[ADP-369] HTTP Caching: ETag

Overview

The ETag header is used for resource versioning and cache validation. For detailed information about ETag, please refer to ADP-128: ETag.

Procedures

1. The client initiates the first GET request without any cache-related headers.
2. The server returns the data with a calculated ETag.
3. The client requests the same GET again with If-None-Match.
4. The server checks the If-None-Match request against the latest data's ETag.
   1. return 200 if mismatch
   2. return 304 if matched

Guidance

• MAY supply ETag to cache the response

Implementation Detail

• There are two kinds of ETag: Weak ETag and Strong ETag.
• In express (nodejs web server framework), it would generate weak ETag by default; could generate strong ETag by changing the configuration
  • weak ETags are generated using CRC32 (source) and strong ETags are generated using MD5 (source).
• In gin (golang web server framework), ETag could be generated via a middleware.

Reference

• https://datatracker.ietf.org/doc/html/rfc7234
• ADP-128: ETag