                    <duration>3600</duration>
                    <queries>0</queries>
                    <errors>0</errors>
                    <result_rows>0</result_rows>
                    <read_rows>0</read_rows>
                    <execution_time>0</execution_time>
                </interval>
            </default>
        </quotas>
    </clickhouse>
    ```

    In this example, the default user is configured without a password for simplicity.
    In practice, this is discouraged.

    <Note>
      In this example, each `users.xml` file is identical for all nodes in the cluster.
    </Note>
  </Step>

  <Step title="Configure ClickHouse Keeper" id="configure-clickhouse-keeper-nodes">
    Next you will configure ClickHouse Keeper, which is used for coordination.

    <h3 id="configuration-explanation">
      Keeper setup
    </h3>

    In order for replication to work, a ClickHouse keeper cluster needs to be set up and
    configured. ClickHouse Keeper provides the coordination system for data replication,
    acting as a stand in replacement for Zookeeper, which could also be used.
    ClickHouse Keeper is, however, recommended, as it provides better guarantees and
    reliability and uses fewer resources than ZooKeeper. For high availability and to
    keep quorum, it is recommended to run at least three ClickHouse Keeper nodes.

    <Note>
      ClickHouse Keeper can run on any node of the cluster alongside ClickHouse, although
      it is recommended to have it run on a dedicated node which allows scaling and
      managing the ClickHouse Keeper cluster independently of the database cluster.
    </Note>

    Create the `keeper_config.xml` files for each ClickHouse Keeper node
    using the following command from the root of the example folder:

    ```bash theme={null}
    for i in {01..03}; do
      touch fs/volumes/clickhouse-keeper-${i}/etc/clickhouse-keeper/keeper_config.xml
    done
    ```

    Modify the empty configuration files which were created in each
    node directory `fs/volumes/clickhouse-keeper-{}/etc/clickhouse-keeper`. The
    highlighted lines below need to be changed to be specific to each node:

    ```xml title="/clickhouse-keeper/keeper_config.xml" highlight={12} theme={null}
    <clickhouse replace="true">
        <logger>
            <level>information</level>
            <log>/var/log/clickhouse-keeper/clickhouse-keeper.log</log>
            <errorlog>/var/log/clickhouse-keeper/clickhouse-keeper.err.log</errorlog>
            <size>1000M</size>
            <count>3</count>
        </logger>
        <listen_host>0.0.0.0</listen_host>
        <keeper_server>
            <tcp_port>9181</tcp_port>
            <server_id>1</server_id>
            <log_storage_path>/var/lib/clickhouse/coordination/log</log_storage_path>
            <snapshot_storage_path>/var/lib/clickhouse/coordination/snapshots</snapshot_storage_path>
            <coordination_settings>
                <operation_timeout_ms>10000</operation_timeout_ms>
                <session_timeout_ms>30000</session_timeout_ms>
                <raft_logs_level>information</raft_logs_level>
            </coordination_settings>
            <raft_configuration>
                <server>
                    <id>1</id>
                    <hostname>clickhouse-keeper-01</hostname>
                    <port>9234</port>
                </server>
                <server>
                    <id>2</id>
                    <hostname>clickhouse-keeper-02</hostname>
                    <port>9234</port>
                </server>
                <server>
                    <id>3</id>
                    <hostname>clickhouse-keeper-03</hostname>
                    <port>9234</port>
                </server>
            </raft_configuration>
        </keeper_server>
    </clickhouse>
    ```

    | Directory                                               | File                                                                                                                                                                                         |
    | ------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
    | `fs/volumes/clickhouse-keeper-01/etc/clickhouse-keeper` | [`keeper_config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-keeper-01/etc/clickhouse-keeper/keeper_config.xml) |
    | `fs/volumes/clickhouse-keeper-02/etc/clickhouse-keeper` | [`keeper_config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-keeper-02/etc/clickhouse-keeper/keeper_config.xml) |
    | `fs/volumes/clickhouse-keeper-03/etc/clickhouse-keeper` | [`keeper_config.xml`](https://github.com/ClickHouse/examples/blob/main/docker-compose-recipes/recipes/cluster_2S_2R/fs/volumes/clickhouse-keeper-03/etc/clickhouse-keeper/keeper_config.xml) |

    Each configuration file will contain the following unique configuration (shown below).
    The `server_id` used should be unique for that particular ClickHouse Keeper node
    in the cluster and match the server `<id>` defined in the `<raft_configuration>` section.
    `tcp_port` is the port used by *clients* of ClickHouse Keeper.

