in reply to Design question: handling hundreds of state machines in a Web context
It appears to me, technically speaking, that you actually have a pretty well-defined situation: each Web session can be identified (through login-information maintained by cookies) to belong to a particular “customer,” hence to a particular (by some means selected...) “state machine” (“workflow”), and hence, per-session, to a particular flow point (“present state”) within that workflow ... by which the current set of POST or GET inputs can be responded-to.
There are already numerous workflow-driven architectures in CPAN, e.g. POE, which, even if they are not entirely applicable, certainly can be used as architectural examples. Yes, you probably will need to “hard-code the actions,” and there are many examples such as these of potential infrastructure. (Edit: AnyEvent? Others? Definitely ... check ’em out. CPAN’s your oyster and your cornucopia ...)
Thinking off-the-shelf about this, I think that the individual request processing sequence (as implemented e.g. by Catalyst and supported by PostGres or whatever-other state backing store), would be:
- Identify the user session in the customary way. (Catalyst handles this...)
- From the session information, determine what state-machine definition is being used and the present state. Instantiate that state-machine and set it to the present state.
- Submit the inputs to the state machine and gather its response.
- Update the new-state information into the session data store.
- Return the information provided by the state-machine to the user.
- Clean-up in anticipation of the forthcoming next request.
In this model, it doesn’t matter how many users there are, nor how many finite state-machines (FSMs) there are, as long as the state machines follow some predictable taxonomy constructed from a reasonably flexible set of underlying, Perl-implemented primitive actions. You instantiate the FSM, feed it inputs, save its new-state, return its outputs to the client, and you’re done. That is certainly a well-trod footpath... and infinitely scalable.