The meaning and usage of ETag(Entity Tag) in HTTP

Understanding ETag: A Caching Mechanism for Web Servers and Clients

ETag is a caching mechanism used in the communication between web servers and clients. It is a unique identifier for identifying the modification status of resources, helping to quickly identify whether resources have been modified. ETag can be used to improve the performance of web applications and reduce network traffic.

How Does ETag Work?

The operation principle of ETag is as follows:

1. When a client requests a resource for the first time, the web server sends the ETag value with the resource in the response header.
2. The client receives the ETag value and stores it in the local cache.
3. When the client requests the same resource again, it sends the If-None-Match request header with the stored ETag value.
4. The web server compares the ETag value in the request header with the ETag value of the resource stored on the current server.
5. If the two values match, the server sends the '304 Not Modified' status code to indicate that the resource has not been modified. The client then fetches the resource from the local cache.
6. If the two values do not match, the server sends the new ETag value and the modified resource. The client stores the new ETag value and updates the cache.

Advantages of ETag

The advantages of ETag are as follows:

• It can reduce the number of repeated resource downloads, thereby reducing network traffic.
• Unmodified resources can be quickly fetched from the local cache, thereby improving the loading speed of web pages.

Implementing ETag

To implement ETag, the web server must add the 'ETag' field to the response header (Response Header) and generate a unique identifier. The unique identifier can be generated based on the file's hash, modification time, etc.

HTTP/1.1 200 OK
Content-Type: text/html
ETag: "d41d8cd98f00b204e9800998ecf8427e"

In the above example, the ETag value is the file's hash value. The file's hash value is a value that uniquely identifies the file's contents, and it changes when the file's contents change. Therefore, the ETag value can be used to check whether the file has been modified.

Conclusion

In conclusion, ETag can be used to reduce the number of repeated resource downloads, thereby reducing network traffic. Unmodified resources can be quickly fetched from the local cache, thereby improving the loading speed of web pages.