In addition to the context service, each Web service participating in an activity may register an Activity Lifecycle Service (or ALS) with the Context Service, which allows that service to be informed about the lifetime of the context. As we shall see, the ALS is the key component in utilizing WS-Context as the base protocol for other higher-level protocols. During execution, when a context is required for the activity associated with the current execution environment, the Context Service calls each registered ALS and obtains additional content for the basic context from it; from this it eventually assembles the entire context document that can be propagated.

The relationship between ALS and context service, application services, and applications is shown in Figure 1.

WS-Context does not mandate how contexts are actually created, but the canonical route is via the Activity Lifecycle services, which "plug-in" to the Context Service. In this respect, the exact structure and semantics of an activity are defined by the combination of ALSs that have been associated with the activity. For example, a Context Service may have a transaction ALS and security ALS registered with it, so that when an activity is started, any context that is created will contain any necessary transaction and security information.

WS-Coordination Framework (WS-CF)
WS-CF is the middle layer in the WSCAF set of specifications and provides an extensible framework that supports a wide range of different coordination protocols (e.g., two-phase or three-phase commit).

While WS-Coordination Framework is ostensibly similar to WS-Coordination, the main differentiator is that WS-CF defines more of the coordinator's architecture than WS-Coordination (which leaves most things up to the services that use it). For example, in WS-CF you can actually inquire as to the status of a coordinator without having to know the details of the protocol (and its implementation). In many ways, WS-CF can be considered a superset of the WS-Coordination.

Figure 2 illustrates how individual Web services as well as composite applications can register as participants with a coordinator, which takes over responsibility for context management and notifying participants of the outcome of a series of related Web services executions. As the figure shows, a coordinator can register itself with another coordinator and become a participant, thereby improving interoperability.

WS-Transaction Management (WS-TXM)
WS-TXM builds on WS-CF to provide transactional coordination. Figure 3 illustrates the layering of WS-TXM protocols. WS-TXM defines a set of pluggable transaction protocols that can be used with the coordinator to negotiate a set of actions for all participants to execute based on the outcome of a series of related Web services executions. The executions are related through the use of shared context (scopes) that can be nested (parent-child relationships) and concurrent.

WS-TXM actually embodies three separate extended transaction protocols. Like WS-Transaction and BTP, WS-TXM provides models that are designed to accommodate multiple use cases, from tightlycoupled intranet-based transactions (TXACID), to Internet-scale, long-lived transactions (TX-LRA), to business process-oriented transactions (TX-BP).

ACID Transactions
This model is designed to support interoperability of existing transaction processing systems via Web services, since such systems already form the backbone of enterprise class applications. Although ACID transactions may not be suitable for all Web services, they are most definitely suitable for some, and particularly highvalue interactions such as those involved in finance. As a result, the ACID transaction model defined in WS-TXM has been designed with interoperability in mind. In the ACID model, each activity is bound to the scope of a transaction, so that the end of an activity automatically triggers the termination (commit or rollback) of the associated transaction.

Long Running Activities (LRA)
The LRA protocol is designed for those business interactions that are long in duration. Within this model, all work performed within the scope of an application should be compensatable. Therefore, an application's work is either performed successfully or undone. How individual Web services perform their work and ensure it can be undone if compensation is required is an implementation choice. The LRA model simply defines the triggers for compensation actions and the conditions under which those triggers are executed.

There is a caveat to this model though. Application services may not be compensatable (e.g., an application-level service that prints and mails checks), or the ability to compensate may be transient. The LRA model allows applications to combine services that can be compensated with those that cannot be compensated. Obviously, by mixing the two service types the user may end up with a business activity that will ultimately not be undone by the LRA model, but which may require outside (application-specific) compensation.

The LRA model defines a protocol actor called a compensator that operates on behalf of a service to undo the work it performs within the scope of an LRA. How compensation is carried out will obviously be dependent upon the service; compensation work may be carried out by other LRAs which themselves have compensators.

When a service performs work that may later have to be compensated within the scope of an LRA, it enlists a compensator participant with the LRA coordinator. The coordinator will send the compensator one of the following messages when the activity terminates:

LRAs may be used both sequentially and concurrently, where the termination of an LRA signals the start of some other unit of work within an application. However, LRAs are units of compensatable work and an application may have as many units of work operating simultaneously as it needs to accomplish its tasks. Furthermore, the outcome of work within LRAs may determine how other LRAs are terminated.

An application can be structured so that LRAs are used to assemble units of compensatable work and then held in the active state while the application performs other work in the scope of different (concurrent or sequential) LRAs. Only when the right subset of work (LRAs) is arrived at by the application will that subset be confirmed; all other LRAs will be told to cancel (complete in a failure state).

Business Process (BP)
The BP protocol is significantly different from any of the other transaction models we have seen to date (and there is no directly comparable model in either WSTransaction or BTP). This model is specifically aimed at tying heterogeneous transaction domains together into a single business- to-business transaction. For example, with the BP model it's possible to have a long-running business transaction span messaging, workflow, and traditional ACID transactions, allowing enterprises to leverage their existing IT investment.

In the business process transaction model, all parties involved in a business process reside within business domains, which may themselves use business processes to perform work. Business process transactions are responsible for managing interactions between these domains. A business process is split into business tasks and each task executes within a specific business domain. A business domain may itself be subdivided into other business domains recursively.

Each domain may represent a different transaction model if such a federation of models is more appropriate to the activity. Each business task (which may be modelled as a scope) may provide implementation- specific countereffects in the event the enclosing scope must cancel. Furthermore, the controlling application may periodically request that all business domains checkpoint their state so that they can either be consistently rolled back to that checkpoint by the application or restarted from the checkpoint in the event of a failure.

Figure 4 shows an online travel agent interacting with its suppliers, each of which resides in its own business domain. The work necessary to obtain each component is modelled as a separate task. In this example, the Flight Reservation task is actually composed of two subtasks - one gets the flight and the other gets the necessary travel insurance.

In this example, the user may interact synchronously with the travel agent to build up the required details of the holiday. Or, the user may submit an order (possibly with a list of alternate requirements, such as destinations, dates, etc.) to the agent, who will call back when it has been filled. Likewise, the travel agent then submits orders to each supplier, requiring them to call back when each component is available (or is known to be unavailable).

Business domains are instructed to perform work within the scope of a global business process. The business process has an overall manager that may be informed by individual tasks when they have completed their work or it may periodically communicate with each task to determine its current status. In addition, each task may make checkpoints of its progress so if a failure occurs, it may be restarted from that point rather than having to start from the beginning. A business process can either terminate in a confirmed (successful) manner, in which case all of the work requested will have been performed, or it will terminate in a cancelled (unsuccessful) manner, in which case all of the work will be undone.

If it cannot be undone, then this fact must be logged.

Summary
From a distance, WS-CAF may be misinterpreted simply as the industry's third attempt at designing a transaction management solution for Web services. However, while one aspect of WS-CAF does address the kind of extended transaction models that are crucial for Web services reliability, there is actually much more to WS-CAF than just transactions. WS-CAF also provides generic context-management and service-coordination frameworks that can form the basis of composite applications, processes, and workflows. These features are exposed to Web services-based applications and can be tailored to build protocols that are specific to particular applications domains.

References