    ```xml theme={null}
    <tcp_port>9181</tcp_port>
    <server_id>{id}</server_id>
    ```

    The following section is used to configure the servers that participate in the
    quorum for the [raft consensus algorithm](https://en.wikipedia.org/wiki/Raft_\(algorithm\)):

    ```xml highlight={6} theme={null}
    <raft_configuration>
        <server>
            <id>1</id>
            <hostname>clickhouse-keeper-01</hostname>
            <!-- TCP port used for communication between ClickHouse Keeper nodes -->
            <port>9234</port>
        </server>
        <server>
            <id>2</id>
            <hostname>clickhouse-keeper-02</hostname>
            <port>9234</port>
        </server>
        <server>
            <id>3</id>
            <hostname>clickhouse-keeper-03</hostname>
            <port>9234</port>
        </server>
    </raft_configuration>
    ```

    <Tip>
      **ClickHouse Cloud simplifies management**

      [ClickHouse Cloud](/docs/products/cloud/getting-started/intro)
      removes the operational burden associated with managing shards and replicas. The
      platform automatically handles high availability, replication, and scaling decisions.
      Compute and storage are separate and scale based on demand without requiring manual
      configuration or ongoing maintenance.

      [Read more](/docs/products/cloud/features/autoscaling/overview)
    </Tip>
  </Step>

  <Step title="Test the setup" id="test-the-setup">
    Make sure that docker is running on your machine.
    Start the cluster using the `docker-compose up` command from the root of the `cluster_2S_2R` directory:

    ```bash theme={null}
    docker-compose up -d
    ```

    You should see docker begin to pull the ClickHouse and Keeper images,
    and then start the containers:

    ```bash theme={null}
    [+] Running 8/8
     ✔ Network   cluster_2s_2r_default     Created
     ✔ Container clickhouse-keeper-03      Started
     ✔ Container clickhouse-keeper-02      Started
     ✔ Container clickhouse-keeper-01      Started
     ✔ Container clickhouse-01             Started
     ✔ Container clickhouse-02             Started
     ✔ Container clickhouse-04             Started
     ✔ Container clickhouse-03             Started
    ```

    To verify that the cluster is running, connect to any one of the nodes and run the
    following query. The command to connect to the first node is shown:

    ```bash theme={null}
    # Connect to any node
    docker exec -it clickhouse-01 clickhouse-client
    ```

    If successful, you will see the ClickHouse client prompt:

    ```response theme={null}
    cluster_2S_2R node 1 :)
    ```

    Run the following query to check what cluster topologies are defined for which
    hosts:

    ```sql title="Query" theme={null}
    SELECT 
        cluster,
        shard_num,
        replica_num,
        host_name,
        port
    FROM system.clusters;
    ```

    ```response title="Response" theme={null}
       ┌─cluster───────┬─shard_num─┬─replica_num─┬─host_name─────┬─port─┐
    1. │ cluster_2S_2R │         1 │           1 │ clickhouse-01 │ 9000 │
    2. │ cluster_2S_2R │         1 │           2 │ clickhouse-03 │ 9000 │
    3. │ cluster_2S_2R │         2 │           1 │ clickhouse-02 │ 9000 │
    4. │ cluster_2S_2R │         2 │           2 │ clickhouse-04 │ 9000 │
    5. │ default       │         1 │           1 │ localhost     │ 9000 │
       └───────────────┴───────────┴─────────────┴───────────────┴──────┘
    ```

