<p>Raised on a replica when an event group cannot be executed in parallel mode, forcing the SQL thread to stop worker threads.</p>
<p>MySQL Error 1755 ER_MTS_CANT_PARALLEL arises when a replica worker thread hits a binlog event group that cannot run in parallel. Temporarily switch to single-threaded replication, let the event execute, then restore the original worker count to clear the error.</p>
Cannot execute the current event group in the parallel
The message Cannot execute the current event group in the parallel mode appears on a MySQL replica running with slave_parallel_workers greater than zero. The SQL thread found a transaction that cannot be safely scheduled across parallel worker threads, so it aborts the group and raises error 1755 to protect data consistency.
Each event group in the binary log must obey dependency rules. When a DDL statement, a cross database transaction, or a statement based row event violates those rules, MySQL halts parallel execution and emits ER_MTS_CANT_PARALLEL.
The replica performs real time dependency checks. If the event group touches the mysql system schema, modifies GTID state, or mixes DDL and DML, the checks fail. This prompts the SQL thread to stop workers and request manual intervention.
Replication lag grows while the error persists because the remaining events cannot progress. Production read traffic that depends on the replica may experience outdated data.
Disable parallel workers temporarily, execute the problematic event serially, then re enable multithreaded replication. Most environments solve the issue with three statements: STOP SLAVE SQL_THREAD, SET GLOBAL slave_parallel_workers=0, START SLAVE SQL_THREAD.
Once the relay log position advances past the blocking event, restore the original worker count and verify Seconds_Behind_Master returns to normal.
Parallel replication boosts throughput, but error 1755 forces a return to single threaded mode, potentially widening lag. Monitoring tools should alert on this condition so you can react quickly.
Schedule schema changes during low traffic windows or use online DDL tools that emit row based events only. Keep replica settings slave_preserve_commit_order and binlog_transaction_dependency_tracking aligned with workload patterns to reduce conflicts.
CREATE, ALTER, or DROP statements within a group prevent safe parallel scheduling.
When a single transaction modifies tables in different schemas, dependency tracking cannot ensure order.
Events that write to the mysql or performance_schema databases are always executed serially.
GTID manipulation and SET statements that affect session context block parallel workers.
Generic replication failure that also stops the SQL thread but may involve different causes.
The IO thread cannot read the master's binary log due to corruption or missing files.
CREATE DATABASE event conflicts with an existing schema on the replica.
You can set sql_slave_skip_counter = 1 but risk data divergence. Prefer serial execution.
No. The variable is dynamic. Just stop and start the SQL thread to apply.
It prevents commit order violations but cannot fix DDL conflicts that still block parallel workers.
Galaxy's editor highlights replication variables and offers AI snippets that set the correct worker counts, reducing human error during recovery.
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