The WS-C framework exposes an Activation Service which supports the creation of coordinators for specific coordination protocols and retrieval of associated contexts. Activation services are invoked synchronously using an RPC style exchange. So, the service WSDL defines a single port declaring a CreateCoordinationContext operation. This operation takes an input specfying the details of the transaction to be created, including the type of coordination required, timeout, and other relevant information. It returns an output containing the details of the newly-created transaction context: the transaction identifier, coordination type, and registration service URL.

<!-- Activation Service portType Declaration --> 

<wsdl:portType name="ActivationCoordinatorPortType"> 

  <wsdl:operation name="CreateCoordinationContext"> 

    <wsdl:input message="wscoor:CreateCoordinationContext"/> 

    <wsdl:output message="wscoor:CreateCoordinationContextResponse"/> 

  </wsdl:operation> 

</wsdl:portType> 

The context returned by the activation service includes the URL of a Registration Service. When a web service receieves a service request accompanied by a transaction context, it contacts the Registration Service to enroll as a participant in the transaction. The registration request includes a participant protocol defining the role the web service wishes to take in the transaction. Depending upon the coordination protocol, more than one choice of participant protocol may be available.

Like the activation service, the registration service assumes synchronous communication. Thus, the service WSDL exposes a single port declaring a Register operation. This operation takes an input specifying the details of the participant which is to be registered, including the participant protocol type. It returns a corresponding output response.

<!-- Registration Service portType Declaration --> 
<wsdl:portType name="RegistrationCoordinatorPortType"> 
  <wsdl:operation name="Register"> 
    <wsdl:input message="wscoor:Register"/> 
    <wsdl:output message="wscoor:RegisterResponse"/> 
  </wsdl:operation> 
</wsdl:portType> 

The role of terminator is generally filled by the client application. At an appropriate point, the client asks the coordinator to perform its particular coordination function with any registered participants, to drive the protocol through to its completion. After completion, the client application may be informed of an outcome for the activity. This outcome may take any form along the spectrum from simple success or failure notification, to complex structured data detailing the activity’s status.

WS-Transaction is layered upon the WS-Coordination protocol, as shown in as shown in Figure 4.3, “WS-Coordination, WS-Transaction, and WS-Business Activity”.

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Figure 4.3. WS-Coordination, WS-Transaction, and WS-Business Activity

WS-C provides a generic framework for specific coordination protocols, like WS-Transaction, used in a modular fashion. WS-C provides only context management, allowing contexts to be created and activities to be registered with those contexts. WS-Transaction leverages the context management framework provided by WS-C in two ways.

WS-Transaction distnguishes the transaction-aware web service in its role executing business-logic, from the web service acting as a participant in the transaction, communicating with and responding to its transaction coordinator. Transaction-aware web services deal with application clients using business-level protocols, while the participant handles the underlying WS-Transaction protocols, as shown in Figure 4.4, “WS-Transaction Global View”.

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Figure 4.4. WS-Transaction Global View

A transaction-aware web service encapsulates the business logic or work that needs to be conducted within the scope of a transaction. This work cannot be confirmed by the application unless the transaction also commits. Thus, control is ultimately removed from the application and given to the transaction.

The participant is the entity that, under the dictates of the transaction coordinator, controls the outcome of the work performed by the transaction-aware Web service. In Figure 4.4, “WS-Transaction Global View”, each web service is shown with one associated participant that manages the transaction protocol messages on behalf of its web service. Figure 4.5, “WS-Transaction Web Services and Participants”, however, shows a close-up view of a single web service, and a client application with their associated participants.

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Figure 4.5. WS-Transaction Web Services and Participants

The transaction-aware web service employs a back end database accessed via a JDBC driver, which sends SQL statements to the database for processing. However, those statements should only commit if the enclosing web service transaction does. For this to work, the web service must employ transaction bridging. Transaction bridging registers a participant with the coordinator for the web service transaction and creates a matching XA transaction within which it can invoke the driver to make tentative changes to the database. The web service ensures that service requests associated with a specific web service transaction are executed in the scope of the corresponding XA transaction, grouping changes common to a given transaction while isolating changes belonging to different transactions. The participant responds to prepare, commit, or rollback requests associated from the web service transaction coordinator by forwarding the same operations to the underlying XA transaction coordinator, ensuring that the local outcome in the database corresponds with the global outcome of the web service transaction as a whole.

