[Animats]

APIs usually create a master/slave relationship. The strong party gets to define the API, and the weak party has to adapt to it. There are few fully symmetrical APIs.

Usually the seller defines the API, but where the buyer is more powerful, the buyer sometimes does. See, for example, General Motors' purchasing system for suppliers. WalMart has something similar. There, the seller must adapt to the buyer's system.

There are a few systems where there are interchange standards good enough to allow new parties to communicate as peers without a new implementation. ARINC does this for the aviation industry.

We have yet to develop systems where both sides enter into communication and figure out how to talk. This is needed. XML schemas were supposed to help with that, but nobody used them that way.