Replication Guide

ReplicatedMergeTree Basics

• Engine: ReplicatedMergeTree('/clickhouse/tables/{shard}/{database}/{table}', '{replica}')
• Requires ZooKeeper or ClickHouse Keeper
• All replicas are equal — any replica can accept writes
• Replication is asynchronous by default

Monitoring Replication

• system.replicas — per-table status: absolute_delay, queue_size, is_leader, is_readonly
• system.replication_queue — pending operations: fetches, merges, mutations
• Key health indicators:
  • absolute_delay = 0 — fully caught up
  • is_readonly = 0 — accepting writes
  • queue_size < 10 — healthy queue
  • active_replicas = total_replicas — all replicas online