Context and Problem
Modern web applications often need to handle unpredictable workloads, offload long-running processes, and ensure high availability.
- Web applications can become overloaded when handling long-running or computationally expensive tasks
- Direct synchronous processing can lead to performance bottlenecks and poor user experience
- Background jobs need reliability, fault tolerance, and scalable execution
Solution
The Web-Queue-Worker architecture decouples web request handling from background processing using a message queue.
- The web tier receives incoming user requests and quickly returns a response
- Requests that require long processing are sent to a message queue
- The worker tier retrieves and processes tasks asynchronously from the queue
- Workers execute tasks independently, scaling as needed based on demand
Benefits
- Scalability
- Web and worker tiers can scale independently
- Resilience
- Message queues provide fault tolerance and retry mechanisms
- Performance
- Web servers stay responsive by offloading heavy processing
- Decoupling
- Separation of concerns improves maintainability and flexibility
Trade-offs
- Complexity
- Requires additional infrastructure for message queues and worker management
- Latency
- Asynchronous processing may delay task completion
- Monitoring
- Requires observability to track queued tasks and worker failures
- Queue Overhead
- Message queue persistence can introduce additional costs
Issues and Considerations
- Message Durability
- Ensuring messages persist in case of failures
- Task Idempotency
- Preventing duplicate processing of the same task
- Queue Congestion
- Avoiding backlog buildup in high-load scenarios
- Worker Scaling
- Adjusting worker count dynamically based on queue length
When to Use This Pattern
- Offloading time-consuming background tasks from web servers
- Processing jobs asynchronously, such as sending emails, video encoding, or data processing
- Handling bursty workloads with scalable worker processes
- Ensuring fault tolerance and retry mechanisms for critical background tasks