Configure OkHttp Cache for Retrofit and force network requests

WHAT THIS TESTS: This question probes whether you treat HTTP caching as a cooperative protocol between client and server rather than a client-side switch. Interviewers want to see that you understand OkHttp owns the cache, Retrofit merely delegates to it, and that cache behavior is ultimately governed by response headers and OkHttp's RFC-compliant policy.

A GOOD ANSWER COVERS: First, constructing a Cache instance with a dedicated directory and a maxSize such as 50 MiB, then attaching it to an OkHttpClient Builder. Second, passing that OkHttpClient into your Retrofit Builder so all requests share the same client and cache. Third, explaining that server-sent Cache-Control headers like max-age, no-store, must-revalidate, and ETag or Last-Modified drive whether OkHttp stores a response, serves it without validation, or performs a conditional GET. Fourth, demonstrating how to force a network request by adding a CacheControl.FORCE_NETWORK header to the Request, which makes OkHttp skip the cache lookup entirely. Fifth, noting that the cache directory must be exclusively owned by one OkHttpClient instance and should persist across app restarts.

COMMON WRONG ANSWERS: Claiming Retrofit has a built-in cache abstraction separate from OkHttp. Stating that adding the Cache object alone guarantees all responses are cached regardless of headers. Forgetting that responses must be fully consumed, closed, or read to completion before OkHttp writes them to disk; cancelled or stalled streams are discarded. Suggesting you invalidate the cache by creating a brand new OkHttpClient instead of using cache evictAll or the urls iterator. Ignoring that a 304 Not Modified response merges the cached body with updated network headers.

LIKELY FOLLOW-UPS: How would you implement a pull-to-refresh force refresh without disabling the cache permanently? What happens when a response contains both a Cache-Control max-age and an Expires header? How would you handle cache size limits when users are offline for extended periods? Can you safely delete the cache directory manually, and what are the risks?

ONE CONCRETE EXAMPLE: Suppose your Retrofit service fetches a user profile. You build an OkHttpClient with Cache(File(context.cacheDir, "http_cache"), 50L * 1024L * 1024L). The server returns Cache-Control: max-age=3600 with an ETag. The first request hits the network. Within the hour, Retrofit serves the cached response instantly with a CacheHit event. After the hour, OkHttp sends a conditional request with If-None-Match. If the profile has not changed, the server returns 304 Not Modified, OkHttp returns the cached body, and you see CacheConditionalHit followed by CacheHit. To force an update after the user pulls to refresh, you build a new request with .cacheControl(CacheControl.FORCE_NETWORK) and remove the specific URL from the cache urls iterator before reissuing the call.

Source: square.github.io/okhttp