Multi Step Processes medium
When a user uploads a photo, you need to: 1. Resize it, 2. Update their profile, 3. Analyze it for AI tags. You want these to happen in parallel. Which pattern fits best?
- A Fan-Out (Publish-Subscribe): Publish one 'PhotoUploaded' event that triggers 3 separate consumers.
- B Batch Processing.
- C Chain of Responsibility (A -> B -> C).
- D Single Threaded Loop.
Answer: A. Fan-Out (Publish-Subscribe): Publish one 'PhotoUploaded' event that triggers 3 separate consumers.
Fan-Out decouples the producer from consumers. The uploader just says 'Photo Ready'. The resize service, profile service, and AI service all subscribe independently.
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?