1. A method for delivering content, the method comprising: receiving at least one specification for a transform; receiving a plurality of input messages associated with input streams from one or more publishers; applying, by a processor, the transform to the plurality of input messages to form an output message, wherein the output message depends on at least two different input messages from the plurality of input messages, and wherein the output message is based on a derived view that is defined by relational expressions for the at least two different input messages; wherein the output message updates a subscribed state requested by a subscriber; wherein the subscribed state is defined using a declarative specification; wherein the declarative specification includes a relational algebraic specification; and delivering the output message to a subscriber.