For the last few weeks I've been trying to put together a framework for creating XMPP components in Erlang. While the code I have now is not by any means covering all possible features the XMPP developer might need, I think it's a good starting point. I'm going to go over the interface and features of gen_component behavior that represents framework's API. Your input is very much appreciated.
gen_component implementation is backed by gen_fsm, so just like for gen_fsm, there is a certain set of callback functions that must be provided by a callback module.
XMPP (external) component could be seen as intermediary between XMPP server that routes messages, and some backend service that needs to be exposed to XMPP server's clients. The service could be anything: a database, a game, a stock ticker, Twitter feeds etc. Hence there are functions handling XMPP server conversations, such as handle_iq/4, handle_presence/4 and handle_message/4, and the functions handling interactions with backend service, such as feed_component/2 and handle_feed/2. XMPP service discovery support for the component is backed by functions disco_info/2 and disco_items/2. There are also functions that deal with life cycle of gen_component, such as start_component/6, stop_component/1 and their respective callbacks init/2 and terminate/1. Interface includs some helper functions such as send_packet/2 and print_packet/1.
As a proof of concept I built a test_component module that implements gen_component callbacks. test_component represents mocked weather alert service Jabber clients could subscribe to. The client browses available weather alerts and registers with ones he wishes to get updated with. That's how it looks like in action:
exmpp library has been used extensively in gen_component, and I found it pretty stable, although there were some gotchas that I attribute to my lack of experience. I'm planning to do a detailed code walkthrough with comments on exmpp usage.
Source code for gen_component is here.