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Background for using the XTS Module of JBoss Transactions to provide Web Services Transactions
Abstract
The XTS Background Guide contains background information, required before you can use JBoss Transaction Service to develop service-based applications, that use transaction technology to manage business processes.
- Preface
- 1. About This Guide
- 2. Introduction
- 3. Transactions Overview
- 4. Overview of Protocols Used by XTS
- A. Revision History
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Publican is a DocBook publishing system.
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package org.jboss.book.jca.ex1;
import javax.naming.InitialContext;
public class ExClient
{
public static void main(String args[])
throws Exception
{
InitialContext iniCtx = new InitialContext();
Object ref = iniCtx.lookup("EchoBean");
EchoHome home = (EchoHome) ref;
Echo echo = home.create();
System.out.println("Created Echo");
System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
}
}
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Warnings should not be ignored. Ignoring warnings will most likely cause data loss.
The XTS Background Guide explains how to add resilience to distributed business processes based on web services, making them reliable in the event of system or network failures. It covers the background theory around Transactions in general and alsot the WS-C, WS-AT and WS-BA Web Services standards.
The JBoss Application Server implements Web Services Transactions standards using XTS (XML Transaction Service). XTS supports development and deployment of transaction-aware web services. It also enables web service clients to create and manage web service transactions from which transactional web services can be invoked. XTS ensures that the client and web services achieve consistent outcomes even if the systems on which they are running crash or temporarily lose network connectivity.
XTS is compliant with the WS-Coordination, WS-Atomic Transaction, and WS-Business Activity specifications. The implementation supports web services and clients which are based on the JaxWS standard. XTS is itself implemented using services based on JaxWS. While this guide discusses many Web Services standards like SOAP and WSDL, it does not attempt to address all of their fundamental constructs. However, basic concepts are provided where necessary.
This guide is most relevant for application developers and Web service developers who are interested in building applications and Web services that are transaction-aware. It is also useful for system analysts and project managers who are unfamiliar with transactions as they pertain to Web services.
This particular guide just provides background information. You should see the 'XTS Development and Administration Guide' for the details on how to use XTS.
JBoss Transaction Service uses the Java programming language and this manual assumes that you are familiar with programming in Java. Additional helpful skills are outlined in Prerequisite Skills for XTS Developers.
Prerequisite Skills for XTS Developers
A Working knowledge of Web Services, including XML, SOAP, and WSDL
A general understanding of transactions
A general understanding of WS-Coordination, WS-Atomic Transaction and WS-Business Activity protocols
This guide presents overview information for all of the above. However, to aid in understanding the Web Services component of JBoss Transaction Service, the WS-C[1], WS-Atomic Transaction[2], and WS-Business Activity[3] specifications are discussed in great detail.
The XML Transaction Service (XTS) component of JBoss Transaction Service supports the coordination of private and public Web Services in a business transaction. Therefore, to understand XTS, you must be familiar with Web Services, and also understand something about transactions. This chapter introduces XTS and provides a brief overview of the technologies that form the Web Services standard. Additionally, this chapter explores some of the fundamentals of transactioning technology and how it can be applied to Web Services. Much of the content presented in this chapter is detailed throughout this guide. However, only overview information about Web Services is provided. If you are new to creating Web services, please see consult your Web Services platform documentation.
JBoss Transaction Service provides the XTS component as a transaction solution for Web Services. Using XTS, business partners can coordinate complex business transactions in a controlled and reliable manner. The XTS API supports a transactional coordination model based on the WS-Coordination, WS-Atomic Transaction, and WS-Business Activity specifications.
Protocols Included in XTS
WS-Coordination (WS-C) is a generic coordination framework developed by IBM, Microsoft and BEA.
WS-Atomic Transaction (WS-AT) and WS-Business Activity (WS-BA) together comprise the WS-Transaction (WS-T) transaction protocols that utilize this framework.
JBoss Transaction Service implements versions 1.1, and 1.2 of these three specifications. Version specifications are available from http://www.oasis-open.org/specs/.
