-The concurrency limits use the concept of semaphores when enqueuing, and work as follows: when a job is enqueued, we check if it specifies concurrency controls. If it does, we check the semaphore for the computed concurrency key. If the semaphore is open, we claim it and we set the job as _ready_. Ready means it can be picked up by workers for execution. When the job finishes executing (be it successfully or unsuccesfully, resulting in a failed execution), we signal the semaphore and try to unblock the next job with the same key, if any. Unblocking the next job doesn't mean running that job right away, but moving it from _blocked_ to _ready_. Since something can happen that prevents the first job from releasing the semaphore and unblocking the next job (for example, someone pulling a plug in the machine where the worker is running), we have the `duration` as a failsafe. Jobs that have been blocked for more than duration are candidates to be released, but only as many of them as the concurrency rules allow, as each one would need to go through the semaphore dance check. This means that the `duration` is not really about the job that's enqueued or being run, it's about the jobs that are blocked waiting.
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