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, wherein only the input and output message patterns are used to model the web service operation and the method is performed using a processor, wherein a first message input to the web service operation includes request for information and the input requirements of the web service operation are the criteria that must be established by the first message in order for the web service operation to be executed to generate the requested information and output the generated information in a first output message, and wherein every one of the variables in the output message pattern must be included in the input message pattern.