A method implemented by a data processing system for processing data items of a stream of data items, including: accessing a specification that represents the executable logic, wherein a state of the specification for a particular value of the key specifies one or more portions of the executable logic that are executable in that state; receiving, over an input device or port, data items of a stream of data; for a first one of the data items of the stream, identifying a first state of the specification for a value of the key associated with that first one of the data items; processing, by the data processing system, the first one of the data items according to one or more portions of executable logic that are represented in the specification as being associated with the first state.