ClickHouse CDC Patterns

Overview

Change Data Capture (CDC) replicates database changes to ClickHouse for analytics. Core principle: Capture changes at source, transform for column-storage, handle deletes gracefully.

Key challenge: ClickHouse isn't designed for updates/deletes. Use ReplacingMergeTree or append-only patterns.

When to Use

Symptoms:

• Need to sync PostgreSQL/MySQL tables to ClickHouse
• Want real-time analytics on transactional data
• Schema changes breaking replication pipeline
• Unsure how to handle DELETE operations

When NOT to use:

• Streaming from Kafka/message queues → See clickhouse-streaming
• One-time data migration → Use batch ETL
• Query optimization → See clickhouse-patterns

Prerequisites

• Understanding of source database (PostgreSQL triggers, MySQL binlog)
• Basic ClickHouse knowledge → See clickhouse-patterns
• Familiarity with ReplacingMergeTree engine

Quick Reference

CDC Method Selection

digraph cdc_methods {
    rankdir=TD;
    node [shape=box, style=rounded];

    start [label="Choose CDC Method", shape=ellipse];
    volume [label="High volume?\n(>10k rows/sec)", shape=diamond];
    complexity [label="Complex schema?\n(many tables)", shape=diamond];

    debezium [label="Debezium\n(production-grade)"];
    airbyte [label="Airbyte\n(managed service)"];
    custom [label="Custom Triggers\n(simple setup)"];

    start -> volume;
    volume -> debezium [label="yes"];
    volume -> complexity [label="no"];
    complexity -> airbyte [label="yes"];
    complexity -> custom [label="no"];
}

Method	Best For	Pros	Cons
Debezium	High volume, production	Log-based, no DB impact	Complex setup
Airbyte	Multi-source, managed	UI, pre-built connectors	Cost, less control
Custom Triggers	Simple, low volume	Easy to understand	DB overhead

Critical Patterns

Pattern	Use Case	Key Technique
Soft Delete	Handle DELETEs	Add is_deleted flag
ReplacingMergeTree	Handle UPDATEs	Use version/timestamp column
Schema Evolution	Add columns	Nullable with defaults
Backfill	Initial sync	Snapshot then CDC

Pattern 1: PostgreSQL with Debezium

Architecture

PostgreSQL (WAL) → Debezium → Kafka → ClickHouse Kafka Engine

ClickHouse Setup

-- 1. Kafka staging table
CREATE TABLE users_kafka (
    id UInt64,
    name String,
    email String,
    updated_at DateTime,
    _operation String  -- INSERT, UPDATE, DELETE
) ENGINE = Kafka()
SETTINGS
    kafka_broker_list = 'kafka:9092',
    kafka_topic_list = 'dbserver1.public.users',
    kafka_group_name = 'clickhouse_consumer',
    kafka_format = 'JSONEachRow';

-- 2. Target table with dedup
CREATE TABLE users (
    id UInt64,
    name String,
    email String,
    updated_at DateTime,
    is_deleted UInt8 DEFAULT 0
) ENGINE = ReplacingMergeTree(updated_at)
ORDER BY id;

-- 3. Transform with materialized view
CREATE MATERIALIZED VIEW users_mv TO users AS
SELECT
    id, name, email, updated_at,
    if(_operation = 'DELETE', 1, 0) AS is_deleted
FROM users_kafka;

-- 4. Query active records
SELECT * FROM users FINAL WHERE is_deleted = 0;

Debezium Connector

{
  "name": "postgres-connector",
  "config": {
    "connector.class": "io.debezium.connector.postgresql.PostgresConnector",
    "database.hostname": "postgres",
    "database.port": "5432",
    "database.user": "replicator",
    "database.dbname": "mydb",
    "database.server.name": "dbserver1",
    "table.include.list": "public.users,public.orders",
    "plugin.name": "pgoutput"
  }
}

Pattern 2: PostgreSQL with Custom Triggers

Use Case

• Low volume (< 1k rows/sec)
• Simple schema (1-5 tables)
• No Kafka infrastructure

PostgreSQL Trigger

CREATE OR REPLACE FUNCTION notify_changes()
RETURNS TRIGGER AS $$
BEGIN
    IF TG_OP = 'INSERT' OR TG_OP = 'UPDATE' THEN
        PERFORM pg_notify('table_changes', json_build_object(
            'table', TG_TABLE_NAME,
            'operation', TG_OP,
            'data', row_to_json(NEW)
        )::text);
        RETURN NEW;
    ELSIF TG_OP = 'DELETE' THEN
        PERFORM pg_notify('table_changes', json_build_object(
            'table', TG_TABLE_NAME,
            'operation', 'DELETE',
            'id', OLD.id
        )::text);
        RETURN OLD;
    END IF;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER users_notify_trigger
AFTER INSERT OR UPDATE OR DELETE ON users
FOR EACH ROW EXECUTE FUNCTION notify_changes();