    Run the following query to check the status of the ClickHouse Keeper cluster:

    ```sql title="Query" theme={null}
    SELECT *
    FROM system.zookeeper
    WHERE path IN ('/', '/clickhouse')
    ```

    ```response title="Response" theme={null}
       ┌─name───────┬─value─┬─path────────┐
    1. │ task_queue │       │ /clickhouse │
    2. │ sessions   │       │ /clickhouse │
    3. │ keeper     │       │ /           │
    4. │ clickhouse │       │ /           │
       └────────────┴───────┴─────────────┘
    ```

    The `mntr` command is also commonly used to verify that ClickHouse Keeper is
    running and to get state information about the relationship of the three Keeper nodes.
    In the configuration used in this example, there are three nodes working together.
    The nodes will elect a leader, and the remaining nodes will be followers.

    The `mntr` command gives information related to performance, and whether a particular
    node is a follower or a leader.

    <Tip>
      You may need to install `netcat` in order to send the `mntr` command to Keeper.
      Please see the [nmap.org](https://nmap.org/ncat/) page for download information.
    </Tip>

    Run the command below from a shell on `clickhouse-keeper-01`, `clickhouse-keeper-02`, and
    `clickhouse-keeper-03` to check the status of each Keeper node. The command
    for `clickhouse-keeper-01` is shown below:

    ```bash theme={null}
    docker exec -it clickhouse-keeper-01  /bin/sh -c 'echo mntr | nc 127.0.0.1 9181'
    ```

    The response below shows an example response from a follower node:

    ```response title="Response" highlight={9} theme={null}
    zk_version      v23.3.1.2823-testing-46e85357ce2da2a99f56ee83a079e892d7ec3726
    zk_avg_latency  0
    zk_max_latency  0
    zk_min_latency  0
    zk_packets_received     0
    zk_packets_sent 0
    zk_num_alive_connections        0
    zk_outstanding_requests 0
    zk_server_state follower
    zk_znode_count  6
    zk_watch_count  0
    zk_ephemerals_count     0
    zk_approximate_data_size        1271
    zk_key_arena_size       4096
    zk_latest_snapshot_size 0
    zk_open_file_descriptor_count   46
    zk_max_file_descriptor_count    18446744073709551615
    ```

    The response below shows an example response from a leader node:

    ```response title="Response" highlight={9,18-19} theme={null}
    zk_version      v23.3.1.2823-testing-46e85357ce2da2a99f56ee83a079e892d7ec3726
    zk_avg_latency  0
    zk_max_latency  0
    zk_min_latency  0
    zk_packets_received     0
    zk_packets_sent 0
    zk_num_alive_connections        0
    zk_outstanding_requests 0
    zk_server_state leader
    zk_znode_count  6
    zk_watch_count  0
    zk_ephemerals_count     0
    zk_approximate_data_size        1271
    zk_key_arena_size       4096
    zk_latest_snapshot_size 0
    zk_open_file_descriptor_count   48
    zk_max_file_descriptor_count    18446744073709551615
    zk_followers    2
    zk_synced_followers     2
    ```

    With this, you have successfully set up a ClickHouse cluster with two shards and two replicas.
    In the next step, you will create a table in the cluster.
  </Step>

  <Step title="Create a database" id="creating-a-database">
    Now that you have verified the cluster is correctly setup and is running, you
    will be recreating the same table as the one used in the [UK property prices](/docs/get-started/sample-datasets/uk-price-paid)
    example dataset tutorial. It consists of around 30 million rows of prices paid
    for real-estate property in England and Wales since 1995.

