## Automaton definition/instantiation

Parts of a system that are nearly identical, are often modeled as nearly identical automata. Having to specify them multiple times can be burdensome. It can also hinder scalability, as changes to one of them usually need to be applied to the others as well. Consider again the producer/consumer example from the lesson that introduced channels:

    event int provide;

    automaton producer:
      disc int nr = 0;

      location:
        initial;
        edge provide!nr do nr := nr + 1;
    end

    automaton consumer1:
      disc list int nrs;

      location:
        initial;
        edge provide? do nrs := nrs + [?];
    end

    automaton consumer2:
      disc list int nrs;

      location:
        initial;
        edge provide? do nrs := nrs + [?];
    end

The producer provides products either to the first consumer or to the second consumer. The consumers are modeled using identical automata. Only the names of the `consumer1` and `consumer2` automata differ. Ideally, we would have a sort of consumer template, and use that template twice, once for each of the actual consumers. This can be achieved in CIF using an automaton definition (the template) and two automaton instantiations (the uses of the template):

    event int provide;

    automaton producer:
      disc int nr = 0;

      location:
        initial;
        edge provide!nr do nr := nr + 1;
    end

    automaton def Consumer():
      disc list int nrs;

      location:
        initial;
        edge provide? do nrs := nrs + [?];
    end

    consumer1: Consumer();
    consumer2: Consumer();

The `Consumer` automaton definition is identical to the original consumers, except that it is an automaton definition rather than an automaton. An automaton definition can be identified by the `def` keyword between the `automaton` keyword and the name of the automaton definition, as well as by the parentheses after its name. As a convention, names of automaton definitions start with an upper case letter (`Consumer` rather than `consumer`).

An automaton definition by itself is not an automaton. The instantiations of the automaton definition (`consumer1` and `consumer2`) however, are automata. Before the colon (`:`), the name of the instantiation is given. This name is also the name of the actual automaton. After the colon, the name of the automaton definition that is instantiated is given.

Using an automaton definition, the above example models the behavior of a consumer only once. Adding a third consumer is as easy as adding another automaton instantiation, which takes only one line of code (`consumer3: Consumer();`). Changing the behavior of all consumers only requires changes to the common automaton definition. Automaton definition/instantiation allows for scalability and reuse, and also improves maintainability.

Automaton definition/instantiation can be eliminated, by replacing all automaton instantiations by the automaton definitions that they instantiate, and changing the automaton definition header (`automaton def Consumer():`) by an automaton header (`automaton consumer1:`). If we do that for the example above, we obtain the original specification from the beginning of this lesson. The two specifications are functionally equivalent. Automaton instantiation `consumer1` is also often referred to as automaton `consumer1`, when there is no confusion.