Design API that aggregates three downstream APIs

You are given three existing HTTP JSON APIs owned by other backend services:

• Service A: GET /service-a?user_id={id} → returns basic user profile data.
• Service B: GET /service-b?user_id={id} → returns a list of the user’s recent orders.
• Service C: GET /service-c?user_id={id} → returns a list of recommended items for the user.

Each service:

• Uses REST over HTTP and returns JSON.
• Has independent latency and failure behavior.
• May return 2xx success, 4xx client errors, or 5xx server errors.

You need to design and implement a new API endpoint (often called an API bootstrap or aggregator) that:

• Exposes a single endpoint, e.g. GET /bootstrap?user_id={id} .
• Internally calls all three existing services.
• Combines their responses into a single JSON response to the client.
• Meets a reasonable latency SLO (for example, p95 < 300 ms).
• Handles partial failures gracefully (e.g., if one service is down).

Assume this will be implemented in a typical backend language (e.g., C++/Java/Go), but focus on the API and system design, not language-specific syntax.

Describe your design in detail, including:

1. API contract for the new /bootstrap endpoint (request parameters and response schema), including how you represent partial failures.
2. Call orchestration :
   • How you will call the three downstream services (in parallel vs sequentially).
   • How you will set timeouts and retries.
3. Error handling and fallback strategies :
   • What you return if one or more downstream services fail or time out.
   • How you distinguish between client errors and server errors from downstream services.
4. Performance and scalability considerations :
   • How your design keeps latency low.
   • How the service scales under high QPS.
5. Reliability and observability :
   • Logging, metrics, and tracing you would add.
   • Any use of patterns like circuit breakers or bulkheads.

Explain your reasoning step by step and call out important tradeoffs and edge cases.