Design a listing connectivity ingestion service

Q: Design a listing connectivity ingestion service

This question evaluates a candidate's ability to design scalable, reliable REST APIs and distributed ingestion pipelines, testing competencies in API specification, idempotency, authentication/authorization, asynchronous processing, observability, and integration with downstream validation services.

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Question

Scenario

You are designing a Connectivity Service that allows external service providers (partners) to create or update listings in your company’s platform.

Partners send listing-creation requests to your service. Your service must forward the listing data to an internal downstream system (the Validation / Domain Service) owned by another team for validation and domain processing. After validation, your service must return a result to the partner.

The interview expects a very detailed REST API specification (endpoints, request/response payloads, headers, authentication, idempotency) and a clear end-to-end flow diagram.

Requirements

Functional

Accept listing create/update requests from external partners.
Validate basic request correctness at the edge (schema, auth, size limits).
Send the request to an internal Validation/Domain Service for business validation.
Return a clear outcome to the partner:
- Accepted/created
- Rejected with validation errors
- Still processing (if async)
Support retries safely (no duplicate listings).

API / Protocol expectations

Use HTTP with JSON payloads.
Specify:
- Required headers (e.g., Content-Type , tracing/correlation)
- Authentication/authorization mechanism
- Idempotency strategy
- Possible HTTP status codes and when each applies

Non-functional

Must scale to high request volume and multiple partners.
Be resilient to partner retries and downstream Validation Service outages/latency.
Provide observability (metrics/logs/tracing) and auditability.

Deliverables

A proposed API design (endpoints + request/response examples + headers).
A sequence/flow description (textual flow diagram is OK).
Notes on scaling, reliability, and key edge cases.