Re: What can you cache? [was: Byte ranges -- formal spec proposal ]

POST/PUT still don't contain enough semantics to deal with side-effect
operations that are transactions.  In order to allow for real
transactions via HTTP, we need a transaction protocol. Clients need to
be able to determine, when they've pushed the 'stop' button, whether
or not the transaction was completed. None of the current mechanisms
implement this. Without transactions, we might want verifiers:

PUT-IF-NOT-MODIFIED-SINCE

i.e., you see a web document, edit it, and do a PUT, but ... if the
thing was edited since you got it last, then you don't just overwrite
the old one.