EC
1755
MariaDBERRORNotableReplicationHIGH confidence

MTS: event cannot be executed in parallel

Production Risk

High — replication may be inconsistent.

What this means

The MTS parallel replication worker could not execute the assigned event in parallel due to a dependency conflict or unsupported event type.

Why it happens
  1. 1Event has dependencies that require sequential execution.
  2. 2Non-transactional DDL event received by MTS worker.
How to reproduce
trigger — this will error
trigger — this will error
-- Internal MTS scheduling error; not directly triggerable

expected output

ERROR 1755 (HY000): Multi-threaded slave: Can not execute event in parallel.

Fix

Stop and restart the slave; consider reducing worker count

Stop and restart the slave; consider reducing worker count
STOP SLAVE;
SET GLOBAL slave_parallel_workers = 1;
START SLAVE;

Why this works

Lower parallelism reduces the likelihood of un-parallelisable event conflicts.

What not to do

✕

Sources
Official documentation ↗

MySQL 8.0 — 1755 ER_MTS_CANT_PARALLEL

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