ClickHouse Horizontal Scaling: Optimal Read-Write Split Configuration and Execution

Introduction

Scaling ClickHouse horizontally while optimizing the split between read and write operations is a multi-step process that involves setting up a cluster with sharded and replicated tables. Here’s a detailed guide, including configuration settings and a run-book.

Understanding ClickHouse Horizontal Scaling

1. Sharding: Distributes the data across multiple nodes to balance the write load.
2. Replication: Creates copies of data on different nodes to increase read capacity and fault tolerance.

Pre-requisites

• Multiple ClickHouse nodes (servers).
• ZooKeeper cluster for coordinating replicas.

Step 1: Configuring the Cluster

Cluster Configuration (config.xml)

1. Define each shard and its replicas. Example for a cluster with 2 shards, each having 2 replicas:

<yandex>
    ...
    <clickhouse_remote_servers>
        <my_cluster>
            <shard>
                <replica>
                    <host>node1a</host>
                    <port>9000</port>
                </replica>
                <replica>
                    <host>node1b</host>
                    <port>9000</port>
                </replica>
            </shard>
            <shard>
                <replica>
                    <host>node2a</host>
                    <port>9000</port>
                </replica>
                <replica>
                    <host>node2b</host>
                    <port>9000</port>
                </replica>
            </shard>
        </my_cluster>
    </clickhouse_remote_servers>
    ...
</yandex>

2. Configure ZooKeeper nodes. Add this section to config.xml:

<zookeeper-servers>
    <node index="1">
        <host>zk1</host>
        <port>2181</port>
    </node>
    <node index="2">
        <host>zk2</host>
        <port>2181</port>
    </node>
    <node index="3">
        <host>zk3</host>
        <port>2181</port>
    </node>
</zookeeper-servers>

3. Repeat this configuration on all ClickHouse nodes.

Step 2: Creating Sharded and Replicated Tables

1. Create a Distributed table that spans the entire cluster:

CREATE TABLE mydb.my_table_global ON CLUSTER my_cluster AS mydb.my_table
ENGINE = Distributed(my_cluster, mydb, my_table, rand());

2. Create a ReplicatedMergeTree table on each shard:

CREATE TABLE mydb.my_table ON CLUSTER my_cluster (
    ...
) ENGINE = ReplicatedMergeTree('/clickhouse/tables/{shard}/my_table', '{replica}')
ORDER BY ...

3. The Distributed table will automatically distribute data among shards.

Step 3: Writing Data

• Write operations should be directed to the Distributed table (my_table_global). It will distribute the inserts across the shards.

Step 4: Reading Data

• Read operations can be directed to either the Distributed table for querying the entire dataset or directly to the ReplicatedMergeTree tables for shard-local reads.
• Load balancers can be used to distribute read queries among replicas for load balancing.

Step 5: Monitoring and Maintenance

• Regularly monitor the performance, balance of the shards, and replication lag.
• Adjust sharding strategy based on data growth and query patterns.

Best Practices

• Test your configuration in a staging environment before deploying to production.
• Monitor query performance and adjust indexes and table schemas as necessary.
• Regularly back up your data.

Conclusion

This run-book outlines the basic steps to scale ClickHouse horizontally by setting up a sharded and replicated cluster. The key is to balance the write load across shards and distribute read queries among replicas. Continuous monitoring and adjustments are crucial for optimal performance in a dynamically scaling environment.

To read more on Horizontal Scaling in ClickHouse, do consider reading the below articles:  

• Comprehensive Guide for ClickHouse Horizontal Scaling and Capacity Planning
• ClickHouse Horizontal Scaling: Setting up 6-node ClickHouse Infrastructure with Replication & Sharding
• ClickHouse Horizontal Scaling: Implementing Parallel Replicas with Dynamic Shards
• ClickHouse Horizontal Scaling: Sharding and Resharding Strategies

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