Things are less complex for the client. Through its API, the client application registers a participant with the transaction, and uses this participant to control termination of the transaction.

It has been established that traditional transaction models are not appropriate for Web Services. No one specific protocol is likely to be sufficient, given the wide range of situations where Web service transactions are likely to be used. The WS-Transaction specification proposes two distinct models, where each supports the semantics of a particular kind of B2B interaction.

The following discussion presents the interactions between the client, web service and the transaction coordinator in great detail for expository purposes only. Most of this activity happens automatically behind the scenes. The actual APIs used to initiate and complete a transaction and to register a participant and drive it through the commit or abort process are described in Chapter 7, The XTS API.

4.2.3.1. Atomic Transactions

An atomic transaction (AT) is similar to traditional ACID transactions, and is designed to support short-duration interactions where ACID semantics are appropriate. Within the scope of an AT, web services typically employ bridging to allow them to access XA resources, such as databases and message queues, under the control of the web service transaction. When the transaction terminates, the participant propagates the outcome decision of the AT to the XA resources, and the appropriate commit or rollback actions are taken by each.

All services and associated participants are expected to provide ACID semantics, and it is expected that any use of atomic transactions occurs in environments and situations where ACID is appropriate. Usually, this environment is a trusted domain, over short durations.

Procedure 4.1. Atomic Transaction Process

1. To begin an atomic transaction, the client application first locates a WS-C Activation Coordinator web service that supports WS-Transaction.

2. The client sends a WS-C CreateCoordinationContext message to the service, specifying http://schemas.xmlsoap.org/ws/2004/10/wsat as its coordination type.

3. The client receives an appropriate WS-Transaction context from the activation service.

4. The response to the CreateCoordinationContext message, the transaction context, has its CoordinationType element set to the WS-Atomic Transaction namespace, http://schemas.xmlsoap.org/ws/2004/10/wsat. It also contains a reference to the atomic transaction coordinator endpoint, the WS-C Registration Service, where participants can be enlisted.

5. The client normally proceeds to invoke Web Services and complete the transaction, either committing all the changes made by the web services, or rolling them back. In order to be able to drive this completion activity, the client must register itself as a participant for the Completion protocol, by sending a Register message to the Registration Service whose endpoint was returned in the Coordination Context.

6. Once registered for Completion, the client application then interacts with Web Services to accomplish its business-level work. With each invocation of a business Web service, the client inserts the transaction context into a SOAP header block, such that each invocation is implicitly scoped by the transaction. The toolkits that support WS-Atomic Transaction-aware Web Services provide facilities to correlate contexts found in SOAP header blocks with back-end operations. This ensures that modifications made by the Web service are done within the scope of the same transaction as the client and subject to commit or rollback by the transaction coordinator.

7. Once all the necessary application-level work is complete, the client can terminate the transaction, with the intent of making any changes to the service state permanent. The completion participant instructs the coordinator to try to commit or roll back the transaction. When the commit or roll-back operation completes, a status is returned to the participant to indicate the outcome of the transaction.

Although this description of the completion protocol seems straightforward, it hides the fact that in order to resolve the transaction to an outcome, several other participant protocols need to be followed.

Volatile2pc

The first of these protocols is the optional Volatile2PC (2PC is an abbreviation referring to the two-phase commit). The Volatile2PC protocol is the WS-Atomic Transaction equivalent of the synchronization protocol discussed earlier. It is typically executed where a Web service needs to flush volatile (cached) state, which may be used to improve performance of an application, to a database prior to the transaction committing. Once flushed, the data is controlled by a two-phase aware participant.

When the completion participant initiates a commit operation, all Volatile2PC participants are informed that the transaction is about to complete, via the prepare message. The participants can respond with one of three messages: prepared, aborted, or readonly. A failure at this stage causes the transaction to roll back.

Durable2PC

The next protocol in the WS-Atomic Transaction is Durable2PC. The Durable2PC protocol is at the core of WS-Atomic Transaction. It brings about the necessary consensus between participants in a transaction, so the transaction can safely be terminated.

