|By Jim Webber||
|September 23, 2002 12:00 AM EDT||
It's a fact: Web services have started to mature. Those emergent standards that once held so much promise are now actually starting to deliver useful implementations. With the basic Web services plumbing mastered, we're starting to see more advanced infrastructure, which enables these second-generation Web services to focus on complex interactions over the Internet. This article, the first of a two-part series, covers one such aspect of the second-generation infrastructure for Web services: transactions.
The OASIS Business Transactions Protocol, or BTP, has become the prominent standard for Web services transactions. BTP is the product of just over a year's work by vendors such as HP, BEA, and Oracle, and has resulted in the development of a transaction model particularly suited to loosely coupled systems like Web services.
In this article, we're going to look at how BTP fits into the whole Web services architecture, and how we can use one of the vendor toolkits (we'll use HP's toolkit, but the underlying principles apply to other vendors' software) to build and consume transaction-aware Web services. But before we do, let's review the architecture in the context of a simple transactional scenario.
The diagram shown in Figure 1 is similar to a typical high-level Web services architecture. The only differences here are that one service, the transaction manager, has been singled out as being distinct from the other Web services (which we assume are responsible for some aspects of a business process), and the fact that we've chosen to identify two distinct categories of messages: control messages (which are used to control transactions) and application messages (which propagate application data around the system).
Of course, if it really were as simple as deploying a transaction manager service into the architecture, then this article wouldn't be necessary. Unfortunately it's not that simple; or at least not quite that simple, as we shall see. To illustrate, it's convenient to use Figure 1 as a point of reference as we work through the architecture, filling in the details. We'll work from left to right, from the client through to the Web services, and cover everything in between.
Consuming Transactional Web Services
Though Web services is a hot technology, we shouldn't lose sight of the fact that it exists to support business processes. With that in mind, the right place to start our investigation is most definitely at the client end of a system - where the results of Web services interactions are brought together and where the value of a business process is ultimately focused. To place this in the proper context, it's useful to see an exploded view of the client-side infrastructure, shown in Figure 2.
In a nontransactional Web services- based application, the client process can be something as simple as a collection of calls (via proxies) to services that are involved in the activity. In a transactional Web services-based application, the same is (surprisingly enough) true, with the caveat that the developer must demarcate any transactions that support business logic, as well as deal with application-specific calls. In this case the transaction demarcation is supported by the client transaction API (the Client Tx API in Figure 2), whereas the business methods supported by service proxies appear to logically remain free of any transactional infrastructure from the point of view of the client application developer. In fact, under the covers there is a mechanism that performs context associations with local threads of control within the client and messages passed between the client and (transactional) Web services. In Figure 2, this is the purpose of the Tx Context Interceptor.
The client API provides the developer with the necessary tools with which to structure and control transactions within the application. The commands available to a developer in a transactional Web services environment are quite familiar to those of us that have used other transaction APIs in the past, with the caveat that BTP supports full control over both phases of the commit process and thus has a larger command set than we might otherwise envision. The UserTransaction API supports the common verbs (and by implication the methods that enact those verbs) for transaction demarcation:
Two verbs allow advanced manual transaction management:
Those who have previously worked with transactions will immediately find themselves at home with this API, since it is in the same spirit as other transaction APIs like JTA. Let's take a look at an example.
In the code shown in Listing 1, we see an atom being used to ensure a consistent outcome across calls to the Web services shown in Figure 1. Initially we obtain a reference to an instance of UserTransaction from a (previously initialized) UserTransactionFactory, which we then use to delimit the scope of the single transaction in our application. Our atomic transaction is started by calling the begin(...) method on the user transaction API and specifying the type of transaction as an atom. From now on the business logic is straightforward and contains no further transaction control primitives; we simply go ahead and make the bookings we want for our night out through the book(...) methods of the service proxies we created.
Once the business logic has completed, we can terminate the transaction by calling prepare(...) and confirm(...) which, in the absence of failures, should confirm to all parties that they should henceforth honor all our booking requests. If there are failures, then all parties are informed and should take the necessary steps to undo any work undertaken on our behalf, while the client application will receive an exception that details what exactly has gone wrong.
The great thing about this example is that it shows just how simple and relatively noninvasive it can be to wrap work with Web services within a transaction. In fact, the business logic aspects of the code would be the same irrespective of whether or not transactions are used.
Under the Covers:
BTP's Two-Pipe Model
To support transactional Web services-based applications, BTP utilizes two distinct types of messages that the client application exchanges with business Web services. The first of these messages is exchanged exclusively within the transaction infrastructure. The other type consists of messages that the client exchanges with business Web services onto which BTP messages might be piggybacked.
The messages that the application exchanges with the transaction infrastructure are encapsulated by the primitives supported by the client API. For example, a begin(...) method being executed by the client causes a corresponding BTP begin message to be sent to a transaction manager via the SOAP server, and for response messages from the transaction manager to be processed in the reverse order. This is shown in Figure 3, and a sample BTP message (begin) is shown in Listing 2. The only slightly unusual aspect to this example is that the response to begin messages (and only begin messages) is cached for later use so local threads of execution can be associated with the BTP transaction under which its work is being carried out.
When transporting application messages, the situation is a little different. Unlike BTP messages in which the message content travels in the body of the SOAP envelope, when application messages are sent, application-specific content travels in the body, while any BTP messages are relegated to the header part of the envelope. We can see this in Listing 3, in which the SOAP body carries the application payload, while the header is used to carry the BTP context.
This scheme works well since most SOAP stacks are well equipped to perform efficient header processing, and placing the BTP content, including the transaction context, in the header means that SOAP actors can pick out the parts of the header space that are of interest without having to parse the whole application payload. From a development point of view, most SOAP servers support pluggable header processors, which means that building BTP context processing into your infrastructure should be straightforward. To demonstrate that point, let's take a look at the general client-side architecture (which is based on Apache Axis in the toolkit we've used), as per the examples in Figure 3 and Listing 2.
Figure 4 shows the outward path of a call to a Web service, starting from the left with the local method call to a service proxy. The call then follows the logical path of being converted to the appropriate SOAP body, which contains the application payload, before it progresses to the outgoing context handler. The context handler takes advantage of the fact that the information supplied in response to the BTP begin message was recorded, and is able to produce a BTP context from that data, which it duly inserts into the SOAP envelope's header. If there is no contextual data stored for the current thread (i.e., it isn't part of a transaction or the transaction has been deliberately suspended), then the context insertion is simply bypassed.
For return messages, the strategy is simply the reverse, as shown in Figure 5, in which the flow is from right to left. Responses are quickly scanned to see if they contain any BTP context entries in their headers. If context data is present, it is stripped out of the message and may be used to resume the transaction locally by associating the current thread while the rest of the message passes through to the service proxies. Once at the service proxies, the local method call returns control to the client, which is unaware of all of the additional processing that has occurred on its behalf.
Having reached the point where we can send application messages with BTP contexts, as well as BTP messages themselves, we're able to follow the messages as they travel across the wire. Following the cables inevitably leads us to business Web services.
The first article in this series on implementing transactional Web services- based applications has shown how client applications can be constructed using off-the-shelf BTP toolkits. We've seen how much of the hard work involved in managing transactions has been delegated to the toolkit and the underlying SOAP infrastructure, leaving to the developer the real value-add work of getting the application logic and transaction structure right. However, this is only half the story. In the next article, we'll investigate what happens at the Web service end, and show how true enterprise-class Web services applications can be made transactional from end to end.
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