Include job_id in order clause when fetching scheduled jobs to dispatch and when dispatching #164
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…ch and when dispatching This was missing and makes the locking prone to deadlocks when we have a bunch of jobs scheduled at the same time with the same priority, since we'd have a non-deterministic ordering. Besides, on the DELETE query, force a order by job_id even if we don't care about that. It turns out, under certain circumstances, when the scheduled_executions table is too small, MySQL might choose not to use any indexes for the DELETE by job_id. This means it might lock other rows besides those it's going to delete. Then, when running more than one dispatcher doing this, we might end up with a deadlock because one dispatcher is deleting and locking some rows that's not deleting, and the other is doing the same, and both are waiting for the other's lock. This deadlock was happening consistently in CI, for MySQL only, but I didn't manage to reproduce it locally, and it has never happened in production for us. Using an INDEX hint in the DELETE query, to ensure the index on job_id was used, also avoided the deadlock.
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This was missing and makes the locking prone to deadlocks when we have a bunch of jobs scheduled at the same time with the same priority, since we'd have a non-deterministic ordering.
Besides, on the
DELETEquery, forceORDER BY job_ideven if we don't care about that. It turns out, under certain circumstances, when the scheduled_executions table is too small, MySQL might choose not to use any indexes for theDELETEbyjob_id. This means it might lock other rows besides those it's going to delete. Then, when running more than one dispatcher doing this, we might end up with a deadlock because one dispatcher is deleting and locking some rows that is not going to delete, and the other is doing the same, and both are waiting for the other's lock.This deadlock was happening consistently in CI, for MySQL only, but I didn't manage to reproduce it locally, and it has never happened in production for us. Using an
INDEXhint in theDELETEquery, to ensure the index onjob_idwas used, also avoided the deadlock.