Note
The 1.1, and 1.2 specifications only differ in a small number of details. The rest of this document employs version 1.1 of these specifications when providing explanations and example code. On the few occasions where the modifications required to adapt these to the 1.1 specifications are not obvious, an explanatory note is provided.
Web Services are modular, reusable software components that are created by exposing business functionality through a Web service interface. Web Services communicate directly with other Web Services using standards-based technologies such as SOAP and HTTP. These standards-based communication technologies enable customers, suppliers, and trading partners to access Web Services, independent of hardware operating system, or programming environment. The result is a vastly improved collaboration environment as compared to today's EDI and business-to-business (B2B) solutions, an environment where businesses can expose their current and future business applications as Web Services that can be easily discovered and accessed by external partners.
Web Services, by themselves, are not fault-tolerant. In fact, some of the reasons that the Web Services model is an attractive development solution are also the same reasons that service-based applications may have drawbacks.
Properties of Web Services
Application components that are exposed as Web Services may be owned by third parties, which provides benefits in terms of cost of maintenance, but drawbacks in terms of having exclusive control over their behavior.
Web Services are usually remotely located, increasing risk of failure due to increased network travel for invocations.
Applications that have high dependability requirements need a method of minimizing the effects of errors that may occur when an application consumes Web Services. One method of safeguarding against such failures is to interact with an application’s Web Services within the context of a transaction. A transaction is a unit of work which is completed entirely, or in the case of failures is reversed to some agreed consistent state. The goal, in the event of a failure, is normally to appear as if the work had never occurred in the first place. With XTS, transactions can span multiple Web Services, meaning that work performed across multiple enterprises can be managed with transactional support.
XTS allows you to create transactions that drive complex business processes, spanning multiple Web Services. Current Web Services standards do not address the requirements for a high-level coordination of services. This is because in today’s Web Services applications, which use single request/receive interactions, coordination is typically not a problem. However, for applications that engage multiple services among multiple business partners, coordinating and controlling the resulting interactions is essential. This becomes even more apparent when you realize that you generally have little in the way of formal guarantees when interacting with third-party Web Services.
XTS provides the infrastructure for coordinating services during a business process. By organizing processes as transactions, business partners can collaborate on complex business interactions in a reliable manner, insuring the integrity of their data - usually represented by multiple changes to a database – but without the usual overheads and drawbacks of directly exposing traditional transaction-processing engines directly onto the web. An Evening On the Town demonstrates how an application may manage service-based processes as transactions:
An Evening On the Town
The application in question allows a user to plan a social evening. This application is responsible for reserving a table at a restaurant, and reserving tickets to a show. Both activities are paid for using a credit card. In this example, each service represents exposed Web Services provided by different service providers. XTS is used to envelop the interactions between the theater and restaurant services into a single (potentially) long-running business transaction. The business transaction must insure that seats are reserved both at the restaurant and the theater. If one event fails the user has the ability to decline both events, thus returning both services back to their original state. If both events are successful, the user’s credit card is charged and both seats are booked. As you may expect, the interaction between the services must be controlled in a reliable manner over a period of time. In addition, management must span several third-party services that are remotely deployed.
Without the backing of a transaction, an undesirable outcome may occur. For example, the user credit card may be charged, even if one or both of the bookings fail.
An Evening On the Town describes the situations where XTS excels at supporting business processes across multiple enterprises. This example is further refined throughout this guide, and appears as a standard demonstrator (including source code) with the XTS distribution.
The WS-Coordination, WS-Atomic Transaction, and WS-Business Activity protocols are based on one-way interactions of entities rather than traditional synchronous request/response RPC-style interactions. One group of entities, called transaction participants, invoke operations on other entities, such as the transaction coordinator, in order to return responses to requests. The programming model is based on peer-to-peer relationships, with the result that all services, whether they are participants, coordinators or clients, must have an active component that allows them to receive unsolicited messages.
