The business activity model has multiple protocols: BusinessAgreement and BusinessAgreementWithComplete. However, unlike the AT protocol, which is driven from the coordinator down to participants, this protocol is driven from the participants upwards.

Under the BusinessAgreement protocol, a child activity is initially created in the Active state; if it finishes the work it was created to do and no more participation is required within the scope of the BA (such as when the activity operates on immutable data), the child can unilaterally send an exited message to the parent. However, if the child task finishes and wishes to continue in the BA, then it must be able to compensate for the work it has performed. In this case it sends a completed message to the parent and waits to receive the final outcome of the BA from the parent. This outcome will be either a close message - the BA has completed successfully - or a compensate message - the parent activity requires that the child task reverse its work.

The BusinessAgreementWithComplete protocol is identical to the BusinessAgreement protocol with the exception that the child cannot autonomously decide to end its participation in the business activity, even if it can be compensated. Rather, the child task relies upon the parent to inform it when the child has received all requests for it to perform work. The parent does this by sending the complete message to the child, which then acts as it does in the BusinessAgreement protocol.

The crux of the BA model, compared to the AT model, is that it allows the participation of services that cannot or will not lock resources for extended periods.

While the full ACID semantics are not maintained by a BA, consistency can be maintained through compensation, although writing correct compensating actions (and thus overall system consistency) is delegated to the developers of the services controlled by the BA. Such compensations may use backward error recovery, but typically employ forward recovery.

Coordinating Business Activities on the Web
However, the real beauty of the Web services model is that it is highly modular. Capitalizing on that modularity, consider the case shown in Figure 1, where a shopping portal uses several suppliers to deliver a richer shopping experience to the customer.

 

In this case, a BA is used since there is no close trust relationship between any of the suppliers (indeed they are probably competitors), and purchases are committed immediately as per the BA model. In the non-failure case, things are straightforward and each child BA reports back that it has completed to the coordinator via a completed message.

The failure case, however, is a little more interesting (see Figure 2). Let's assume that Supplier 2 could not source the tie that the customer wanted and its corresponding BA fails. It reports the failure back to the coordinator through a faulted message. On receiving this message, the logic driving the BA, which we assume to be a workflow script residing in the portal service, is invoked to deal with the fault. In this case, the logic uses forward error recovery to try to obtain the item from an alternative supplier.

 

If the forward error recovery works, and the alternate supplier's Web service confirms that it is able to source the desired item, then the BA proceeds normally, executing subsequent child BAs until completion. If, however, the BA cannot make forward progress and it thus has no option but to go backwards and compensate previous successfully completed activities. Note that failed activities are not compensated because their state is, by definition, unknown.

Once the compensation has taken place successfully (remember that an added complexity is that compensations can themselves fail), the system should be in a state that is semantically equivalent to the state it was in before the purchase operations were carried out. The shopping portal service knows the status of the transaction from the coordinator, and can then report back to the customer application that the order didn't complete.

Business Activities and BPEL4WS
During the execution of a business process, like our shopping portal example, data in the various systems that the process encompasses changes. Normally such data is held in mission-critical enterprise databases and queues, which have ACID transactional properties to ensure data integrity. This can lead to a situation whereby a number of valid commits to databases could have been made during the course of a process, but where the overall process might fail, leaving work partially completed. In such situations the reversal of partial work cannot rely on backward error recovery mechanisms - rollback - supported by the databases since the updates to the database will have been long since committed. Instead, we must compensate at the application level by performing the logical reverse of each activity that was executed as part of our process, from the most recently executed scope back to the earliest executed scope. This model is known as a saga, and is the default compensation model supported by BPEL4WS.

The BPEL4WS specification suggests WS-Transaction Business Activity as the protocol of choice for managing transactions that support the interactions of process instances running within different enterprise systems. A business activity is used both as the means of grouping distributed activities into a single logical unit of work and the dissemination of the outcome of that unit of work - whether all scopes completed successfully or need to be compensated.

If each of the Web services in our shopping portal example were implemented as BPEL4WS workflow scripts, the messages from the BA protocol messages from the coordinator could be consumed by those workflow scripts and used to instigate any compensating activities for those activities. The execution of compensating activities caused by the coordinator sending compensate messages to the participants returns the process as a whole to the same state logically as it was before the process executed.

Relationship to OASIS BTP
The OASIS Business Transactions Protocol (BTP) was developed by a consortium of companies, including Hewlett-Packard, Oracle, and BEA, to tackle a similar problem to WS-Transaction: business-to-business transactions in loosely coupled domains. BTP was designed with loose coupling of services in mind and integration with existing enterprise transaction systems was not a high priority. Web services were also not the only deployment environment considered by the BTP developers so the specification only defines an XML protocol message set, and leaves the binding of this message set to specific deployment domains.

BTP defines two transaction models: atoms, which guarantee atomicity of decision among participants; and cohesions, which allow relaxed atomicity such that subsets of participants can see different outcomes in a controlled manner. Both models use a two-phase completion protocol, which deliberately does not require ACID semantics: although it is similar to the 2PC protocol used by WS-Transaction Atomic Transactions, it is used purely to attain consensus and no semantics can be inferred from higher-level services that use atoms. An implementer of a BTP participant is free to use compensation techniques in the second-phase operations to guarantee atomicity if that model best suits the business.

Both atoms and cohesions also use the open-top coordination protocol, whereby both phases of the two-phase protocol must be explicitly executed by users. Because no time limit is implied between the two phases of the completion protocol, this explicit separation of the phases is intended to allow businesses to better model their business processes.

Although at least in theory WS-Transaction and BTP are intended to address the same problem domain, there are significant differences between them. BTP allows business-level negotiation to occur during many points in the protocol in its Qualifier mechanism; WS-Transaction does not have such a capability.

Summary
Over the course of these articles, we've seen both the atomic AT protocol and the non-ACID BA designed to support long-running transactions. While both the AT and BA models will be available to Web services developers directly through toolkits, it is the BA model that is supported by the BPEL4WS standard to provide distributed transaction support for business processes.