Problem 42

Build a Simple Job Queue Processor

EASYBUILD

Queues / Background Jobs+2

Implement a JobQueue class that processes background jobs in first-in, first-out order.

Jobs move through a simple lifecycle: they start as pending, become done when their handler succeeds, or failed if anything goes wrong. There is no retry logic — a failed job stays failed.

This is the foundation. A more complex retry system (like the one in problem 0030) builds on top of exactly this shape.

Requirements

Implement a JobQueue class in main.py.
register_handler(job_type, fn) registers a callable to invoke when a job of that type is processed.
enqueue(job_type, payload) adds a job with status 'pending' to the queue and returns a unique string job ID.
process_next() takes the next pending job in FIFO order, calls its registered handler with (payload), and returns the job ID. Returns None if the queue is empty.
If the handler raises any exception, the job status becomes 'failed'. If no handler is registered for the job type, the job status also becomes 'failed'. In both cases process_next must not propagate the exception.
get_job(job_id) returns a dict with keys id, type, payload, and status reflecting the current state of that job.

Examples

Example 1

Input

q = JobQueue()
q.register_handler('email', lambda p: None)
job_id = q.enqueue('email', {'to': 'a@b.com'})
processed = q.process_next()
q.get_job(job_id)

Output

processed == job_id
{"id": job_id, "type": "email", "payload": {"to": "a@b.com"}, "status": "done"}

Note

The handler succeeds, so the job transitions to 'done'.

Example 2

Input

q.register_handler('risky', lambda p: (_ for _ in ()).throw(ValueError('boom')))
job_id = q.enqueue('risky', {})
q.process_next()
q.get_job(job_id)['status']

Output

'failed'

Note

The handler raises an exception. process_next catches it and marks the job failed without re-raising.

Example 3

Input

result = q.process_next()  # called on an empty queue

Output

None

Note

Empty queue returns None.

Constraints

Job IDs must be unique across all calls to enqueue within a single JobQueue instance.
Do not use any external libraries — stdlib only (uuid, collections, etc. are fine).
process_next must not raise even if the handler raises.

Follow-up

This queue loses all jobs when the process restarts. What would you need to change to make it durable — so jobs survive a crash?

Hints

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