Contract-oriented programming with timed session types

Contract-oriented programming is a software engineering paradigm which proposes the use of behavioural contracts to discipline the interaction among software components. In a distributed setting, the various components of an application may be developed and run by untrustworthy parties, which could opportunistically diverge from the expected behaviour when they find it convenient. The use of contracts in this setting is essential: by binding the behaviour of each component to a contract, and by sanctioning contract violations, components are incentivized to behave in a correct and cooperative manner. This chapter is a step-by-step tutorial on programming contract-oriented distributed applications. The glue between components is a middleware which establishes sessions between services with compliant contracts, and monitors sessions to detect and punish violations. Contracts are formalised as timed session types, which describe timed communication protocols between two components at the endpoints of a session. We illustrate some basic primitives of contract-oriented programming: advertising contracts, performing contractual actions, and dealing with violations. We then show how to exploit these primitives to develop some small distributed applications.