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HTTP Conditional Requests: ETags, Last-Modified & 304

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Conditional requests: checking freshness without downloading

HTTP caching has two modes: strong caching (serve from cache without asking the server) and weak/conditional caching (ask the server if the content changed, skip download if it hasn't).

Conditional requests use validators — ETags (content hashes) or Last-Modified timestamps — to check in a single round trip whether a cached response is still valid. A 304 Not Modified response has no body, saving bandwidth for unchanged content.

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