Nicholas,

>if you build into an app checking a currency service for rates how do you cater for that web service disappearing or altering the way their site works.
>
>Its interesting that when building an app we try to make it bulletproof but basing part of the app on something over which you have no control (externa; web information) goes away from that.

You've raised very good questions about the concept of Web Services, and I don't think anyone yet knows the answers. In Microsoft's view, Web Services will be part of every developer's arsenal, and they (MS) will be providing many of them, such as Passport authentication. It's their overall direction to move toward a service-based business model.

Basically, you'll have to deal with this problem as you would any other situation where your program depends on outside info. You'll need some kind of "fall-back" plan for those times when the service is not available or the user's internet connection is down. In many cases that can involve caching a local copy of remote data so the program can keep functioning.

That might work somewhat OK for short periods of time if you're caching inventory quantities or something like that. In this case, either turn off currency conversion and mark the transactions as needing later correction or use a cached rate.

When you stop and think about it, your program can easily be designed to instantiate a local class with the same parameters, etc as the Web Service if the call to the service times out or returns an error condition.

As to Web Services keeping a standard interface, the companies that plan to continue providing such services will surely realize the devastating effect of changing the interface -- loss of goodwill and future business. Of course, if they're offering them free or just for fun, that might not be much of a motivating factor.

That brings me to a final point: Know the people you're depending on for info, whether through a Web Service or not. Deal with the well-established companies who can prove they know the consequences of interface changes or intermittent service.

For example, several online Credit Card verification services are available for use on your eCommerce sites, but some are better than others. It's probably not a great idea to deal with something like "ABC Real Good Verification Services," (I hope that's not a real company) operating out of a basement with 1 employee, as the core of your eCommerce strategy.

Let us all know when you get it figured out.