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(gnus)Back End Interface

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Back End Interface
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   Gnus doesn't know anything about NNTP, spools, mail or virtual
groups.  It only knows how to talk to "virtual servers".  A virtual
server is a "back end" and some "back end variables".  As examples of
the first, we have `nntp', `nnspool' and `nnmbox'.  As examples of the
latter we have `nntp-port-number' and `nnmbox-directory'.

   When Gnus asks for information from a back end--say `nntp'--on
something, it will normally include a virtual server name in the
function parameters.  (If not, the back end should use the "current"
virtual server.)  For instance, `nntp-request-list' takes a virtual
server as its only (optional) parameter.  If this virtual server hasn't
been opened, the function should fail.

   Note that a virtual server name has no relation to some physical
server name.  Take this example:

     (nntp "odd-one"
           (nntp-address "ifi.uio.no")
           (nntp-port-number 4324))

   Here the virtual server name is `odd-one' while the name of the
physical server is `ifi.uio.no'.

   The back ends should be able to switch between several virtual
servers.  The standard back ends implement this by keeping an alist of
virtual server environments that they pull down/push up when needed.

   There are two groups of interface functions: "required functions",
which must be present, and "optional functions", which Gnus will always
check for presence before attempting to call 'em.

   All these functions are expected to return data in the buffer
`nntp-server-buffer' (` *nntpd*'), which is somewhat unfortunately
named, but we'll have to live with it.  When I talk about "resulting
data", I always refer to the data in that buffer.  When I talk about
"return value", I talk about the function value returned by the
function call.  Functions that fail should return `nil' as the return
value.

   Some back ends could be said to be "server-forming" back ends, and
some might be said not to be.  The latter are back ends that generally
only operate on one group at a time, and have no concept of "server" -
they have a group, and they deliver info on that group and nothing more.

   In the examples and definitions I will refer to the imaginary back
end `nnchoke'.

Required Back End Functions

Functions that must be implemented.

Optional Back End Functions

Functions that need not be implemented.

Error Messaging

How to get messages and report errors.

Writing New Back Ends

Extending old back ends.

Hooking New Back Ends Into Gnus

What has to be done on the Gnus end.

Mail-like Back Ends

Some tips on mail back ends.