ETag
An ETag (Entity Tag) is an HTTP response header used for web cache validation. It is a unique identifier assigned by a web server to a specific version of a resource. ETags help improve efficiency by allowing browsers to make conditional requests, reducing unnecessary data transfer.
Key Points
Purpose: ETags enable caching systems to determine if the content of a resource has changed.
Format: An ETag is typically a string enclosed in double quotes, e.g.,
ETag: "686897696a7c876b7e".Types:
- Strong ETags: Change if there's any difference in the response body.
- Weak ETags: Change only if there are significant semantic changes.
Usage:
- Server sends ETag with response.
- Client includes ETag in subsequent requests using
If-None-Matchheader. - Server compares ETags to determine if resource has changed.
Benefits:
- Reduces bandwidth usage.
- Improves server performance.
- Ensures data consistency.
Considerations:
- Generation method can impact performance.
- Should be used in conjunction with other caching mechanisms.
Guidance
- SHOULD use ETags for all cacheable resources.
- MUST generate unique ETags for each version of a resource.
- SHOULD use strong ETags unless performance considerations necessitate weak ETags.
- MUST include the ETag in the response headers for GET and HEAD requests.
- SHOULD support conditional requests using If-Match and If-None-Match headers.
- MUST return a 304 Not Modified response if the ETag matches in a conditional GET request.
- SHOULD use consistent ETag generation methods across distributed systems.
Example
http
HTTP/1.1 200 OK
Content-Type: text/html
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
[Resource content]Use the Shared Headers defined in ADP-767
yaml
headers:
ETag:
- "$ref": "https://vivotek-it.github.io/api-design-principles/definitions.yaml#/parameters/Etag"Related ADPs
- ADP-121: Common HTTP Headers
- ADP-361: HTTP Caching
- ADP-369: HTTP Caching: ETag
- ADP-352: Conditional Requests