In XTS, the active component is achieved through deployment of JaxWS endpoints. Each XTS endpoint that is reachable through SOAP/XML is published via JaxWS, without developer intevention. The only requirement is that transactional client applications and transactional web services must reside within a domain capable of hosting JaxWS endpoints, such as an application server. JBoss Application Server can provide this functionality.
SOAP has emerged as the de facto message format for XML-based communication in the Web Services arena. It is a lightweight protocol that allows the user to define the content of a message and to provide hints as to how recipients should process that message.
Web Services Description Language (WSDL) is an XML-based language used to define Web service interfaces. An application that consumes a Web service parses the service’s WSDL document to discover the location of the service, the operations that the service supports, the protocol bindings the service supports (SOAP, HTTP, etc), and how to access them. For each operation, WSDL describes the format that the client must follow.
Note
This chapter deals with the theory of transactional Web Services. If you are familiar with these principles, consider this chapter a reference.
Transactions have emerged as the dominant paradigm for coordinating interactions between parties in a distributed system, and in particular to manage applications that require concurrent access to shared data. Much of the JBoss Transaction Service Web Service API is based on contemporary transaction APIs whose familiarity will enhance developer productivity and lessen the learning curve. While the following section provides the essential information that you should know before starting to use XTS for building transactional Web Services, it should not be treated as a definitive reference to all transactional technology.
A transaction is a unit of work that encapsulates multiple database actions such that that either all the encapsulated actions fail or all succeed.
Transactions ensure data integrity when an application interacts with multiple datasources.
The main components involved in using and defining transactional Web Services using XTS are illustrated in Figure 3.1, “Components Involved in an XTS Transaction”.
Every transaction is associated with a coordinator, which is responsible
for governing the outcome of the transaction. When a client begins a Web
Service transaction it posts a create request to
a coordination service, which creates the coordinator and returns its
details to the client. This service may be located in its own container or
may be colocated with the application client or with one of the
transactional web services for improved performance. The coordination
service is typically responsible for managing many transactions in
parallel, so each coordinator is identified by a unique transaction
identifier.
The coordinator is responsible for ensuring that the web services invoked by the client arrive at a consistent outcome. When the client asks the coordinator to complete the transaction, the coordinator ensures that each web service is ready to confirm any provisional changes it has made within the scope of the transaction. It then asks them all to make their changes permanent. If any of the web services indicates a problem at the confirmation stage, the coordinator ensures that all web services reject their provisional changes, reverting to the state before the transaction started. The coordinator also reverts all changes if the client asks it to cancel the transaction.
The negotiation between the coordinator and the web services is organized to ensure that all services will make their changes permanent, or all of them will revert to the previous state, even if the coordinator or one of the web services crashes part of the way through the transaction."
In order for a transaction to span a number of services, certain information has to be shared between those services, to propagate information about the transaction. This information is known as the Context. The coordination service hands a context back to the application client when it begins a transaction. This context is passed as an extra, hidden parameter whenever the client invokes a transactional web service. The XTS implementation saves and propagates this context automatically with only minimal involvement required on the part of the client. However, it is still helpful to understand what information is captured in a context. This information is listed in Contents of a Context.
Contents of a Context
- Transaction Identifier
Guarantees global uniqueness for an individual transaction.
- Transaction Coordinator Location
The endpoint address participants contact to enroll.
The coordinator cannot know the details of how every transactional service is implemented. In fact this knowledge is not even necessary for it to negotiate a transactional outcome. It treats each service taking part in a transaction as a participant and communicates with it according to some predefined participant coordination models appropriate to the type of transaction. When a web service receives its first service request in some given transaction, it enrolls with the coordinator as a participant, specifying the participant model it wishes to follow. The context contains a URL for the endpoint of the coordination service which handles enrollment requests. So, the term participant merely refers a transactional service enrolled in a specific transaction using a specific participant model.