The Durable2PC protocol ensures atomicity between participants, and is based on the classic technique of two-phase commit with presumed abort.

Procedure 4.2. Durable2PC Procedure

1. During the first phase, when the coordinator sends the prepare message, a participant must make durable any state changes that occurred during the scope of the transaction, so these changes can either be rolled back or committed later. None of the original state information can be lost at this point, since the atomic transaction may still roll back. If the participant cannot prepare, it must inform the coordinator, by means of the aborted message. The transaction will ultimately roll back. If the participant is responsible for a service that did not change any of the transaction's data,, it can return the readonly message, causing it to be omitted from the second phase of the commit protocol. Otherwise, the prepared message is sent by the participant.

2. If no failures occur during the first phase, Durable2PC proceeds to the second phase, in which the coordinator sends the commit message to participants. Participants then make permanent the tentative work done by their associated services, and send a committed message to the coordinator. If any failures occur, the coordinator sends the rollback message to all participants, causing them to discard tentative work done by their associated services, and delete any state information saved to persistent storage at prepare, if they have reached that stage. Participants respond to a rollback by sending an aborted message to the coordinator.

Note

The semantics of the WS-Atomic Transaction protocol do not include the one-phase commit optimization. A full two-phase commit is always used, even where only a single participant is enlisted.

Figure 4.6, “WS-Atomic Two-Phase Participant State Transitions” shows the state transitions of a WS-Atomic Transaction and the message exchanges between coordinator and participant. Messages generated by the coordinator are represented by solid lines, while the participants' messages use dashed lines.

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Figure 4.6. WS-Atomic Two-Phase Participant State Transitions

Once the Durable2PC protocol completes, the Completion protocol that originally began the termination of the transaction can complete, and inform the client application whether the transaction was committed or rolled back. Additionally, the Volatile2PC protocol may complete.

Like the prepare phase of Volatile2PC, the final phase is optional and can be used to inform participants about the transaction's completion, so that they can release resources such as database connections.

Any registered Volatile2PC participants are invoked after the transaction terminates, and are informed about the transaction's completion state by the coordinator. Since the transaction has terminated, any failures of participants at this stage are ignored, since they have no impact on outcomes.

Figure 4.7, “” illustrates the intricate interweaving of individual protocols comprising the AT as a whole.

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Figure 4.7. 

4.2.3.2. Business Activities

Most B2B applications require transactional support in order to guarantee consistent outcome and correct execution. These applications often involve long-running computations, loosely coupled systems, and components that do not share data, location, or administration. It is difficult to incorporate atomic transactions within such architectures.

For example, an online bookshop may reserve books for an individual for a specific period of time. However, if the individual does not purchase the books within that period, they become available again for purchase by other customers. Because it is not possible to have an infinite supply of stock, some online shops may seem, from the user's perspective, to reserve items for them, while actually allow others to preempt the reservation. A user may discover, to his disappointment, that the item is no longer available.

A Business Activity (BA) is designed specifically for these kinds of long-duration interactions, where it is impossible or impractical to exclusively lock resources.

Procedure 4.3. BA Process Overview

1. Services are requested to do work.

2. Where those services have the ability to undo any work, they inform the BA, in case the BA later decides the cancel the work. If the BA suffers a failure. it can instruct the service to execute its undo behavior.

The key to BA is that how services do their work and provide compensation mechanisms is not the responsibility of the WS-BA specification. It is delegated to the service provider.

The WS-BA defines a protocol for Web Services-based applications to enable existing business processing and work-flow systems to wrap their proprietary mechanisms and interoperate across implementations and business boundaries.

Unlike the WS-AT protocol model, where participants inform the coordinator of their state only when asked, a child activity within a BA can specify its outcome to the coordinator directly, without waiting for a request. A participant may choose to exit the activity or may notify the coordinator of a failure at any point. This feature is useful when tasks fail, since the notification can be used to modify the goals and drive processing forward, without the need to wait until the end of the transaction to identify failures. A well-designed Business Activity should be proactive.

