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Load balancing ensures queries are distributed evenly across ClickHouse nodes. With Spring Boot, you can easily configure and use it.

1. Cluster Setup in ClickHouse

Define replicas under a shard for balancing: 
<remote_servers>
  <my_cluster>
    <shard>
      <replica>
        <host>node1</host>
        <port>9000</port>
      </replica>
      <replica>
        <host>node2</host>
        <port>9000</port>
      </replica>
    </shard>
  </my_cluster>
</remote_servers>
ClickHouse automatically balances queries among replicas.

2. Create a Distributed Table

CREATE TABLE logs_local (
  id UInt32,
  message String,
  ts DateTime
) ENGINE = MergeTree()
ORDER BY id;

CREATE TABLE logs_dist AS logs_local
ENGINE = Distributed('my_cluster', 'default', 'logs_local', rand());
rand() ensures random replica selection for load balancing.

3. Use in Spring Boot

List<Map<String, Object>> logs = jdbcTemplate.queryForList(
  "SELECT * FROM logs_dist WHERE ts > now() - INTERVAL 1 DAY"
);
Spring Boot queries the distributed table, and ClickHouse handles balancing.

4. Connection Pooling

Add HikariCP to manage connections efficiently: 
spring.datasource.hikari.maximum-pool-size=20
spring.datasource.hikari.minimum-idle=5
This helps Spring Boot apps perform better under load.

5. Failover and Reliability

If one replica is down, ClickHouse routes queries to others automatically. Your Spring Boot app stays responsive.
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