Traditionally, transaction processing systems support ACID properties. ACID is an acronym for Atomic, Consistent, Isolated, and Durable. A unit of work has traditionally been considered transactional only if the ACID properties are maintained, as describe in ACID Properties.
ACID Properties
- Atomicity
The transaction executes completely, or not at all.
- Consistency
The effects of the transaction preserve the internal consistency of an underlying data structure.
- Isolated
The transaction runs as if it were running alone, with no other transactions running, and is not visible to other transactions.
- Durable
The transaction's results are not lost in the event of a failure.
The classical two-phase commit approach is the bedrock of JBoss Transaction Service, and more generally of Web Services transactions. Two-phase commit provides coordination of parties that are involved in a transaction. The general flow of a two-phase commit transaction is described in Figure 3.3, “Two-Phase Commit Overview”.
Note
During two-phase commit transactions, coordinators and resources keep track of activity in non-volatile data stores so that they can recover in the case of a failure.
Besides the two-phase commit protocol, traditional transaction processing systems employ an additional protocol, often referred to as the synchronization protocol. With the original ACID properties, Durability is important when state changes need to be available despite failures. Applications interact with a persistence store of some kind, such as a database, and this interaction can impose a significant overhead, because disk access is much slower to access than main computer memory.
One solution to the problem disk access time is to cache the state in main memory and only operate on the cache for the duration of a transaction. Unfortunately, this solution needs a way to flush the state back to the persistent store before the transaction terminates, or risk losing the full ACID properties. This is what the synchronization protocol does, with Synchronization Participants.
Synchronizations are informed that a transaction is about to commit. At that point, they can flush cached state, which might be used to improve performance of an application, to a durable representation prior to the transaction committing. The synchronizations are then informed about when the transaction completes and its completion state.
Procedure 3.1. The "Four Phase Protocol" Created By Synchronizations
Synchronizations essentially turn the two-phase commit protocol into a four-phase protocol:
Step 1
Before the transaction starts the two-phase commit, all registered Synchronizations are informed. Any failure at this point will cause the transaction to roll back.
Steps 2 and 3
The coordinator then conducts the normal two-phase commit protocol.
Step 4
Once the transaction has terminated, all registered Synchronizations are informed. However, this is a courtesy invocation because any failures at this stage are ignored: the transaction has terminated so there’s nothing to affect.
The synchronization protocol does not have the same failure requirements as the traditional two-phase commit protocol. For example, Synchronization participants do not need the ability to recover in the event of failures, because any failure before the two-phase commit protocol completes cause the transaction to roll back, and failures after it completes have no effect on the data which the Synchronization participants are responsible for.
There are several variants to the standard two-phase commit protocol that are worth knowing about, because they can have an impact on performance and failure recovery. Table 3.1, “Variants to the Two-Phase Commit Protocol” gives more information about each one.
Table 3.1. Variants to the Two-Phase Commit Protocol
Variant | Description |
|---|---|
Presumed Abort | If a transaction is going to roll back, the coordinator may record this information locally and tell
all enlisted participants. Failure to contact a participant has no effect on the transaction outcome. The
coordinator is informing participants only as a courtesy. Once all participants have been contacted, the
information about the transaction can be removed. If a subsequent request for the status of the transaction
occurs, no information will be available and the requester can assume that the transaction has aborted. This
optimization has the benefit that no information about participants need be made persistent until the transaction
has progressed to the end of the |
One-Phase | If only a single participant is involved in the transaction, the coordinator does not need to drive
it through the |
Read-Only | When a participant is asked to prepare, it can indicate to the coordinator that no information or data that it controls has been modified during the transaction. Such a participant does not need to be informed about the outcome of the transaction since the fate of the participant has no affect on the transaction. Therefore, a read-only participant can be omitted from the second phase of the commit protocol. |
Note
The WS-Atomic Transaction protocol does not support the one-phase commit optimization.
In order to guarantee atomicity, the two-phase commit protocol is blocking. As a result of failures, participants may remain blocked for an indefinite period of time, even if failure recovery mechanisms exist. Some applications and participants cannot tolerate this blocking.
