What are the http caching mechanisms?

http caching mechanisms include cache headers, cache strategies, cache hits, cache invalidations, cache recycling, cache consistency, cache replacement strategies, proxy caching, browser caching, compression and encoding, CDN caching, etc. Detailed introduction: 1. Cache header, which is metadata contained in HTTP requests and responses, used to control cache behavior; 2. Cache policy, the web server uses the cache policy to determine how to handle the cache request; 3. Cache hit, when browsing When the browser requests the same resource again, if the resource is already available in the cache, the browser will directly retrieve it from the cache and so on.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

HTTP caching mechanism is a technology that improves the performance of web applications by storing common or reused data. The HTTP caching mechanism can help reduce unnecessary network transmission and server load, and speed up the response speed of web applications. The following are the main contents of the HTTP caching mechanism:

1. Cache Headers: Cache headers are metadata contained in HTTP requests and responses, which are used to control cache behavior. The following are some common cache headers:

Expires: Sent by the server, it defines the time when the response expires, telling the browser that it can continue to use the cached copy before the expiration time.
• Max-Age: Sent by the client, defines the maximum time that the response can be cached, telling the server not to send a new copy, but to use the cached copy.
• If-Modified-Since: Sent by the client, tells the server to send a new copy if the resource has not been modified since the last request.
• ETag: Sent by the server, it is a version identifier of a specific resource and is used to compare whether the version of the resource has changed.

2. Cache Policies : Web servers use cache policies to determine how to handle cache requests. Here are some common caching strategies:

• Force refresh (Force-reload): Force the browser to ignore the cache and reload resources. Enter the URL in the browser address bar or use the Shift F5 key combination to force the browser to refresh the page.
• Conditional Caching: Use If-Modified-Since and ETag headers to compare whether the version of the resource has changed, and only send a new copy when the resource changes.
• Persistent Caching: Define the expiration time of the resource in the Expires header, telling the browser that it can continue to use the cached copy before the expiration time.

3. Cache Hits : When the browser requests the same resource again, if the resource is already available in the cache, the browser will directly retrieve it from the cache. Get resources from the server instead of sending a request to the server. This is called a cache hit.

4. Cache Misses : Cache misses occur when the browser cannot find the requested resource in the cache. In this case, the browser will send a request to the server to get the latest resource.

5. Cache Revalidation : When the browser needs to update resources in the cache, it can use the cache recycling mechanism to avoid unnecessary network transmission. The browser will send a request with the If-Modified-Since header, telling the server to send a new copy if the resource has not been modified since the last request. The server checks to see if the resource's version has changed and accordingly sends a new copy or confirms that the resource has not changed.

6. Cache Consistency: In a distributed system, multiple cache nodes may store copies of the same resources. In order to ensure cache consistency, some mechanism needs to be used to synchronize data between different cache nodes. Common cache consistency protocols include HTTP/1.1's ETag and If-None-Match headers, Redis protocol, etc.

7. Cache Replacement Policies: Cache space is limited, so a strategy is needed to replace old cache entries to accommodate new entries. Common cache replacement strategies include least recently used (LRU), least frequently used (LFU), etc. These policies can determine which entry should be replaced based on factors such as frequency of access, resource size, resource type, and more.

8. Proxy Caching: In a distributed system, the proxy server can serve as an intermediate agent between the client and the server to help cache and forward requests. Proxy servers can share cached data across multiple users and requests, reducing network traffic and improving performance.

9. Browser Caching : The browser is responsible for managing local cache, including storing HTTP responses, link resources in HTML documents, etc. The browser can decide how to handle cache requests based on the Cache-Control directive in the HTTP header, such as obtaining resources directly from the cache, sending requests to the server to obtain new resources, etc.

10. Compression and Encoding : The HTTP protocol supports compression and encoding of responses, such as Gzip compression, Deflate compression, etc. These techniques can reduce the size of responses, thereby reducing network transfer time and bandwidth consumption. At the same time, the browser can also decode and decompress the response for local use.

11, CDN Caching(CDN Caching): Content delivery network (CDN) can cache resources on edge servers around the world to provide services to users faster . CDN caching technology can reduce network latency and improve user experience while reducing the load on the origin server.

In summary, HTTP caching mechanisms include various cache headers, cache strategies, cache hits, cache invalidations, cache recycling, cache consistency, cache replacement strategies, proxy caches, browser caches and CDN caches, etc. technology. These technologies work together to improve web application performance, reduce network transmission and server load, improve user experience, and more. Properly configuring and using HTTP caching mechanisms can help developers optimize the performance and responsiveness of web applications.

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