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Choreography

By Fabrizio Montesi. Created on 2023-05-17. Last updated on 2023-06-24.
See also: Introduction to Choreographies

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In computer science, a choreography is a coordination plan for processes that participate in concurrent and distributed systems [Montesi 2023]. It defines the communications that these processes are expected to perform, usually with the intention that these processes collaborate in order to achieve a joint goal.

Choreographies are typically given in a choreographic language, which feature high-level primitives for expressing communications between processes. The design of these languages often depends on the desired application. When a choreographic language is powerful enough to express executable distributed programs, it is called a choreographic programming language.

buyer.product -> seller.x;
seller.priceOf(x) -> buyer.price;
if buyer.likes(price) then
	buyer -> seller[Accept];
	seller -> shipper[PrepareShipping];
	buyer.address -> shipper.destination;
else
	buyer -> seller[Reject];
	seller -> shipper[NoShipping];

A simple choreography where a buyer requests the price of a product from a seller and then decides whether to go forward with the product's shipping by a third-party shipper, given in Recursive Choreographies. (Example adapted from [Carbone et al. 2012].)

sequenceDiagram
	participant b as buyer
	participant s as seller
	participant sh as shipper
	b->>s: product
	s->>b: price
	alt buyer.likes(price)
		b->>s: Accept
		s->>sh: PrepareShipping
		b->>sh: address
	else
		b->>s: Reject
		s->>sh: NoShipping
	end

The buyer-seller-shipper example revisited as a sequence diagram.

Data Choreographies

Choreographies in computer science are not to be confused with choreographies in other fields. When disambiguation is necessary, the choreographies in computer science can be called 'Data Choreographies' (https://youtu.be/B7MNZk1v37g).

Decentralised nature

By nature, choreographies do not require central control. They are often contrasted with the concept of orchestration, whereby a central coordinator (the orchestrator) invokes and composes the operations offered by the other processes in the system.

However, while choreographies do not need centralised control, this does not mean that all choreographies describe decentralised systems. It is indeed conceivable to write a choreography that describes an orchestrated system.

References

Carbone, M., Honda, K., Yoshida, N. [2012], 'Structured Communication-Centered Programming for Web Services', ACM Trans. Program. Lang. Syst. 34(2): 8:1-8:78. https://doi.org/10.1145/2220365.2220367

Montesi, F. [2023], 'Introduction to Choreographies', Cambridge University Press. https://doi.org/10.1017/9781108981491