To break this blocking nature, participants that are past the prepare phase are allowed
to make autonomous decisions about whether to commit or rollback. Such a participant must record its decision, so
that it can complete the original transaction if it eventually gets a request to do so. If the coordinator
eventually informs the participant of the transaction outcome, and it is the same as the choice the participant
made, no conflict exists. If the decisions of the participant and coordinator are different, the situation is
referred to as a non-atomic outcome, and more specifically as a heuristic outcome.
Resolving and reporting heuristic outcomes to the application is usually the domain of complex, manually driven system administration tools, because attempting an automatic resolution requires semantic information about the nature of participants involved in the transactions.
Precisely when a participant makes a heuristic decision depends on the specific implementation. Likewise, the choice the participant makes about whether to commit or to roll back depends upon the implementation, and possibly the application and the environment in which it finds itself. The possible heuristic outcomes are discussed in Table 3.2, “Heuristic Outcomes”.
Table 3.2. Heuristic Outcomes
Outcome | Description |
|---|---|
Heuristic Rollback | The commit operation failed because some or all of the participants unilaterally rolled back the transaction. |
Heuristic Commit | An attempted rollback operation failed because all of the participants unilaterally committed. One
situation where this might happen is if the coordinator is able to successfully |
Heuristic Mixed | Some participants commit ed, while others were rolled back. |
Heuristic Hazard | The disposition of some of the updates is unknown. For those which are known, they have either all been committed or all rolled back. |
Heuristic decisions should be used with care and only in exceptional circumstances, since the decision may possibly differ from that determined by the transaction service. This type of difference can lead to a loss of integrity in the system. Try to avoid needing to perform resolution of heuristics, either by working with services and participants that do not cause heuristics, or by using a transaction service that provides assistance in the resolution process.
Interposition is a scoping mechanism which allows coordination of a transaction to be delegated across a hierarchy of coordinators. See Figure 3.4, “Interpositions” for a graphical representation of this concept.
Interposition is particularly useful for Web Services transactions, as a way of limiting the amount of network
traffic required for coordination. For example, if communications between the top-level coordinator and a web
service are slow because of network traffic or distance, the web service might benefit from executing in a
subordinate transaction which employs a local coordinator service. In Figure 3.4, “Interpositions”,to
prepare, the top-level coordinator only needs to send one
prepare message to the subordinate coordinator, and receive one
prepared or aborted reply. The subordinate coordinator forwards
a prepare locally to each participant and combines the results to decide whether to send
a single prepared or aborted reply.
Many component technologies offer mechanisms for coordinating ACID transactions based on two-phase commit semantics. Some of these are CORBA/OTS, JTS/JTA, and MTS/MSDTC. ACID transactions are not suitable for all Web Services transactions, as explained in Reasons ACID is Not Suitable for Web Services.
Reasons ACID is Not Suitable for Web Services
Classic ACID transactions assume that an organization that develops and deploys applications owns the entire infrastructure for the applications. This infrastructure has traditionally taken the form of an Intranet. Ownership implies that transactions operate in a trusted and predictable manner. To assure ACIDity, potentially long-lived locks can be kept on underlying data structures during two-phase commit. Resources can be used for any period of time and released when the transaction is complete.
In Web Services, these assumptions are no longer valid. One obvious reason is that the owners of data exposed through a Web service refuse to allow their data to be locked for extended periods, since allowing such locks invites denial-of-service attacks.
All application infrastructures are generally owned by a single party. Systems using classical ACID transactions normally assume that participants in a transaction will obey the directives of the transaction manager and only infrequently make unilateral decisions which harm other participants in a transaction.
Web Services participating in a transaction can effectively decide to resign from the transaction at any time, and the consumer of the service generally has little in the way of quality of service guarantees to prevent this.
