Design aggregator for multiple downstream services

Design a backend service for a large retailer (similar to Walmart) that must call **many downstream services** to fulfill a single user request. When a client calls this "aggregator" service: - It must fan out requests to several downstream services (for example: pricing, inventory, user profile, recommendations, shipping options, promotions). - It needs to **wait for responses from all required downstream services** before sending a combined response back to the client. Assume a microservices environment, potentially implemented with a framework like Spring Boot. Discuss and design the system. In your answer, cover: 1. **High-level architecture** - How the client interacts with this aggregator service. - How the aggregator interacts with multiple downstream services. - Any supporting components (API gateway, cache, message broker, etc.). 2. **Request handling and concurrency** - How the aggregator should make calls to downstream services (synchronously vs. asynchronously; parallel vs. sequential). - How to aggregate all responses efficiently. - How to handle timeouts and retries. 3. **Failure handling and resilience** - What happens if one or more downstream services are slow, fail, or return errors. - How to avoid the aggregator becoming a bottleneck or single point of failure. - Use of patterns such as circuit breakers, bulkheads, and fallbacks. 4. **Performance and scalability** - How to ensure low latency for end users, even when many downstream calls are involved. - How to scale the aggregator service as traffic grows. - Possible use of caching and batching. 5. **API and data model considerations** - How you would structure the aggregator’s API and response schema. - How to version the API as downstream services evolve. 6. **Implementation considerations in Spring Boot (or similar)** - How you might use async/non-blocking patterns (e.g., thread pools, reactive programming) to implement fan-out/fan-in. - How you would configure timeouts, retries, and observability (logging, metrics, tracing). Provide a detailed, step-by-step design, calling out the key trade-offs and alternatives you consider.