A method and system for executing programs. The execution system executes a program with interactions. Each interaction has one or more commands that each have one or more attributes and a behavior. The execution system identifies interaction of the program to be performed and performs the commands of each indentified interaction. For each command of an identified interaction, the execution system identifies the input attributes based on the values of output attributes of a previously performs command of an interaction of the program. The execution then performs the command to generate values of output attributes of the command in accordance with the behavior of the command.