The BA protocols employ a compensation-based transaction model. When a participant in a business activity completes its work, it may choose to exit the activity. This choice does not allow any subsequent rollback. Alternatively, the participant can complete its activity, signaling to the coordinator that the work it has done can be compensated if, at some later point, another participant notifies a failure to the coordinator. In this latter case, the coordinator asks each non-exited participant to compensate for the failure, giving them the opportunity to execute whatever compensating action they consider appropriate. For instance, participant might credit a bank account which it previously debited. If all participants exit or complete without failure, the coordinator notifies each completed participant that the activity has been closed.

Underpinning all of this are three fundamental assumptions, detailed in Assumptions of WS-BA.

Assumptions of WS-BA

• All state transitions are reliably recorded, including application state and coordination metadata (the record of sent and received messages).

• All request messages are acknowledged, so that problems are detected as early as possible. This avoids executing unnecessary tasks and can also detect a problem earlier when rectifying it is simpler and less expensive.

• As with atomic transactions, a response is defined as a separate operation, not as the output of the request. Message I/O implementations typically have timeout requirements too short for BA responses. If the response is not received after a timeout, it is re-sent, repeatedly, until a response is received. The receiver discards all but one identical request received.

The BA model has two participant protocols: BusinessAgreementWithParticipantCompletion and BusinessAgreementWithCoordinatorCompletion. Unlike the AT protocols which are driven from the coordinator down to participants, this protocol takes the opposite approach.

BusinessAgreementWithParticipantCompletion

1. A participant is initially created in the Active state.

2. If it finishes its work and it is no longer needed within the scope of the BA (such as when the activity operates on immutable data), the participant can unilaterally decide to exit, sending an exited message to the coordinator. However, if the participant finishes and wishes to continue in the BA, it must be able to compensate for the work it has performed. In this case, it sends a completed message to the coordinator and waits for the coordinator to notify it about the final outcome of the BA. This outcome is either a close message, meaning the BA has completed successfully, or a compensate message indicating that the participant needs to reverse its work.

BusinessAgreementWithCoordinatorCompletion

The BusinessAgreementWithCoordinatorCompletion differs from the BusinessAgreementWithParticipantCompletion protocol in that the participant cannot autonomously decide to complete its participation in the BA, even if it can be compensated.

1. Instead, the completion stage is driven by the client which created the BA, which sends a completed message to the coordinator.

2. The coordinator sends a complete message to each participant, indicating that no further requests will be sent to the service associated with the participant.

3. The participant continues on in the same manner as in the BusinessAgreementWithParticipantCompletion protocol.

The advantage of the BA model, compared to the AT model, is that is allows the participation of services that cannot lock resources for extended periods.

While the full ACID semantics are not maintained by a BA, consistency can still be maintained through compensation. The task of writing correct compensating actions to preserve overall system consistency is the responsibility of the developers of the individual services under control of the BA. Such compensations may use backward error recovery, but forward recovery is more common.

Figure 4.8, “” shows the state transitions of a WS-BA BusinessAgreementWithParticipantCompletion participant and the message exchanges between coordinator and participant. Messages generated by the coordinator are shown with solid lines, while the participants' messages are illustrated with dashed lines.

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Figure 4.8. 

Figure 4.9, “” shows the state transitions of a WS-BA BusinessAgreementWithCoordinatorCompletion participant and the message exchanges between coordinator and participant. Messages generated by the coordinator are shown with solid lines, while the participants' messages are illustrated with dashed lines.

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Figure 4.9. 

Application messages are the requests and responses sent between parties, that constitute the work of a business process. Any such messages are considered opaque by XTS, and there is no mandatory message format, protocol binding, or encoding style. This means that you are free to use any appropriate Web Services protocol. In XTS, the transaction context is propagated within the headers of SOAP messages.

XTS ships with support for service developers building WS-Transactions-aware services on the JBoss Application Server. Interceptors are provided for automatic context handling at both client and service, which significantly simplifies development, allowing you to concentrate on writing the business logic without being sidetracked by the transactional infrastructure. The interceptors add and remove context elements to application messages, without altering the semantics of the messages themselves. Any service which understands what to do with a WS-C context can use it. Services which are not aware of WS-C, WS-Atomic Transaction and WS-Business Activity can ignore the context. XTS manages contexts without user intervention.