In a perfect world, all organizations would agree on how internal processes should be structured, so that should they ever have to interoperate, they would already have their automation solutions in perfect alignment.
Though this vision has about a zero percent chance of ever becoming reality, the requirement for organizations to interoperate via services is becoming increasingly real and increasingly complex. This is especially true when interoperation requirements extend into the realm of collaboration, where multiple services from different organizations need to work together to achieve a common goal.
The Web Services Choreography Description Language (WS-CDL) is one of several specifications that attempts to organize information exchange between multiple organizations (or even multiple applications within organizations), with an emphasis on public collaboration (Figure 6.37). It is the specification we've chosen here to represent the concept of choreography and also the specification from which many of the terms discussed in this section have been derived.
Figure 6.37. A choreography enables collaboration between its participants.
In Plain English
After a few months in operation, our little car washing enterprise achieves some success. With our flexible and adaptive system, we have been able to efficiently wash enough cars to make some profit. Once word in the car washing circles gets around, we are contacted by a nearby car washing company.
Even though this team of car washers is located only a kilometer away, they are not considered competitors. We positioned ourselves at a gas station located at the off ramp of a highway, and they are on the other side. Our customers originate from North-bound traffic, whereas theirs come from cars heading South. As a result, we have different peak hours corresponding directly to the traffic patterns of the highway. Our volume peaks during the morning rush hours, whereas theirs peaks in the afternoon.
It is suggested to us that we could form a partnership whereby we pool our respective resources (workers) to allow each of our companies to maximize the potential of each rush hour period. This form of collaboration appeals to us, as so far we've never been able to wash as many cars as we could at peak times. When customers entering the gas station grounds see a line up to our car wash, they often change their minds and drive away.
We decide to join forces with the other team. However, this arrangement soon affects our original business process. We now have to introduce a process that imposes new conditions and constraints. At the same time, though, we want to protect our existing system because it has been successful. After discussing these issues with our new partner, we come to an agreement that results in a flexible collaboration process.
A choreography is essentially a collaboration process designed to allow organizations to interact in an environment that is not owned by any one partner.
An important characteristic of choreographies is that they are intended for public message exchanges. The goal is to establish a kind of organized collaboration between services representing different service entities, only no one entity (organization) necessarily controls the collaboration logic. Choreographies therefore provide the potential for establishing universal interoperability patterns for common inter-organization business tasks.
While the emphasis on choreography is B2B interaction, it also can be applied to enable collaboration between applications belonging to a single organization. The use of orchestration, though, is far more common for this requirement.
6.7.2. Roles and participants
Within any given choreography, a Web service assumes one of a number of predefined roles. This establishes what the service does and what the service can do within the context of a particular business task. Roles can be bound to WSDL definitions, and those related are grouped accordingly, categorized as participants (services).
6.7.3. Relationships and channels
Every action that is mapped out within a choreography can be broken down into a series of message exchanges between two services. Each potential exchange between two roles in a choreography is therefore defined individually as a relationship. Every relationship consequently consists of exactly two roles.
Now that we've defined who can talk with each other, we require a means of establishing the nature of the conversation. Channels do exactly that by defining the characteristics of the message exchange between two specific roles.
Further, to facilitate more complex exchanges involving multiple participants, channel information can actually be passed around in a message. This allows one service to send another the information required for it to be communicated with by other services. This is a significant feature of the WS-CDL specification, as it fosters dynamic discovery and increases the number of potential participants within large-scale collaborative tasks.
6.7.4. Interactions and work units
Finally, the actual logic behind a message exchange is encapsulated within an interaction. Interactions are the fundamental building blocks of choreographies because the completion of an interaction represents actual progress within a choreography. Related to interactions are work units. These impose rules and constraints that must be adhered to for an interaction to successfully complete.
6.7.5. Reusability, composability, and modularity
Each choreography can be designed in a reusable manner, allowing it to be applied to different business tasks comprised of the same fundamental actions. Further, using an import facility, a choreography can be assembled from independent modules. These modules can represent distinct sub-tasks and can be reused by numerous different parent choreographies (Figure 6.38).
Figure 6.38. A choreography composed of two smaller choreographies.
Finally, even though a choreography in effect composes a set of non-specific services to accomplish a task, choreographies themselves can be assembled into larger compositions.
6.7.6. Orchestrations and choreographies
While both represent complex message interchange patterns, there is a common distinction that separates the terms "orchestration" and "choreography." An orchestration expresses organization-specific business workflow. This means that an organization owns and controls the logic behind an orchestration, even if that logic involves interaction with external business partners. A choreography, on the other hand, is not necessarily owned by a single entity. It acts as a community interchange pattern used for collaborative purposes by services from different provider entities (Figure 6.39).
Figure 6.39. A choreography enabling collaboration between two different orchestrations.
One can view an orchestration as a business-specific application of a choreography. This view is somewhat accurate, only it is muddled by the fact that some of the functionality provided by the corresponding specifications (WS-CDL and WS-BPEL) actually overlaps. This is a consequence of these specifications being developed in isolation and submitted to separate standards organizations (W3C and OASIS, respectively).
An orchestration is based on a model where the composition logic is executed and controlled in a centralized manner. A choreography typically assumes that there is no single owner of collaboration logic. However, one area of overlap between the current orchestration and choreography extensions is the fact that orchestrations can be designed to include multi-organization participants. An orchestration can therefore effectively establish cross-enterprise activities in a similar manner as a choreography. Again, though, a primary distinction is the fact that an orchestration is generally owned and operated by a single organization.
6.7.7. Choreography and SOA
The fundamental concept of exposing business logic through autonomous services can be applied to just about any implementation scope. Two services within a single organization, each exposing a simple function, can interact via a basic MEP to complete a simple task. Two services belonging to different organizations, each exposing functionality from entire enterprise business solutions, can interact via a basic choreography to complete a more complex task. Both scenarios involve two services, and both scenarios support SOA implementations.
Choreography therefore can assist in the realization of SOA across organization boundaries (Figure 6.40). While it natively supports composability, reusability, and extensibility, choreography also can increase organizational agility and discovery. Organizations are able to join into multiple online collaborations, which can dynamically extend or even alter related business processes that integrate with the choreographies. By being able to pass around channel information, participating services can make third-party organizations aware of other organizations with which they already have had contact.
Figure 6.40. Choreography relating to other parts of SOA.
TLS owns the Sampson Steel manufacturing plant, a factory that originally produced various metal parts for automobile and airline companies. TLS uses this factory to build parts for its own railways but also continues to support the manufacture of custom parts for other clients.
A relatively significant client has surfaced over the past year, requiring specific types of steel parts for its line of products. To determine the exact design specifications of a single part, the client needs to collaborate with the manufacturing specialists that were formerly employed by Sampson Steel and that now work for TLS.
To achieve the design specification of a single part, numerous factors are taken into consideration, including:
As a result, many drafts of a specification go back and forth between the client and the TLS specialists. These documents undergo automated processing steps during each review cycle, relating to privacy, patents, chemical composition, and the processing of mathematical formulas that pertain to the actual part design.
To facilitate this process, TLS and the client agree to bridge their respective automation environments with a choreography.
The participants of this choreography govern:
The choreography achieves a cross-organization process that automates the collaboration cycle required by TLS and its client to negotiate and finalize specifications for custom manufactured steel parts.
SUMMARY OF KEY POINTS