1. A method of executing a user defined portion of an executable graph comprising executable modules, the method comprising: executing the executable graph in a first execution of the executable graph; providing to a user a graphical representation of the executable graph; receiving user input changing a portion of the executable graph after the first execution of the executable graph; receiving from the user a selection of a sub-portion of the executable graph, including the user changed portion of the executable graph; executing the user selected sub-portion of the graph in isolation from other portions of the executable graph in a second execution of the executable graph, such that only sub-portion of the executable graph is executed in the second execution of the executable graph, and such that the sub-portion is executed without executing other portions of the executable graph that would normally be executed when executing modules in the sub-portion of the executable graph, as a result of the user selecting only the sub-portion of the executable graph for execution.