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Retrofit fundamentals

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Retrofit Fundamentals Deep Dive

Overview

This topic is essential for Android networking interviews because it combines robust API design, converter behavior, and coroutine execution over OkHttp.

Core Concepts

  • Keep API contracts explicit and versioned.
  • Separate transport errors from domain errors.
  • Prefer deterministic request/response mapping.

Internal Implementation

  • Retrofit handles interface parsing and adapter wiring.
  • OkHttp performs connection management and interceptors.
  • Converters map payloads into stable models.

Request/Response Flow

  • Build request from endpoint + headers + body.
  • Execute call through interceptor chain.
  • Map HTTP response into success/error result types.

Caching & Persistence

  • Use HTTP cache directives where possible.
  • Persist critical data for offline reads.
  • Define stale-read and refresh strategy explicitly.

Code Examples

interface UserApi {
    @GET("users")
    suspend fun users(): List<UserDto>
}

Common Interview Questions

  • Q: How do you structure API interfaces for long-term maintainability? A: Answer with API contract and failure semantics: timeout and retry policy, idempotency, error mapping, and observability for client and server behavior.
  • Q: Where should retry and timeout logic live? A: Answer with API contract and failure semantics: timeout and retry policy, idempotency, error mapping, and observability for client and server behavior.
  • Q: How do you expose failures to the UI layer? A: Answer with API contract and failure semantics: timeout and retry policy, idempotency, error mapping, and observability for client and server behavior.

Production Considerations

  • Add telemetry around status codes and latency percentiles.
  • Guard against schema drift with robust parsing defaults.
  • Use idempotency where retries can duplicate side effects.

Performance Insights

  • Reuse connections aggressively through OkHttp pooling.
  • Minimize payload size and serialization overhead.

Senior-Level Insights

  • Strong answers include migration strategy and backward compatibility policy.