Quickbooks SDK

 Let me just say that the Quickbooks SDK when used as the 'web service' method or the Quickbooks Web Connector (qbwc), is like hitting yourself in the face with a dead trout.  A large one. 

How it works (in breif) is they have a program on the client's machine that contacts your web service and asks it what work needs to be done, and then you tell a single bit of information you need.  Then it calls in again returning that bit of information and if you return a int < 100 then it will call in again asking for a bit of work that needs to be done. 

The issue with this is that it requires you to build what ends up being a complex finite state machine (FSM).  I have a simple program that integrates items, customers, and invoices with Quickbooks and I have 14 states so far. 

I also can't think of a good way to do the without requiring the web service to use sessions, another risk I wish to avoid.

Anyway, on the good side, this being done this way (the client making the calls to the webservice) means that you don't have to worry about NAT issues or creating SSL connections (SSL is required) or transport security issues, and that is good.  Just requires a mind-set change.  (Last FSM I worked on was a robots project that I really enjoyed).