In the world of the web, one of the most important concepts related to performance and optimization of the network is the concept of ETag. ETag, or "Entity Tag," is a unique identifier for the version of a resource on the network. This tag helps the server determine whether a file has changed or not and provides the updated content as needed.
ETags allow us to avoid retrieving unnecessary data from the server as long as there hasn't been a change in a resource, thereby improving content delivery. Each time a file changes, its ETag also gets updated.
According to RFC 9110, ETags can be both weak and strong. A weak version is used for minor and insignificant changes in the file that do not require a complete redelivery of the content. However, a strong version is used when significant changes occur that merit a full re-delivery of the content.
The correct use of ETags can significantly impact the efficiency of web applications. Consequently, we can avoid unnecessary data transmissions from the server to the client, and thus improve both bandwidth efficiency and response time.
Here is an example of how to work with ETags in code:
HTTP/1.1 200 OK
ETag: "688247d06a4a123"
Cache-Control: max-age=3600
If-None-Match: "688247d06a4a123"
Line one: HTTP/1.1 200 OK
This response indicates that the request was successfully processed.
Line two: ETag: "688247d06a4a123"
This shows the current version identifier of the resource, which the server communicates to the client.
Line three: Cache-Control: max-age=3600
This sets the cache duration to 3600 seconds (one hour).
Line four: If-None-Match: "688247d06a4a123"
This checks whether the ETag has changed; if it hasn't changed, the cached version is used.