Introduction

Replication lag in MySQL can cause data inconsistency and read-after-write failures. This guide covers identifying the root cause and implementing fixes.

Symptoms

  • SHOW SLAVE STATUS shows Seconds_Behind_Master > 0
  • Reads from replicas return stale data
  • Replication SQL thread lagging behind IO thread

Step-by-Step Fix

  1. 1.Check replication status:
  2. 2.```sql
  3. 3.SHOW SLAVE STATUS;
  4. 4.SHOW MASTER STATUS;
  5. 5.SELECT * FROM performance_schema.replication_applier_status;
  6. 6.`
  7. 7.Identify slow queries on replica:
  8. 8.```sql
  9. 9.SELECT * FROM performance_schema.events_statements_summary_by_digest
  10. 10.ORDER BY SUM_TIMER_WAIT DESC LIMIT 10;
  11. 11.`
  12. 12.Enable parallel replication:
  13. 13.```ini
  14. 14.[mysqld]
  15. 15.slave_parallel_type=LOGICAL_CLOCK
  16. 16.slave_parallel_workers=8
  17. 17.slave_preserve_commit_order=ON
  18. 18.`