13. A computing system for facilitating a transaction, comprising: a first information source comprising one or more data stores; and one or more computing devices configured to: receive input initiating the transaction; generate one or more queries from the input, each of the queries including one or more variables representing unknown information required for the transaction to proceed; for each variable in each query, determine whether the unknown information represented by the variable can be obtained from the first information source; for each variable for which the unknown information cannot be obtained from the first information source, determine whether the unknown information represented by the variable can be retrieved from one or more alternate information sources; receive first responsive information corresponding to a first variable from a first alternate information source; identify alternate interpretations of the first responsive information using an ontology associated with the first information source, the ontology representing knowledge stored in the data stores of the first information source using classes and relationships among the classes; and identify a correct one of the alternate interpretations as the unknown information represented by the first variable using contextual information generated during a current interaction with the user; determine the unknown information represented by each variable other than the first variable with further responsive information from the first information source or the one or more alternate information sources; and proceed with the transaction.
13. A computing system for facilitating a transaction, comprising: a first information source comprising one or more data stores; and one or more computing devices configured to: receive input initiating the transaction; generate one or more queries from the input, each of the queries including one or more variables representing unknown information required for the transaction to proceed; for each variable in each query, determine whether the unknown information represented by the variable can be obtained from the first information source; for each variable for which the unknown information cannot be obtained from the first information source, determine whether the unknown information represented by the variable can be retrieved from one or more alternate information sources; receive first responsive information corresponding to a first variable from a first alternate information source; identify alternate interpretations of the first responsive information using an ontology associated with the first information source, the ontology representing knowledge stored in the data stores of the first information source using classes and relationships among the classes; and identify a correct one of the alternate interpretations as the unknown information represented by the first variable using contextual information generated during a current interaction with the user; determine the unknown information represented by each variable other than the first variable with further responsive information from the first information source or the one or more alternate information sources; and proceed with the transaction. 14. The computing system of claim 13 , wherein the one or more queries include a guard query and one or more fallback queries, the guard query and the fallback queries representing different mechanisms for determining the unknown information represented by the same variable, the one or more computing devices being further configured to only execute one of the one or more fallback queries as determined by execution of the guard query.