    Connect to the client of each host by running each of the following commands from separate terminal
    tabs or windows:

    ```bash theme={null}
    docker exec -it clickhouse-01 clickhouse-client
    docker exec -it clickhouse-02 clickhouse-client
    docker exec -it clickhouse-03 clickhouse-client
    docker exec -it clickhouse-04 clickhouse-client
    ```

    You can run the query below from clickhouse-client of each host to confirm that there are no databases created yet,
    apart from the default ones:

    ```sql title="Query" theme={null}
    SHOW DATABASES;
    ```

    ```response title="Response" theme={null}
       ┌─name───────────────┐
    1. │ INFORMATION_SCHEMA │
    2. │ default            │
    3. │ information_schema │
    4. │ system             │
       └────────────────────┘
    ```

    From the `clickhouse-01` client run the following **distributed** DDL query using the
    `ON CLUSTER` clause to create a new database called `uk`:

    ```sql highlight={2} theme={null}
    CREATE DATABASE IF NOT EXISTS uk 
    ON CLUSTER cluster_2S_2R;
    ```

    You can again run the same query as before from the client of each host
    to confirm that the database has been created across the cluster despite running
    the query only from `clickhouse-01`:

    ```sql theme={null}
    SHOW DATABASES;
    ```

    ```response highlight={6} theme={null}
       ┌─name───────────────┐
    1. │ INFORMATION_SCHEMA │
    2. │ default            │
    3. │ information_schema │
    4. │ system             │
    5. │ uk                 │
       └────────────────────┘
    ```
  </Step>

  <Step title="Create a table on the cluster" id="creating-a-table">
    Now that the database has been created, next you will create a table with replication.

    Run the following query from any of the host clients:

    ```sql highlight={2,19} theme={null}
    CREATE TABLE IF NOT EXISTS uk.uk_price_paid_local
    ON CLUSTER cluster_2S_2R
    (
        price UInt32,
        date Date,
        postcode1 LowCardinality(String),
        postcode2 LowCardinality(String),
        type Enum8('terraced' = 1, 'semi-detached' = 2, 'detached' = 3, 'flat' = 4, 'other' = 0),
        is_new UInt8,
        duration Enum8('freehold' = 1, 'leasehold' = 2, 'unknown' = 0),
        addr1 String,
        addr2 String,
        street LowCardinality(String),
        locality LowCardinality(String),
        town LowCardinality(String),
        district LowCardinality(String),
        county LowCardinality(String)
    )
    ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/{table}/{shard}', '{replica}')
    ORDER BY (postcode1, postcode2, addr1, addr2);
    ```

    Notice that it is identical to the query used in the original `CREATE` statement of the
    [UK property prices](/docs/get-started/sample-datasets/uk-price-paid) example dataset tutorial,
    except for the `ON CLUSTER` clause and use of the `ReplicatedMergeTree` engine.

    The `ON CLUSTER` clause is designed for distributed execution of DDL (Data Definition Language)
    queries such as `CREATE`, `DROP`, `ALTER`, and `RENAME`, ensuring that these
    schema changes are applied across all nodes in a cluster.

    The [`ReplicatedMergeTree`](/docs/reference/engines/table-engines/mergetree-family/replication#converting-from-mergetree-to-replicatedmergetree)
    engine works just as the ordinary `MergeTree` table engine, but it will also replicate the data.
    It requires two parameters to be specified:

    * `zoo_path`: The Keeper/ZooKeeper path to the table's metadata.
    * `replica_name`: The table's replica name.

    <br />

    The `zoo_path` parameter can be set to anything you choose, although it is recommended to follow
    the convention of using prefix

    ```text theme={null}
    /clickhouse/tables/{shard}/{database}/{table}
    ```

    where:

    * `{database}` and `{table}` will be replaced automatically.
    * `{shard}` and `{replica}` are macros which were [defined](#macros-config-explanation)
      previously in the `config.xml` file of each ClickHouse node.

    You can run the query below from each host's client to confirm that the table has been created across the cluster:

    ```sql title="Query" theme={null}
    SHOW TABLES IN uk;
    ```

    ```response title="Response" theme={null}
       ┌─name────────────────┐
    1. │ uk_price_paid_local │
       └─────────────────────┘
    ```
  </Step>

  <Step title="Insert data into a distributed table" id="inserting-data-using-distributed">
    To insert data into the table, `ON CLUSTER` can't be used as it does
    not apply to DML (Data Manipulation Language) queries such as `INSERT`, `UPDATE`,
    and `DELETE`. To insert data, it is necessary to make use of the
    [`Distributed`](/docs/reference/engines/table-engines/special/distributed) table engine.
    As you learned in the [guide](/docs/guides/oss/deployment-and-scaling/examples/2-shards-1-replica) for setting up a cluster with 2 shards and 1 replica, distributed tables are tables which have access to shards located on different
    hosts and are defined using the `Distributed` table engine.
    The distributed table acts as the interface across all the shards in the cluster.

