EC
1668
MySQLERRORCommonReplicationHIGH confidence

Multi-threaded replication worker reset required

Production Risk

High — replica cannot proceed until workers are reset.

What this means

MTS parallel replication workers need to be reset because the worker state is inconsistent and cannot be recovered without a full reset.

Why it happens
  1. 1An unclean shutdown left worker thread states in an inconsistent condition.
  2. 2RESET SLAVE was not performed before changing the number of parallel workers.
How to reproduce
trigger — this will error
trigger — this will error
-- Starting replica after unclean MTS shutdown

expected output

ERROR 1668 (HY000): Slave workers need to be reset before proceeding.

Fix

Reset the replica and reconfigure

Reset the replica and reconfigure
STOP SLAVE;
RESET SLAVE;
CHANGE MASTER TO MASTER_AUTO_POSITION=1;
START SLAVE;

Why this works

RESET SLAVE clears the worker state, allowing a clean restart.

Version notes

Sources
Official documentation ↗

MySQL 8.0 — 1668 ER_MTS_RESET_WORKERS

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