Entity tags (ETags) are a mechanism used by web servers to help browsers and other caches determine if a resource has changed since it was last fetched. They act as a unique identifier for a specific version of a resource, allowing for more efficient caching by reducing unnecessary data transfers.
Here’s a more detailed explanation:
How ETags Work:
- 1. Server-Generated:When a web server sends a resource (like an image or a webpage), it includes an ETag in the HTTP response header.
- 2. Client Storage:The client (browser or cache) stores the resource and the associated ETag.
- 3. Conditional Request:When the client wants to check for updates to the resource, it includes the ETag in a conditional request to the server.
- 4. Server Validation:The server compares the ETag in the request with the current ETag for the resource.
- 5. Response:
- If the ETags match, the server sends a “304 Not Modified” response, indicating that the resource has not changed. The client can use the cached version.
- If the ETags don’t match, the server sends the full resource with a new ETag. The client updates its cache with the new version.
Benefits of Using ETags:
- Reduced Bandwidth:By avoiding unnecessary full resource downloads, ETags save bandwidth and improve website performance.
- Faster Loading Times:Clients can use cached resources, reducing the need to wait for the server to send a new copy.
- Content Validation:ETags help ensure that the client’s cache contains the most up-to-date version of a resource.
- Conflict Resolution:ETags can be used to detect and resolve conflicts when multiple users are modifying the same resource.
Example:
Imagine you visit a website and download an image. The server sends an ETag, say “W/\”12345678\””, along with the image. You store the image and the ETag in your browser’s cache.
Later, you visit the same page and your browser sends a request for the image, including the ETag “W/\”12345678\””. If the server still has the same ETag for that image, it will send a “304 Not Modified” response, and you can use the cached version. If the ETag has changed, the server will send the new image with a new ETag, and your browser will update its cache.
