Context and Problem
Client applications often need to request information from backend services where the processing time is significant or unpredictable. Synchronous requests can lead to poor client responsiveness, timeouts, and inefficient resource utilization as connections stay open during long-running operations.
- Client UI remains blocked
- Unpredictable timeouts
- Wasted server resources
- Connection management overhead
- Poor scalability
Solution
Implement an asynchronous request-reply pattern where the client submits a request message to a broker, receives an acknowledgment immediately, and later checks for or receives a notification when the response is ready. This decouples the client from the service processing time constraints.
- Configure a message broker or queue system to handle asynchronous communication
- Implement a request queue for incoming client requests
- Create a response queue or notification mechanism for completed operations
- Generate and track correlation IDs to match requests with their corresponding responses
- Develop a client-side polling mechanism or callback handler for receiving responses
- Implement timeout handling and retry logic for reliability
- Set up monitoring and tracking for request-response lifecycle management
Benefits
- Non-Blocking Operation
- Improves client responsiveness by not blocking during processing
- Loose Coupling
- Enhances system resilience through loosely coupled components
- Enhanced Scalability
- Enables better scalability and throughput for backend services
- Built-in Reliability
- Provides natural retry and failure handling mechanisms
- Long-Running Process Management
- Simplifies handling of long-running operations
- Resource Optimization
- Reduces resource consumption from holding open connections
Trade-offs
- Increased Complexity
- Adds more components and interactions to the system architecture
- Infrastructure Requirements
- Additional components (message broker, storage) required to implement the pattern
- Eventual Consistency
- Results are not available immediately, requiring client adaptation to asynchronous responses
- Correlation Management
- Need to handle correlation between requests and responses across distributed components
- Complex Error Handling
- More complicated error handling and timeout scenarios across the request-reply lifecycle
Issues and Considerations
- Client Adaptation
- Clients must handle the asynchronous nature of responses
- Message Correlation
- Need for a correlation ID mechanism to match requests with responses
- Message Orphaning
- Potential for orphaned messages if services fail during processing
- Distributed State Management
- Managing timeouts and error states across distributed components
- Observability Challenges
- Additional monitoring complexity for tracking request status across system components
When to Use This Pattern
- When operations take significant or unpredictable time to complete
- For computationally intensive operations
- When building scalable, high-throughput systems
- For improving user experience during long-running processes
- When client and service availability windows don't align
- To implement resilient communication across unreliable networks