Mechanisms for enforcing a message exchange pattern. When two computing systems communicate in order to accomplish a particular task, they engage in a particular message exchange pattern. Given certain progress through the message exchange pattern, the message exchange pattern may restrict which computing system may send what kind of messages. Each computing system tracks progress through the message exchange pattern by, for example, using a state transition tree in which each node represents a state of the message exchange pattern, and in which transmission or receipt of certain messages may cause state transitions. A computing system then only transmits messages if appropriate given the current tracked progress through the message exchange pattern.