1. A method executed by a processor for message-based distributed computing, the method comprises: receiving a plurality of instructions for an application, wherein: the instructions are organized into a plurality of message responses, each message response comprising: an expression; and a set instance definition which identifies a plurality of members of the set for use by the expression, wherein: the set is a data structure having a plurality of dimensions; and each member of the set is stored in a unique location defined by dimension values unique to each member, and comprises a scalar value which does not define the member's location in the set; and the set instance definition identifies members of the set for use by the expression using dimension values; building an application from the received plurality of instructions; initiating execution of the application; creating a set in memory for the application; receiving, during execution of the application, an electronic message from a message stream, wherein the message comprises dimension values; matching the message to a message response; and executing the expression of the matched message response on the members of the set identified by the set instance definition, wherein the set instance definition identifies members using dimension values derived from the message.