basic module questions

[Shaun savage]

I am starting to learn how to write nginx module.  I have a few basic
questions, these are more reference questions.

1> how do you set/create request variable?
2> how do you read a request variable?
3> can an authorization and a filter be in the same module
4> memcached checks at what phase?  I fails does it check again after
the cache is loaded.
5> srcache first checks cache then does a subrequest?  can it do this?

        check cache     -> no -> load cache, retry 
            yes

        check cache for more information -> no -> ERROR
            yes

        check $url or $request_filename with information from cache ->
DECLINE
         OK