Extended transaction models which relax the ACID properties have been proposed over the years. WS-T provides a new transaction protocol to implement these concepts for the Web Services architecture. XTS is designed to accommodate four underlying requirements inherent in any loosely coupled architecture like Web Services. These requirements are discussed in Requirements of Web Services.
Requirements of Web Services
Ability to handle multiple successful outcomes to a transaction, and to involve operations whose effects may not be isolated or durable.
Coordination of autonomous parties whose relationships are governed by contracts, rather than the dictates of a central design authority.
Discontinuous service, where parties are expected to suffer outages during their lifetimes, and coordinated work must be able to survive such outages.
Interoperation using XML over multiple communication protocols. XTS uses SOAP encoding carried over HTTP.
This section discusses fundamental concepts associated with the WS-Coordination, WS-Atomic Transaction and WS-Business Activity protocols, as defined in each protocol's specification. Foundational information about these protocols is important to understanding the remaining material covered in this guide.
Note
If you are familiar with the WS-Coordination, WS-Atomic Transaction, and WS-Business Activity specifications you may only need to skim this chapter.
In general terms, coordination is the act of one entity,known as the coordinator, disseminating information to a number of participants for some domain-specific reason. This reason could be to reach consensus on a decision by a distributed transaction protocol, or to guarantee that all participants obtain a specific message, such as in a reliable multicast environment. When parties are being coordinated, information, known as the coordination context, is propagated to tie together operations which are logically part of the same coordinated work or activity. This context information may flow with normal application messages, or may be an explicit part of a message exchange. It is specific to the type of coordination being performed.
The fundamental idea underpinning WS-Coordination (WS-C) is that a coordination infrastructure is needed in a Web Services environment. The WS-C specification defines a framework that allows different coordination protocols to be plugged in to coordinate work between clients, services, and participants, as shown in Figure 4.1, “WS-C Architecture”.
Whatever coordination protocol is used, and in whatever domain it is deployed, the same generic requirements are present.
Generic Requirements for WS-C
Instantiation, or activation, of a new coordinator for the specific coordination protocol, for a particular application instance.
Registration of participants with the coordinator, such that they will receive that coordinator’s protocol messages during (some part of) the application’s lifetime.
Propagation of contextual information between Web Services that comprise the application.
An entity to drive the coordination protocol through to completion.
The first three of the points in Generic Requirements for WS-C are the direct responsibility of WS-C, while the fourth is the responsibility of a third-party entity. The third-party entity is usually the client component of the overall application. These four WS-C roles and their relationships are shown in Figure 4.2, “Four Roles in WS-C”.
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.
Example 4.1.
<!-- 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.
Example 4.2. Registration ServiceWSDL Interface
<!-- 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> Once a participant is registered with a coordinator through the registration service, it receives coordination messages from the coordinator. Typical messages include such things as “prepare to complete” and “complete” messages, if a two-phase protocol is used. Where the coordinator’s protocol supports it, participants can also send messages back to the coordinator.
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 (WS-T) comprises the pair of transaction coordination protocols, WS-Atomic Transaction (WS-AT) and WS-Business Activity (WS-BA), which utilize the coordination framework provided by WS-Coordination (WS-C).
WS-Transactions was developed to unify existing traditional transaction processing systems, allowing them to communicate reliably with one another without changes to the systems' own function.
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”.
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.
It extends the WS-C context to create a transaction context.
It augments the activation and registration services with a number of additional services (Completion, Volatile2PC, Durable2PC, BusinessAgreementWithParticipantCompletion, and BusinessAgreementWithCoordinatorCompletion) and two protocol message sets (one for each of the transaction models supported in WS-Transaction), to build a fully-fledged transaction coordinator on top of the WS-C protocol infrastructure.
An important aspect of WS-Transaction that differs from traditional transaction protocols is that a synchronous request/response model is not assumed. Sequences of one way messages are used to implement communications between the client/participant and the coordination services appropriate to the transaction's coordination and participant protocols. This is significant because it means that the client and participant containers must deploy XTS service endpoints to receive messages from the coordinator service.
