Re: Backend writing

[prj@po.cwru.edu (Paul Jarc)]

Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai Großjohann) writes:
>| Secondly, it is not allowed for later articles to `re-use' older
>| article numbers.

I would suggest "it is normally not possible [...] without confusing
Gnus", or something like that, to avoid giving the impression that
Gnus is capable of detecting and reporting this.

>| That is, if a group has ever contained a message numbered 42, then
>| no other message may get that number, or Gnus will get mightily
>| confused.@footnote{??? Is this still true with the
>| nnchoke-request-*-mark* functions?}

You can reassign article numbers, but it takes some extra work.  You
let Gnus notify your backend about article marks by implementing
nnchoke-request-set-mark.  That function must store marks somehow,
typically in a way similar to how articles themselves are stored.  The
stored marks must identify the articles they belong to by some other
identification that will never change during an article's lifetime.
Your backend must then hand the marks back to Gnus via
nnchoke-request-update-info, mapping the persistent article
identification to article numbers.  Your backend must maintain the
mapping between numbers and the persistent identification, so that
-request-set-mark will know which marks belong to which articles.  You
can reassign numbers at each call to -request-update-info, but
depending on the details of your backend, it may be easier to assign
them only when opening the server and scanning for new articles, and
to let that numbering remain in effect until the server is closed.

>| But it seems that it might be useful to assign @emph{consecutive}
>| article numbers, for Gnus gets quite confused if there are holes in
>| the article numbering sequence.

It's also useful to start from 1.  Otherwise, you'll run out of
article numbers earlier than you would have.

Another useful addition to the description of each function would be a
list of user actions that cause that function to be called.  A backend
author will probably know when to perform certain backend actions in
terms of user actions, but figuring out the mapping between user
actions and backend function calls takes some work.


paul