The specific, labeled implementation of a communication pattern from a template, also called a Network Microservice Instance.
A contract is an abstraction of a data exchange instance. The contract abstraction is comparable to the notion of topics in a messaging systems. All communications in the system are organized in the form of contracts. Each party to a contract- called a role - acquires rights and duties relative to the rights and duties of the other parties. If a service wants to send data, it must enter into a data-specific contract in the desired role (e.g., sender). If another service wants to receive that data, it must enter the same contract in a different role (e.g., receiver).