HTTP caching is a mechanism that allows web browsers and intermediate servers to store copies of web resources like images, scripts, and HTML pages. Caching enables the content to be served faster and more efficiently, reducing the number of requests that need to be made to the server.
The HTTP protocol defines a set of response headers that control how and for how long web resources are cached. These headers include:
1. Cache-Control: This header is used to specify caching directives that must be obeyed by all HTTP caches along the request/response chain. The directives can be used to specify how long the content should be cached, whether it can be cached at all, and under what conditions it can be revalidated or served from the cache.
1. Expires: The Expires header specifies the date and time when the content expires and should be considered stale. Once the content has expired, it must be re-validated with the server.
1. Last-Modified: This header specifies the last time the content was modified on the server. The browser can use this value to determine whether the content in the cache is still fresh, and whether the server needs to be contacted again.
1. ETag: The ETag header is an opaque string token that represents a specific version of the content on the server. If the content is updated, the ETag is also updated, and the browser can use the ETag to determine whether the cached version is still valid.
When a web resource is requested, the browser first checks its cache to see if it already has a copy of the content. If it does, and the cache policy (as specified by the HTTP response headers) allows for it, the cached version is served immediately without contacting the server. If the cached version has expired or is no longer valid, the browser sends a request to the server to re-validate the content. If the server confirms that the cached version is still up-to-date, it sends a 304 Not Modified response, and the browser serves the cached copy. If the content has changed, the server responds with a fresh copy, which the browser uses to update its cache.
Intermediate servers like proxies, load balancers, and CDNs also use HTTP caching to reduce the load on the server, speed up content delivery, and reduce network bandwidth usage. These servers act as both clients and servers in the HTTP request/response chain, caching and serving content on behalf of the origin server. Their caching policies are also governed by the same set of HTTP response headers, which allow them to cache content based on their own configurable parameters.
