How the browser's built-in caching mechanism works

The browser's built-in caching mechanism is designed to improve web performance by storing copies of resources such as HTML, CSS, JavaScript, and images, so that they do not need to be downloaded repeatedly. Here’s a detailed explanation of how this caching works!

1. Caching Basics

When a browser makes a request for a resource (like a webpage or an image), it checks its cache to see if a fresh copy of the resource is available. If the resource is already cached and is still considered valid, the browser can use the cached version, which reduces load times and network traffic.

2. Cache-Control Headers

Cache behavior is primarily controlled by HTTP headers. The key headers involved in caching are:

• Cache-Control: This header is used to specify caching directives for both the browser and intermediate caches (like proxies). Some common directives include:

  • max-age=<seconds>: Specifies the maximum amount of time a resource is considered fresh. After this period, the resource is considered stale and must be revalidated.
  • no-cache: Indicates that the resource must be revalidated with the server before being used from the cache.
  • no-store: Specifies that the resource should not be cached at all.
  • public: Indicates that the response can be cached by any cache, even if the response is normally non-cacheable or authenticated.
  • private: Indicates that the response is intended for a single user and should not be cached by shared caches (like proxies).

• Expires: This header specifies an absolute date and time after which the resource is considered stale. It's an older form of caching control, mostly replaced by Cache-Control.

• ETag: This is a unique identifier assigned by the server to a specific version of a resource. The browser uses this to validate whether the cached resource is still up to date.

• Last-Modified: This header indicates the date and time when the resource was last modified. The browser can use this to check if the cached resource is still valid by sending an If-Modified-Since request header.

3. How Caching Works

Here’s a step-by-step breakdown of how the browser caching process typically works:

1. Initial Request: The browser requests a resource from the server.

2. Server Response:

   • Cache-Control Headers: The server includes caching headers in the response to indicate how the resource should be cached.
   • Storing in Cache: The browser stores the resource in its cache along with the caching headers.

3. Subsequent Requests:

   • Check Cache: For subsequent requests, the browser first checks if the resource is in the cache and whether it is still valid based on the caching headers.
   • Use Cached Resource: If the cached resource is valid, the browser uses it directly, avoiding a network request.

4. Revalidation:

   • ETag/Last-Modified: If the cached resource is stale or requires revalidation (based on Cache-Control), the browser sends a conditional request to the server using If-None-Match (for ETag) or If-Modified-Since (for Last-Modified) to check if the resource has changed.
   • Server Response: If the resource has not changed, the server responds with a 304 Not Modified status and the browser continues to use the cached resource. If the resource has changed, the server responds with the new version of the resource.

4. Cache Storage

• In-Memory Cache: For faster access, some browsers cache resources in memory. This is useful for frequently accessed resources.
• Disk Cache: Resources are also stored on disk to persist across browser sessions.

5. Cache Expiration and Invalidation

• Cache Expiration: When the cache expires, the browser must fetch a fresh copy of the resource from the server.
• Manual Cache Control: Developers can use headers like Cache-Control: no-cache to enforce revalidation or Cache-Control: no-store to prevent caching altogether.

6. Example

Consider the following HTTP response headers for a static resource:

HTTP/1.1 200 OK
Cache-Control: public, max-age=3600
ETag: "123456789"
Last-Modified: Wed, 21 Jul 2024 12:00:00 GMT

• Cache-Control: The resource can be cached for up to 3600 seconds (1 hour).
• ETag: The resource has an ETag value of "123456789".
• Last-Modified: The resource was last modified on July 21, 2024, at 12:00 PM GMT.

For a subsequent request within an hour, the browser uses the cached resource. After an hour, the browser may revalidate the resource with the server using the ETag or Last-Modified headers.