Multi Step Processes hard
A worker processes a message for 5 minutes. The Queue Visibility Timeout is 1 minute. The queue redelivers the message to a second worker. Now two workers are processing the same data concurrently. How do you prevent the first worker from corrupting the state when it finally finishes?
- A Increase the timeout to 10 minutes.
- B Kill the first worker.
- C Optimistic Locking (Version Numbers) on the database rows.
- D Use a FIFO queue.
Answer: C. Optimistic Locking (Version Numbers) on the database rows.
You can't always perfectly tune timeouts (GC pauses, network lag). Optimistic Locking (`UPDATE ... WHERE version = 1`) ensures that if Worker 2 finished first (bumping version to 2), Worker 1's late write fails safely.
One system design question a day, with spaced repetition so the ones you miss come back.
Get the appRelated questions
- A user registration process involves two steps: inserting a row into the 'Users' database table and inserting a row into the 'Profiles' table. If the first insert succeeds but the second fails, the database is left in an inconsistent state. What mechanism prevents this?
- In a multi-step workflow across different services (Order Service -> Payment Service), you cannot use a single database transaction. If the Payment fails after the Order was created, how do you fix the Order state?
- You are writing a script to process 10,000 images. The script runs on a server that might restart unexpectedly. To ensure you don't have to start over from image #1 after a crash, what should you implement?
- A client app sends a 'Pay $50' request to your API. The server processes the payment but the network response is lost. The client automatically retries the request. What prevents the user from being charged $100?
- A user uploads a large video file that needs to be transcoded (converted), which takes 5 minutes. Why is it bad practice to keep the HTTP connection open while the server processes the video?
- How does CDC (reading the database transaction log) solve the 'Dual Write' problem better than the Application-level Outbox pattern?