1. A method for modeling a web service operation, comprising: defining an input message pattern that describes input requirements of a web service operation, wherein the input message pattern includes a set of variables representing data elements that must be contained in a message input to the web service operation, and a graph pattern that semantically describes the data elements that must be contained in the message input to the web service operation; and defining an output message pattern that describes an output message of the web service operation, wherein the output message pattern includes a set of variables and exemplars that represent data elements contained in the output message, and a graph pattern that semantically describes the data elements contained in the output message.