This requirement is visible in the details of the
RegisterandRegisterResponsemessages declared in the Registration Service WSDL in Example 4.2, “Registration ServiceWSDL Interface”. TheRegistermessage contains the URL of an endpoint in the client or web service container. This URL is used when a WS-Transaction coordination service wishes to dispatch a message to the clinet or web service. Similarly, theRegisterResponsemessage contains a URL iendtifying an endpoint for the protocol-specific WS-Transaction coordination service for which the client/web service is registered, allowing messages to be addressed to the transaction coordinator.
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”.
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.
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 ???.
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
To begin an atomic transaction, the client application first locates a WS-C Activation Coordinator web service that supports WS-Transaction.
The client sends a WS-C
CreateCoordinationContextmessage to the service, specifying http://schemas.xmlsoap.org/ws/2004/10/wsat as its coordination type.The client receives an appropriate WS-Transaction context from the activation service.
The response to the
CreateCoordinationContextmessage, the transaction context, has itsCoordinationTypeelement 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.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
Completionprotocol, by sending aRegistermessage to the Registration Service whose endpoint was returned in the Coordination Context.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.
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
commitoperation, all Volatile2PC participants are informed that the transaction is about to complete, via thepreparemessage. The participants can respond with one of three messages:prepared,aborted, orreadonly. 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
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 theabortedmessage. 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 thereadonlymessage, causing it to be omitted from the second phase of the commit protocol. Otherwise, thepreparedmessage is sent by the participant.If no failures occur during the first phase, Durable2PC proceeds to the second phase, in which the coordinator sends the
commitmessage to participants. Participants then make permanent the tentative work done by their associated services, and send acommittedmessage to the coordinator. If any failures occur, the coordinator sends therollbackmessage to all participants, causing them to discard tentative work done by their associated services, and delete any state information saved to persistent storage atprepare, if they have reached that stage. Participants respond to a rollback by sending anabortedmessage 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.
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.
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
Services are requested to do work.
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
undobehavior.
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
A participant is initially created in the Active state.
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
exitedmessage 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 acompletedmessage to the coordinator and waits for the coordinator to notify it about the final outcome of the BA. This outcome is either aclosemessage, meaning the BA has completed successfully, or acompensatemessage indicating that the participant needs to reverse its work.
- BusinessAgreementWithCoordinatorCompletion
The
BusinessAgreementWithCoordinatorCompletiondiffers from theBusinessAgreementWithParticipantCompletionprotocol in that the participant cannot autonomously decide to complete its participation in the BA, even if it can be compensated.Instead, the completion stage is driven by the client which created the BA, which sends a
completedmessage to the coordinator.The coordinator sends a
completemessage to each participant, indicating that no further requests will be sent to the service associated with the participant.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.
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.
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.
Although the application or service developer is rarely interested in the messages exchanged by the transactional infrastructure, it is useful to understand what kinds of exchanges occur so that the underlying model can be fitted in to an overall architecture.
WS-Coordination, WS-Atomic Transaction and WS-Business Activity-specific messages are transported using SOAP
messaging over HTTP. The types of messages that are propagated include instructions to perform standard transaction
operations like begin and prepare.
Note
XTS messages do not interfere with messages from the application, an application need not use the same transport as the transaction-specific messages. For example, a client application might deliver its application-specific messages using SOAP RPC over SMTP, even though the XTS messages are delivered using a different mechanism.
XTS provides a coordination infrastructure which allows transactions to run between services owned by different business, across the Internet. That infrastructure is based on the WS-C, WS-Atomic Transaction and WS-Business Activity specifications. It supports two kinds of transactions: atomic transactions and business activities, which can be combined in arbitrary ways to map elegantly onto the transactional requirements of the underlying problem. The use of the whole infrastructure is simple, because its functionality is exposed through a simple transactioning API. XTS provides everything necessary to keep application and transactional aspects of an application separate, and to ensure that a system's use of transactions does not interfere with the functional aspects of the system itself.