An activity diagram to describe the resolution of HTTP response status codes, given various headers, implemented via semantical callbacks.
And it goes on Twitter as #httpdd - HTTP Decision Diagram.
This is part of a bigger effort: for-GET HTTP.
The diagram follows the indications in RFC7230 RFC7231 RFC7232 RFC7233 RFC7234 RFC7235, and fills in the void where necessary. Under no circumstances does this diagram override the HTTP specifications. If it does, please file an issue as soon as possible.
cache-retrieve and cache-store
Darrel Miller started something similar to HTTP decision diagram, in terms of deciding whether to cache (store) or not a HTTP response, and whether to respond to a HTTP request with a cached HTTP response or not.