Join Specifications

By default, a join lets execution pass on its outward flow when all its input flows have arrived at the join. (Or in more formal speak, it emits a token on its output flow when a token has arrived on each input flow.) In some cases, particularly when you have a flow with multiple tokens, it's useful to have a more involved rule.

A join specification is a Boolean expression attached to a join. Each time a token arrives at the join, the join specification is evaluated and if true, an output token is emitted. So in Figure 11.12, whenever I select a drink or insert a coin, the machine evaluates the join specification. The machine slakes my thirst only if I've put in enough money. If, as in this case, you want to indicate that you have received a token on each input flow, you label the flows and include them in the join specification.