Intermediatebehavioral
Describe a situation where you had to collaborate closely with frontend or mobile engineers to deliver a backend feature. How did you handle API design and iteration?
Backend Developer
General

Sample Answer

(S) We were building a new “saved items” feature across web and mobile, and the existing APIs were inconsistent and chatty, causing ~1.5s load times on mobile. (T) I was the backend lead responsible for designing a unified API that worked well for all clients. (A) I started with a joint design session with frontend and mobile engineers to understand their data needs and constraints. We agreed on a single `/saved-items` endpoint that returned a tailored DTO including product details, availability, and user metadata in one round-trip. I introduced pagination and an `updated_since` filter to reduce payload sizes by about 40%. We used an OpenAPI spec as the contract and iterated in a mock server before backend implementation. I added versioning (`v2`) and feature flags so clients could migrate gradually. (R) After rollout, the mobile “saved items” screen load time dropped from ~1.5s to 600–700ms on average, and API calls per page decreased from 4–5 to 1. We completed the feature within the sprint, and follow-up bugs related to this endpoint dropped by 60% compared to similar features, thanks to the shared contract and early collaboration.

Keywords

Cross-functional collaborationAPI contract-first approachHandling performance and payload concernsVersioning and gradual rolloutMetrics on performance and quality
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