The ETag (Entity Tag) HTTP header is a mechanism that allows web servers and clients to efficiently manage the cache and conditional requests of resources. This article will explore the purpose and usage of the ETag header, along with request and response examples to help illustrate its functionality.
Purpose of the ETag Header
The primary purpose of the ETag header is to provide a unique identifier for a specific version of a resource. This identifier, known as the entity tag, is generated by the server and sent to the client along with the resource. The client can then use this entity tag in subsequent requests to determine whether the resource has changed since it was last fetched.
By using the ETag header, clients can avoid downloading a resource that hasn’t changed, thus saving bandwidth and improving performance. Additionally, the ETag header enables optimistic concurrency control, which helps prevent conflicts when multiple clients are attempting to modify a resource simultaneously.
ETag Generation
The server generates the entity tag based on the content of the resource. Common generation methods include:
- Strong ETag: A strong ETag is generated based on the exact content of the resource. This means that even a single byte change in the resource will result in a different ETag. Strong ETags are ideal for ensuring the integrity of the resource and are typically used for static resources, such as images and stylesheets.
- Weak ETag: A weak ETag is generated based on a less precise representation of the resource, such as the modification timestamp. Weak ETags are less strict in validating the resource’s integrity and are often used for dynamic resources, such as HTML pages generated by a server-side script.
The server indicates the type of ETag by prefixing the entity tag with W/ for weak ETags. If no prefix is present, the ETag is considered strong.
ETag in Action
Server Response with ETag
When a server sends a resource to the client, it includes the ETag header in the response. Here’s an example of an HTTP response containing an ETag header:
HTTP/1.1 200 OK
Content-Type: image/jpeg
Content-Length: 1024
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"In this example, the server sends an image resource with a strong ETag.
Client Request with If-None-Match
When a client wants to check if a resource has changed since it was last fetched, it includes the If-None-Match header in the request, along with the previously received ETag value. Here’s an example of an HTTP request containing the If-None-Match header:
GET /image.jpg HTTP/1.1
Host: example.com
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"Server Response with 304 Not Modified
If the server determines that the resource has not changed, it responds with a 304 Not Modified status code and omits the resource content. This response informs the client that it can use its cached version of the resource. Here’s an example of an HTTP response with a 304 Not Modified status:
HTTP/1.1 304 Not Modified
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"Server Response with Updated Resource
If the resource has changed, the server responds with a 200 OK status code and includes the updated resource along with a new ETag. The client can then update its cache with the new resource and ETag.
HTTP/1.1 200 OK
Content-Type: image/jpeg
Content-Length: 1024
ETag: "a7c07d9121d3b8c20333a352f3b045e9b5e271f7"Summary
The ETag HTTP header plays a crucial role in cache management and conditional requests for web servers and clients. By utilizing the ETag header, clients can efficiently determine whether a resource has changed since it was last fetched, saving bandwidth and improving performance. Furthermore, the ETag header enables optimistic concurrency control, preventing conflicts when multiple clients attempt to modify a resource simultaneously.
