8. A system comprising: a computer having a conversation controller for controlling a conversation between a Web service and an external entity, and a communications interface for exchanging documents with a Web service and an external entity, wherein the conversation controller comprises: (a) a plurality of defined document type descriptions, each document type description specifying a type of document within a common document structure that can be used; (b) a set of defined interactions describing respective document exchanges, each interaction specifying any expected inbound document types and any resulting outbound document types; (c) a transition structure that maps all permissible flows for a given conversation by identifying interactions from the set of defined interactions and specifying transitions between the identified interactions; (d) a set of transformations, each transformation specifying how to convert one document type in the common document structure to another document type, also in the common document structure, wherein each of at least some of the interactions has a transformation sub-element to describe a corresponding one of the transformations; (e) a control processor for exchanging documents with the Web service and with the external entity through the communications interface in accordance with the transition structure; and (f) a transformation component that maps document types using the set of transformations to compensate for mismatches between documents actually received from the external entity and expected inbound document types, wherein each of (a)-(d) is a separately defined component of said conversation controller.
8. A system comprising: a computer having a conversation controller for controlling a conversation between a Web service and an external entity, and a communications interface for exchanging documents with a Web service and an external entity, wherein the conversation controller comprises: (a) a plurality of defined document type descriptions, each document type description specifying a type of document within a common document structure that can be used; (b) a set of defined interactions describing respective document exchanges, each interaction specifying any expected inbound document types and any resulting outbound document types; (c) a transition structure that maps all permissible flows for a given conversation by identifying interactions from the set of defined interactions and specifying transitions between the identified interactions; (d) a set of transformations, each transformation specifying how to convert one document type in the common document structure to another document type, also in the common document structure, wherein each of at least some of the interactions has a transformation sub-element to describe a corresponding one of the transformations; (e) a control processor for exchanging documents with the Web service and with the external entity through the communications interface in accordance with the transition structure; and (f) a transformation component that maps document types using the set of transformations to compensate for mismatches between documents actually received from the external entity and expected inbound document types, wherein each of (a)-(d) is a separately defined component of said conversation controller. 17. A system according to claim 8 , wherein the common document structure comprises an extensible markup language (XML).