developer-delphi-horse-etag

To enable HTTP ETag caching in a Horse web server, add the ETag middleware to your pipeline. It computes a hash of the response body, attaches the ETag header, and returns 304 Not Modified for unchanged client requests.

Conditional HTTP caching uses an ETag header to let clients validate cached data. You need ETag middleware when GET endpoints return stable data and you want to avoid unnecessary data transfers by returning 304 Not Modified responses.

You should place ETag middleware in the Horse pipeline after JSON serialization. This ensures the middleware computes the hash of the final response body and accurately sets the ETag header before the response is sent.

The middleware handles If-None-Match headers by comparing the client's value with the computed response body hash. When they match, it automatically returns a 304 Not Modified response, supporting Vary headers for nuanced cache invalidation.

You should apply ETag headers only to Horse GET endpoints that return stable data. Computing hashes for highly dynamic responses adds overhead without providing meaningful caching benefits or valid 304 Not Modified responses.