Node.js CDC Service

import { Client } from 'pg';
import { ClickHouse } from 'clickhouse';

const pgClient = new Client({ connectionString: process.env.PG_URL });
const clickhouse = new ClickHouse({ url: process.env.CH_URL });

async function startCDC() {
  await pgClient.connect();
  await pgClient.query('LISTEN table_changes');

  pgClient.on('notification', async (msg) => {
    const { table, operation, data, id } = JSON.parse(msg.payload);

    if (operation === 'INSERT' || operation === 'UPDATE') {
      await clickhouse.insert(table, [{ ...data, is_deleted: 0 }]);
    } else if (operation === 'DELETE') {
      await clickhouse.query(`
        ALTER TABLE ${table} UPDATE is_deleted = 1 WHERE id = ${id}
      `).toPromise();
    }
  });
}

Pattern 3: MySQL Binlog

{
  "name": "mysql-connector",
  "config": {
    "connector.class": "io.debezium.connector.mysql.MySqlConnector",
    "database.hostname": "mysql",
    "database.user": "debezium",
    "database.server.id": "184054",
    "database.server.name": "mysql-server",
    "table.include.list": "mydb.orders,mydb.customers"
  }
}

Key differences: Uses binlog (not WAL), requires server.id, schema changes tracked separately.

Handling DELETE Operations

Option 1: Soft Delete (Recommended)

CREATE TABLE orders (
    id UInt64,
    user_id UInt64,
    amount Decimal(10, 2),
    is_deleted UInt8 DEFAULT 0
) ENGINE = ReplacingMergeTree(updated_at)
ORDER BY id;

SELECT * FROM orders FINAL WHERE is_deleted = 0;

Option 2: CollapsingMergeTree

CREATE TABLE orders (
    id UInt64,
    amount Decimal(10, 2),
    sign Int8  -- 1 = active, -1 = deleted
) ENGINE = CollapsingMergeTree(sign)
ORDER BY id;

-- Insert
INSERT INTO orders VALUES (1, 50.00, 1);
-- Delete (insert negative)
INSERT INTO orders VALUES (1, 50.00, -1);
-- Query
SELECT * FROM orders FINAL WHERE sign = 1;

Schema Evolution

Add Columns as Nullable

ALTER TABLE users ADD COLUMN phone String DEFAULT '';
ALTER TABLE users ADD COLUMN country LowCardinality(String) DEFAULT 'UNKNOWN';

Schema Registry Pattern

const schemaRegistry = {
  'users': { version: 2, columns: ['id', 'name', 'email', 'phone'] }
};

function transformRow(table: string, row: any) {
  return {
    ...row,
    phone: row.phone || '',
    country: row.country || 'UNKNOWN'
  };
}

Backfill Strategy

// 1. Snapshot existing data
async function backfillTable(tableName: string) {
  const stream = pgClient.query(`COPY (SELECT * FROM ${tableName}) TO STDOUT`);
  await pipeline(stream, csvParser(), clickhouse.insert(tableName).stream());
}

// 2. Start CDC from snapshot position
const lsn = await pgClient.query(`SELECT pg_current_wal_lsn()`);

Common Mistakes

Mistake	Why It Fails	Fix
No deduplication	Duplicate events	Use ReplacingMergeTree
Hard deletes	Not supported well	Soft delete with flag
Ignoring order	Out-of-order updates	Use version/timestamp
No backfill	Missing historical data	Snapshot before CDC

Performance Tips

-- Batch Kafka consumption
SETTINGS kafka_max_block_size = 65536, kafka_poll_timeout_ms = 1000;

-- Async inserts
SET async_insert = 1, wait_for_async_insert = 0;

Monitoring

-- Replication lag
SELECT table, max(synced_at) AS last_sync,
       now() - max(synced_at) AS lag_seconds
FROM system.parts WHERE active GROUP BY table;

-- Kafka consumer errors
SELECT database, table, exceptions, last_exception_time
FROM system.kafka_consumers;

Best Practices

Design:

• Use ReplacingMergeTree for tables with updates
• Always include updated_at/version column
• Implement soft deletes, not hard deletes

Operations:

• Monitor replication lag (< 1 minute target)
• Alert on Kafka consumer errors
• Test schema changes in staging

Performance:

• Batch Kafka consumption (kafka_max_block_size)
• Use async inserts
• Partition by time

Red Flags

• ❌ "ALTER TABLE DELETE" → Extremely slow
• ❌ "Updates without version" → Race conditions
• ❌ "No lag monitoring" → Stale analytics
• ❌ "Hard deletes" → Data inconsistency

When to Escalate

• Replication lag > 5 minutes consistently
• Kafka consumer crashes
• Schema changes breaking pipeline
• Multi-datacenter replication needed

Resources: Debezium docs (https://debezium.io), ClickHouse Kafka engine docs

Related Skills

• ClickHouse fundamentals: See clickhouse-patterns
• Streaming from queues: See clickhouse-streaming

---

Remember: CDC is about consistency. Always handle deletes, monitor lag, and test schema changes thoroughly.