ETag Header
The ETag header is a mechanism used in HTTP to efficiently cache responses and avoid unnecessary data transfers. It works by generating a unique identifier, or tag, for a resource, and then sending this tag along with the response to the client.
Here’s how it works:
- Resource Modification: When a resource on the server is modified, the server generates a new ETag for that resource.
- ETag in Response: The server includes the ETag in the HTTP response header for the resource.
- Client Caching: The client stores the ETag along with the cached resource.
- Subsequent Requests: When the client makes a subsequent request for the same resource, it includes the previously stored ETag in the If-None-Match header of the request.
- Server Comparison: The server compares the ETag in the If-None-Match header with the ETag of the current resource.
- Conditional Response:
- If the ETags match, the server responds with a 304 Not Modified status code, indicating that the resource has not been modified since the last request. The client can use its cached copy without downloading the entire resource again.
- If the ETags do not match, the server sends the updated resource along with a new ETag. The client caches the new resource and ETag.
Benefits of ETag:
- Reduced network traffic: By avoiding unnecessary data transfers, ETag can significantly reduce network bandwidth usage.
- Improved performance: By leveraging cached resources, ETag can improve the perceived performance of web applications.
- Enhanced efficiency: ETag helps to optimize resource management and reduce server load.
When to use:
ETags are generally preferred over Last-Modified headers because they provide a more accurate and reliable way to determine if a resource has changed. They can handle scenarios where the content of a resource has changed but the modification timestamp remains the same.
Key points to remember:
- ETag is a mechanism for efficient caching in HTTP.
- It generates a unique identifier for a resource.
- It is used to determine if a resource has been modified since the last request.
- It helps to reduce network traffic and improve performance.