The function onMigrationCheckTick of onlineDDL scheduler will pick one ready task to run once time by calling runNextMigration.

And every ready onlineDDL task need to check conflicts with running tasks by the func isAnyConflictingMigrationRunning: if the ready one has same table with the running one, or both of them has not set the allowConcurrent option, then they can not run concurrently.

What's more, if the ReadyToComplete flag of running task is false, they can not run concurrently also. ReadyToComplete flag is set true in the func reviewRunningMigrations of onlineDDL scheduler: when the func isVReplMigrationReadyToCutOver returns true, which means the onlineDDL task is ready to cut over, the ReadyToComplete flag is set true. And the onlineDDL will start cutover by calling the func cutOverVReplMigration.

To make a conclusion: Only when two onlineDDL tasks act on different tables, and one of them has set the allowConcurrent option, and the running onlineDDL task is ready to cutover or is already doing cutover, then they can be executed in parallel.