Explain why a claimed queue job is not a completed job

By Batu · English technical notes

Also published in our Blogger archive.

Claiming work does not complete it

The conditional UPDATE changes queued to claimed, and commit persists that state. The assertions then show claimed and not completed because no completion operation appears in the script. A worker can therefore truthfully record ownership before it performs any external effect.

If a worker exits after the claim, treating it as completion would be false. This one-row model has no lease, completion time, or downstream idempotency record; those govern recovery of uncertain attempts.

Example

import sqlite3
from pathlib import Path
from tempfile import TemporaryDirectory

with TemporaryDirectory() as directory:
    connection = sqlite3.connect(Path(directory) / "queue.db")
    connection.execute("CREATE TABLE job(id INTEGER PRIMARY KEY, state TEXT)")
    connection.execute("INSERT INTO job VALUES (1, 'queued')")
    connection.execute("UPDATE job SET state = 'claimed' WHERE id = 1 AND state = 'queued'")
    connection.commit()
    state = connection.execute("SELECT state FROM job WHERE id = 1").fetchone()[0]
    assert state == "claimed"
    assert state != "completed"
    print("state=claimed completed=no")

Expected stdout:

state=claimed completed=no

Sources

- SQLite Transaction Control

- SQLite CREATE TABLE

Prepared with AI assistance. The example uses synthetic data; its stated limits apply.