Break it on purpose.

What you are looking at

A job queue is a to-do list that survives its program being killed. Work is written down first, then done later by something else — so a crash loses nothing that was already promised.

Everything on this page is that list, shown twice:

Jobs is what the queue believes — one card per job, with its state. The log file is what is actually on disk, record by record, with real byte offsets. The second one is the truth: the Jobs panel is rebuilt from it every time you press Recover. Keep an eye on both, because the whole design is that they can never disagree after a crash.

Nothing here is sent anywhere. It runs entirely in this tab, and reloading starts over.

A five-minute tour

1. Put work on the list Press Enqueue. A card appears under Jobs, and one ENQUEUE record appears in the log. Look at the byte offsets — that record is now on the disk, and the real thing would not have returned until it was.
2. Take the work, but do not remove it Press Lease. The job goes leased: hidden from everyone else for 30 seconds, but still on the list. This is the whole trick. If it were removed and the worker died, the job would be gone forever.
3. Kill the worker without saying so Instead of Ack, press +35s to run the clock past the 30-second deadline. The job comes back by itself. Nothing had to notice the worker died — the deadline is just a timestamp that passed.
4. Break the disk mid-write Press Crash mid-write, then read the last line of the log: half a record, cut off exactly where the write stopped. Now press Recover. The torn piece is detected and thrown away, and every complete record before it survives. That is the entire crash-safety story in two clicks.
5. Corrupt a byte that is already written Press Flip a bit, then Recover. The record is still the right length, so only the checksum can catch it — and it does. Without that, the queue would happily act on damaged data.

The five buttons at the top of the page do these sequences for you if you would rather watch than click. Hover any button for a one-line explanation of what it does.

The words on the buttons

Enqueue

Add a job to the list.

Lease

Claim a job for 30 seconds without removing it.

Ack

"Done" — now it can leave the list.

Nack

"That failed" — put it back, after a wait.

Extend

"Still working on it" — push the deadline out.

Backoff

Waiting longer before each retry: 1s, 2s, 4s. Stops a failing job hammering something already broken.

DLQ

Dead-letter queue: where a job goes after failing too many times, so a person can look at it instead of it retrying forever.

Compact

Rewrite the log with only the jobs that still matter. Finished ones are dropped; failed ones are kept.

CRC-32

A number computed from a record's bytes. If any bit changes, it stops matching — which is how corruption is caught.

fsync

Forcing bytes out of memory and onto the physical disk. The one thing this page cannot do; see below.