Browser caching mechanism and its classification


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The browser caching mechanism means that the browser stores some resources (such as page content, style sheets, scripts, images, etc.) locally when accessing a web page, so that pages and resources can be loaded faster on future visits. This helps improve the loading speed and performance of web pages and reduces the load on the server.

Browser caching is generally divided into two broad categories: strong caching and negotiated caching.


⭐ Strong caching (Cache-Control and Expires)

Strong caching means that when the browser requests a resource, it first checks the local cache. If the cached resource is still valid (not expired), the browser will directly use the cached resource without sending a request to the server. Common strong caching strategies include:

  • Cache-Control: Use the Cache-Control field in the HTTP header to specify the cache policy. Common values ​​include:

    • no-store:No content is cached.
    • no-cache: Forces the cache server to verify whether resources have expired.
    • public: The resource can be cached by any cache (including proxy servers).
    • private: The resource can only be cached by the client, and proxy servers are not allowed to cache it.
  • Expires: Use the Expires field in the HTTP header to specify the expiration time of the resource, which is an absolute point in time.


⭐ Negotiation cache (ETag and Last-Modified)

If the resource misses the strong cache, the browser will send a request to the server, but the server will decide whether to return the complete resource or tell the browser to use the cache directly based on the status of the resource. Negotiated caching strategies include:

  • ETag (entity tag): The server generates a unique identifier (usually a hash value) for each resource and includes the ETag header in the response. The browser sends this identifier to the server in subsequent requests, and the server determines whether the resource has changed by comparing the resource's identifier.

  • Last-Modified: The server includes the Last-Modified header in the response to indicate the last modification time of the resource. The browser sends the If-Modified-Since header to the server in subsequent requests, and the server determines whether the resource has changed by comparing the modification time of the resource.

The browser caching mechanism helps reduce unnecessary network requests and improves user experience and page loading speed. But when developing and maintaining websites, developers need to carefully handle caching strategies to ensure that updated content can be delivered correctly to users. Cache behavior can be controlled to match the needs of the website by setting appropriate cache headers.


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