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22. What is HTTP Cache? How does it work?

HTTP Cache is a technique used to store previously requested data (such as web pages, images, or other files) so that it can be quickly retrieved when that same data is requested again. This is done by storing the data on the client-side or intermediary servers (like a CDN), instead of requesting again from the server.

There are several mechanisms that make up HTTP caching, including:

  1. Cache-Control Header: This header is sent in the response by the server to instruct the client on how to cache the resource. It includes directives like public, private, no-cache, max-age, and others.

  2. ETag Header: This header is generated by the server to uniquely identify a version of a resource. The client can use this to check for updates to the cached version.

  3. Last-Modified Header: This header is a timestamp indicating the last time the resource was modified on the server. The client can compare this with the cached version to check for updates.

  4. Conditional Requests: The client can send a conditional request to the server using the If-Modified-Since or If-None-Match headers, which ask the server to return the resource only if it has been modified since the last request.

HTTP Cache works by checking the cache on the client-side, CDN or intermediary servers, and if the requested resource is present and valid (according to Cache-Control, ETag or Last-Modified header), the cache is used to serve that resource instead of requesting it from the server. This saves network bandwidth and server processing time, thus improving the overall performance and user experience.