Ticket #138 (closed design: fixed)
The role of Warning and Semantic Transparency in Caching
|Reported by:||firstname.lastname@example.org||Owned by:|
|Component:||p6-cache||Severity:||Active WG Document|
Part 6 (Caching) makes warning one of the pivotal features that provide semantic transparency in HTTP caching;
Requirements for performance, availability, and disconnected operation require us to be able to relax the goal of semantic transparency. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary. However, because non-transparent operation may confuse non-expert users, and might be incompatible with certain server applications (such as those for ordering merchandise), the protocol requires that transparency be relaxed
- only by an explicit protocol-level request when relaxed by client or origin server
- only with an explicit warning to the end user when relaxed by cache or client
In particular, any of several conditions (e.g., transforming content, using stale responses, failure to revalidate) invokes a requirement upon caches to generate a Warning header. User-Agents are then required to make these Warnings apparent to the end user. Additionally, there are some places where direct notifications to the end user are required, as well as relaxation of MUST-level requirements, upon the condition that the end user is warned (leading to situations where 2616 allows a strongly-stated MUST NOT to be violated; e.g., must-revalidate).
The issue is that very few cache and intermediary implementations (approaching zero) generate Warning headers, and very few user-agent implementations (again, approaching if not at zero) display warnings to users.
As a side note, Warning headers carry very little unique information that's relevant to end users; if the response is stale, this can be determined from examining expiry and Age headers; likewise if a heuristic freshness algorithm is used. While the information that the cache is disconnected or that revalidation failed is potentially interesting to automated agents, it is seldom useful to end users.
Furthermore, "Semantic Transparency" muddles together the roles of intermediaries and caches; while primarily defined in the caching section of the specification, it is more appropriate to discuss it in the context of intermediaries only. Cache-specific transparency is adequately covered by existing requirements.