Preface
By relieving the brain of all unnecessary work, a good notation sets it free to concentrate
on more advanced problems.
Alfred North Whitehead
This book provides an introduction to Communicating Sequential Processes (CSP)
and its use as a formal method for concurrency. The CSP approach has been widely used
in the specification, analysis and verification of concurrent and real-time systems, and for
understanding the particular issues that can arise when concurrency is present. It provides a
good notation which enables specifications and designs to be clearly expressed and understood,
together with a supporting theory which allows them to be analyzed and shown to be correct.
Concurrent systems are complicated: they consist of many components which may
execute in parallel, and the complexity arises from the combinations of ways in which their
parts can interact. The design of such systems requires ways of keeping these interactions
under control. Concurrency by its very nature introduces phenomena not present in sequential
systems, such as deadlock and livelock. Deadlock can arise when a number of components are
each awaiting an interaction from some other component before they can themselves continue.
Livelock arises when components descend into an endless sequence of interaction among
themselves, excluding any other components and the outside world. These properties arise
not from individual components but from the way they are combined. Nondeterminism can
also arise naturally in parallel compositions, for example when race conditions arise. The
presence of time adds another dimension to the complexity. A theory of concurrency such as
CSP provides a way of understanding and thereby controlling such phenomena.
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