mv can atomically claim a queue item only when the source and destination are on the same filesystem—and even then, it does not make the work itself happen exactly once. To prevent duplicate effects after a restart, give each item a stable ID, make the worker safe to retry, record completion durably, and define how to recover items left in processing/. launchd can start or relaunch the worker; the queue protocol must handle repeated starts.
What a same-filesystem mv does—and does not—guarantee
On macOS, a same-filesystem rename provides a useful atomic pathname transition: an item is named in the ready directory or the processing directory, rather than appearing half-moved between the two. Apple’s rename(2) documentation says the old and new paths must be on the same filesystem and that the new name remains present if the system crashes during the operation. That guarantee concerns the name transition, not whether the worker finished handling the item or whether an external service received an effect.
The command mv is not always a rename. The macOS mv(1) manual documents a copy-and-remove fallback when a rename cannot operate across filesystems. Keep the queue’s ready and processing directories on the same filesystem if the claim step relies on rename semantics. Do not extend the local-filesystem guarantee to network filesystems or other storage layers without testing them.
Most importantly, a successful claim is not exactly-once processing. If the worker sends a payment, posts a message, or performs another external action and then crashes before recording completion, recovery may run that action again. A pathname change cannot transact atomically with arbitrary application work.
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Use a queue protocol that survives repeated starts
- Publish only complete items. Have the producer write the full payload to a temporary file on the queue’s filesystem, close it, then rename it to a unique ready filename. A separate ready marker is another option. launchd filesystem triggers are not proof that a producer has finished writing.
- Claim one item at a time. Scan the ready directory and move an item to a unique processing path that retains its stable item ID, for example
processing/<item-id>. Treat a failed move as a failed claim: another worker may have claimed it, or the source state may have changed. Do not process an item unless the worker has established ownership. - Make the effect retry-safe. Pass the stable item ID as an idempotency key to downstream systems that support deduplication. If they do not, keep durable completion state and design reconciliation for the case where the external action succeeded but the worker did not record that success.
- Record outcomes explicitly. On success, transition the item to a completed or archive location. On failure, record attempt and error information and decide whether the failure is retryable. Avoid a shared fixed destination: ordinary
mvcan overwrite an existing destination. - Recover processing items by policy. At startup, inspect stale entries in
processing/and choose whether to retry them, quarantine them, or reconcile their effects. Do not blindly assume that every such item was untouched; a crash may have happened after the effect but before completion was recorded.
For example, a worker can use an item ID in its paths:
mv "$queue/ready/$item_id" "$queue/processing/$item_id"This is an illustrative claim step, not a complete queue implementation. It assumes the directories share a filesystem, the item ID is unique and safely validated, and the worker stops if the command fails. The protocol must also deal with destination collisions and concurrent workers; do not use a single reusable destination name for every item.
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Choose launchd triggers for the worker’s lifecycle
QueueDirectories, StartInterval, and KeepAlive describe different start or restart conditions. None turns a worker into a transactional queue engine. Apple’s launchd documentation describes job triggers and process management; it does not promise exactly-once completion of arbitrary work.
| Setting | What prompts a launch or restart | Operational fit and caveat |
|---|---|---|
QueueDirectories |
Queued path state, such as files appearing in a watched directory. | Useful for a directory-backed queue. The worker should drain available work and tolerate repeated invocations. Filesystem event monitoring can miss changes or start the job before a file is consistent, so producers must publish complete items. |
StartInterval |
A periodic timer. | Can provide a reconciliation opportunity if a filesystem trigger is missed. The manual notes that intervals can be coalesced around sleep and a firing can be missed while a job is already running; it is not a precise deadline or a substitute for recovery logic. |
KeepAlive |
Process-lifecycle conditions that keep or relaunch a job. | Use cautiously for a worker designed to drain the queue and exit successfully. Restarting on exit can produce unnecessary launches if successful exit is the normal behavior; repeated launches still need to be harmless. |
RunAtLoad can start a job when launchd loads it, which is useful when the worker should inspect existing queue state at startup. Select triggers according to desired latency and recovery behavior, then make the worker safe if triggers overlap or recur.
The Tool Desk
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For daemons, Apple’s archived launchd guidance says not to daemonize: let launchd manage the process directly. A shell worker should not fork itself into the background and leave launchd managing only a wrapper or an exited parent.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test the crash windows, not just the happy path
- Run two worker instances against the same ready item and verify that only the successful claimant proceeds.
- Stop the worker immediately after it claims an item, then verify the documented stale-processing recovery policy.
- Simulate a crash after the external action but before completion is recorded; confirm that retrying cannot duplicate the effect, or that reconciliation identifies the ambiguous outcome.
- Have a producer pause midway through writing and confirm that the worker cannot mistake the temporary file for a ready item.
- Test collisions, failed moves, and the actual target filesystem and macOS release. A cross-filesystem move may copy and remove rather than perform one atomic rename.
These checks expose the distinction that matters: claim ownership with a reliable state transition, and make processing and recovery safe when that transition or the work is attempted more than once.
Quick Recap
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