Orchestration & Choreography

I generally encounter discussions and questions from various Architects, Designers and Business Persons discussing Orchestration and Choreography. It seems to me that these two topics hotly debated and misunderstood in the context of SOA. I also want to add my voice to the noise.

I understand Orchestration and Choreography from their dictionary meanings.

Orchestration:

1. Noun: To arrange something carefully, and sometimes unfairly, so as to achieve a desired result (Cambridge Advanced Learner's Dictionary)

2. Noun: The arrangement of a musical composition for performance by an orchestra; also: orchestral treatment of a musical composition (Merriam-webster’s online dictionary)

Orchestration Examples:

1. In a concert conductor is giving instructions to players of violin, piano and other instruments to modulate pitch and volume to create music. Conductor is essentially passing instructions at the time of actual performance.
2. A road crossing at which vehicles are following traffic rules in the presence of traffic police personnel. In real time depending upon traffic flow in different directions, traffic policeman takes action and ensures smooth flow of traffic.

In both the cases there is a central body which controls the performance of participants (performers and vehicles) for successful execution of a task

Choreography:

1. Noun: the skill of combining movements into dances to be performed (Cambridge Advanced Learner's Dictionary)
2. Noun: the composition and arrangement of dances especially for ballet (Merriam-webster’s online dictionary)

Choreography Examples:

1. Sandeep Soparkar choreographed a dance sequence Britney Spears stage performance. In this scenario Sandeep has defined every step of dance well ahead of the performance so Britney and her stage co-performers are just enacting the same.
2. A road crossing at which vehicles are following traffic rules without presence of traffic light or traffic police. These rules are defined in advance. Each vehicle is just following them for smoother and safer movement.

In both cases the participants (performers and vehicles) are following some predefined rules for successful staging of drama.

In SOA context, I will carry forward the argument and define the Orchestration and Choreography on same lines.

Orchestration:

1. It is done at run time.
2. There must be some central body, which controls the flow of the message and behavior of each participating service. This central body may be logical entity or set of rules/protocol.

Participating services are not aware of each other and even they are not aware of their involvement in orchestration.

Choreography:

1. It is done at design time
2. There is no logical entity to control the behavior of participating services but a set of rules/protocol exists which controls interaction of services.
3. Participating services are aware of each other and know when to execute their operations



So, how in real life Orchestration and Choreography played out in SOA universe.

In real life SOA implementation of services are done in the form of small business processes, which are state aware. In these processes services remain stateless but not the whole process. Individual services are not aware of their involvement in any process. This is the case for Orchestration. The central hub/rules/protocol decides how different services will interact. Most of the routing is content aware and depends upon result of execution of participating services.

Choreography can be thought of integration of small business processes, which are directly made up of services. The resultant grand business process might involve workflow. So in this grand process smaller constituent business process need not to expose all of its functionality beyond boundaries of itself. Small business processes have some private functionality and some of publicly exposed. Publicly exposed functionalities are exploited to integrate them and reach at grand business process where no central hub is defined so no run time decision. All decisions have been made at design time.