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| CSP (Communicating Sequential Processes) | |
|---|---|
| Name | Communicating Sequential Processes |
| Developer | Tony Hoare |
| First publication | 1978 |
| Paradigm | Concurrency, process algebra |
| Influenced by | ACP (Algebra of Communicating Processes), Calculus of Communicating Systems |
| Influenced | Pi-calculus, Erlang (programming language), Go (programming language), Occam (programming language) |
CSP (Communicating Sequential Processes) is a formal language for describing patterns of interaction in concurrent systems. Developed as a process algebra, it provides compositional notation and mathematical semantics to model synchronization and communication among independent agents, and it underpins numerous verification techniques, programming languages, and industrial tools.
CSP was introduced by Tony Hoare to reason about concurrent processes and synchronous message-passing; it complements other formalisms such as Milner's Calculus of Communicating Systems, Robin Milner, John Backus's functional programming influence, and Leslie Lamport's work on distributed algorithms. CSP's notation and theory relate to algebraic approaches like Jan Bergstra's ACP, and to models used in David Harel's statecharts, Edsgar Dijkstra's guarded commands, and Edsger W. Dijkstra's concurrency constructs. CSP emphasizes deterministic and nondeterministic choice, synchronization on events, and compositional reasoning comparable to Alonzo Church's lambda calculus for sequential computation. The model supports verification techniques connected to the Z notation, B-Method, and temporal logics such as Zohar Manna and Amir Pnueli's linear-time temporal logic.
CSP's origins trace to Tony Hoare's work at Oxford University and interactions with researchers at Bell Labs, Oxford Computer Laboratory, and the Royal Society. Early publications appeared in venues associated with International Conference on Concurrent Systems and proceedings edited for conferences including IFIP. The 1980s saw extensions and tool development influenced by collaborations with Bill Roscoe at University of Oxford, and by parallel efforts at INRIA, MIT, Cambridge University, and University of Edinburgh. CSP's refinement concepts echo ideas from E. W. Dijkstra and from the Refinement Calculus community, while later integrations connected CSP to model checking advances at IBM Research, Bell Labs, and Microsoft Research. Industrial uptake occurred in projects at Siemens, Ericsson, NASA, and British Aerospace, and standards discussions involved bodies like ISO and IEEE.
CSP is equipped with multiple semantic models: traces, failures, and failures-divergences, developed by Tony Hoare and Bill Roscoe; these models parallel denotational semantics efforts by Dana Scott and operational semantics traditions from Gordon Plotkin. The traces model records sequences of events similar to traces in Leslie Lamport's temporal models; failures add refusal sets connecting to refusal testing by Robin Milner and observational equivalences by G. D. Plotkin. The failures-divergences model handles infinite internal activity and liveness issues akin to fairness notions analyzed by Alvy Ray Smith and Leslie Lamport. CSP relates to bisimulation equivalences studied by R. Milner and D. Park. Semantic foundations drew on set theory traditions epitomized by Georg Cantor and logic frameworks influenced by Alfred Tarski and Kurt Gödel.
Core primitives include event prefixing, external choice, internal choice, parallel composition with synchronization, hiding, and recursion, which echo constructs in Robin Milner's calculi and in languages like Occam (programming language). CSP's synchronous channels contrast with asynchronous message-passing in Erlang (programming language) and actor models developed by Carl Hewitt. Operators such as interleaving and alphabetized parallel relate to algebraic operators in Jan Bergstra's ACP and to CCS constructs from Robin Milner. Guarded commands and conditional constructs show lineage from Edsger W. Dijkstra. CSP's notions of refusal sets and stability connect to testing theories by Robin Milner and observational theories developed by Dana Scott.
Verification methods include model checking, refinement checking, and theorem proving; prominent tools include FDR (Formal Development with Refinement), developed by Bill Roscoe and collaborators, and model checkers influenced by Edmund Clarke's Model checking work at Carnegie Mellon University. Proof assistants integrating CSP include efforts with Isabelle (proof assistant), HOL (proof assistant), and links to Z notation tools developed by Anthony Hall. Industrial verification projects used CSP techniques in conjunction with SPIN and tools from NASA and Siemens, and interacted with temporal logic model checking advances by Edmund M. Clarke, E. Allen Emerson, and Joseph Sifakis. Refinement calculi connected CSP to specifications in B-Method and to verification environments at University of Oxford and INRIA.
CSP influenced concurrent programming languages and runtime systems: Occam (programming language) implemented CSP-style constructs on transputer hardware designed by Inmos, while Erlang (programming language) adopted actor concurrency for telecommunication systems at Ericsson. CSP influenced the design of Go (programming language)'s channels and goroutines at Google. Applications span protocol verification in NASA missions, railway interlocking systems at Thales Group and Siemens, and safety-critical avionics projects with British Aerospace. Implementations and tools have been applied in projects at Microsoft and IBM, and in standards work involving IEC committees for industrial control systems.
CSP sits alongside Pi-calculus by Robin Milner, Actor model by Carl Hewitt, and Petri nets by Carl Adam Petri as foundational concurrency formalisms. CSP's synchronous rendezvous contrasts with asynchronous message-passing in Erlang (programming language) and Actor model, while its algebraic operators relate to ACP (Algebra of Communicating Processes) and Calculus of Communicating Systems. CSP's impact extends to programming languages like Occam (programming language), Go (programming language), and Ada (programming language), and to verification communities led by institutions such as University of Oxford, INRIA, CMU, and MIT. Theoretical developments influenced subsequent process calculi including Spi calculus and Mobile ambients by Pierpaolo Degano, Gordon Plotkin's operational semantics work, and security protocol analyses by Ross Anderson and Bruce Schneier.