    From any of the host clients, run the following query to create a distributed table
    using the existing replicated table we created in the previous step:

    ```sql theme={null}
    CREATE TABLE IF NOT EXISTS uk.uk_price_paid_distributed
    ON CLUSTER cluster_2S_2R
    ENGINE = Distributed('cluster_2S_2R', 'uk', 'uk_price_paid_local', rand());
    ```

    On each host you will now see the following tables in the `uk` database:

    ```response theme={null}
       ┌─name──────────────────────┐
    1. │ uk_price_paid_distributed │
    2. │ uk_price_paid_local       │
       └───────────────────────────┘
    ```

    Data can be inserted into the `uk_price_paid_distributed` table from any of the
    host clients using the following query:

    ```sql theme={null}
    INSERT INTO uk.uk_price_paid_distributed
    SELECT
        toUInt32(price_string) AS price,
        parseDateTimeBestEffortUS(time) AS date,
        splitByChar(' ', postcode)[1] AS postcode1,
        splitByChar(' ', postcode)[2] AS postcode2,
        transform(a, ['T', 'S', 'D', 'F', 'O'], ['terraced', 'semi-detached', 'detached', 'flat', 'other']) AS type,
        b = 'Y' AS is_new,
        transform(c, ['F', 'L', 'U'], ['freehold', 'leasehold', 'unknown']) AS duration,
        addr1,
        addr2,
        street,
        locality,
        town,
        district,
        county
    FROM url(
        'http://prod1.publicdata.landregistry.gov.uk.s3-website-eu-west-1.amazonaws.com/pp-complete.csv',
        'CSV',
        'uuid_string String,
        price_string String,
        time String,
        postcode String,
        a String,
        b String,
        c String,
        addr1 String,
        addr2 String,
        street String,
        locality String,
        town String,
        district String,
        county String,
        d String,
        e String'
    ) SETTINGS max_http_get_redirects=10;
    ```

    Run the following query to confirm that the data inserted has been evenly distributed
    across the nodes of our cluster:

    ```sql theme={null}
    SELECT count(*)
    FROM uk.uk_price_paid_distributed;

    SELECT count(*) FROM uk.uk_price_paid_local;
    ```

    ```response theme={null}
       ┌──count()─┐
    1. │ 30212555 │ -- 30.21 million
       └──────────┘

       ┌──count()─┐
    1. │ 15105983 │ -- 15.11 million
       └──────────┘
    ```
  </Step>
</Steps>

<h2 id="conclusion">
  Conclusion
</h2>

The advantage of this cluster topology with 2 shards and 2 replicas is that it provides both scalability and fault tolerance.
Data is distributed across separate hosts, reducing storage and I/O requirements per node, while queries are processed in parallel across both shards for improved performance and memory efficiency.
Critically, the cluster can tolerate the loss of one node and continue serving queries without interruption, as each shard has a backup replica available on another node.

The main disadvantage of this cluster topology is the increased storage overhead—it requires twice the storage capacity compared to a setup without replicas, as each shard is duplicated.
Additionally, while the cluster can survive a single node failure, losing two nodes simultaneously may render the cluster inoperable, depending on which nodes fail and how shards are distributed.
This topology strikes a balance between availability and cost, making it suitable for production environments where some level of fault tolerance is required without the expense of higher replication factors.

To learn how ClickHouse Cloud processes queries, offering both scalability and fault-tolerance, see the section ["Parallel Replicas"](/docs/products/cloud/features/infrastructure/parallel-replicas).