Which HTTP Caching Headers Should I Use to Improve Browser Caching and Reduce Load Times for Repeat Visitors?
Summary
To improve browser caching and reduce load times for repeat visitors, certain HTTP caching headers are key. The major headers include `Cache-Control`, `Expires`, `Last-Modified`, `ETag`, and `Pragma`. By properly using these headers, you can significantly improve user experience by decreasing load times.
Understanding HTTP Caching Headers
`Cache-Control` Header
The `Cache-Control` header is the most important header for controlling web cache. It provides directives (instructions) that can be used by browser to understand the caching policy. For example, you can set how long a resource is considered fresh, or mark a resource as `no-store` -- which means it should never be cached. An example of usage is `Cache-Control: max-age=3600`, which tells the browser to cache the resource for an hour. Learn more at MDN Web Docs (Cache-Control).
`Expires` Header
The `Expires` directive allows you to specify a specific point in time when the cached resource becomes stale. While `Cache-Control: max-age` is preferred, `Expires` may be used for older browsers. Example usage: `Expires: Thu, 01 Dec 1994 16:00:00 GMT` (MDN Web Docs - Expires).
`Last-Modified` Header
This header specifies the date and time when the resource was last modified. The browser can then use this value to check if the version it has in cache is still up-to-date. Example: `Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT` (MDN Web Docs - Last-Modified).
`ETag` Header
The `ETag` header enables more efficient refreshing of cache. When a server sends a new `ETag` for a resource, the browser uses it in an `If-None-Match` header on the next request to the server. If they match, the server can tell the browser to keep using its cached version. For example: `ETag: "3e86-410-3596fbbc"` (MDN Web Docs - ETag).
`Pragma` Header
The `Pragma` header is a legacy header for backwards compatibility with HTTP/1.0 caches where `Cache-Control` was not yet present. It's used to specify `no-cache` to prevent caching. Example: `Pragma: no-cache` (MDN Web Docs - Pragma).
Conclusion
Through the proper use of HTTP caching headers, developers can control how and when a browser caches resources, leading to improved load times and user experience. While `Cache-Control` is the preferred and most extensive header, understanding each available option will enable you to provide the best results for your specific needs.