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.
Overview
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.
Client API
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:
Begin: Creates a new top-level transaction (or
subtransaction) for either atomic or cohesive transactions
Prepare: Instructs an atomic transaction to prepare its associated participating services when the transaction is to terminate
Prepare Inferiors: Instructs a cohesive transaction to prepare one or more of its participating services at transaction
termination time
Confirm: Instructs an atomic transaction to confirm all of its participating services, and confirms all participant services
that voted to confirm in the case of a cohesive transaction
Cancel: Instructs all participating services in an atomic transaction, or those services specified in the parameter to the
method call in a cohesive transaction, to cancel
In addition to these demarcation verbs, a number of other commands
can be used to inquire about a transaction:
Status: Asks the transaction manager to return the state (e.g., committed, preparing) of the current transaction
Transaction type: Exposes the type of the current transaction (i.e., atom or cohesion)
Transaction name: Exposes the name of the current transaction in string form
Two verbs allow advanced manual transaction management:
Suspend: Disassociates the current thread from the current transaction
Resume: (Re)associates the current thread with the current transaction
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.
Summary
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.
Author Bio
Jim Webber is a senior engineer with Hewlett-Packard, based at the
Arjuna Laboratory in Newcastle upon Tyne. Within the Arjuna Lab, HP's
center of excellence for transactioning, Jim is involved with
research and development on transactional Web services, and
architecting HP's Web Service Transactioning solution. Jim is also
involved with standards processes for transactional Web services, and
is currently participating in the OASIS BTP effort.
jim_webber@hp.com
Web Services Infrastructure, by Jim Webber
WSJ Vol 02 Issue 10 - pg.54
Listing 1: Controlling Atoms from a Web Service Client
// obtain a UserTransaction object (assume it is previously
// initialized)
userTransaction = UserTransactionFactory.getTransaction();
// obtain references to the Web services we are going to use:
restaurant = new RestaurantService();
taxi = new TaxiService();
theatre = new TheatreService();
// Start a new transaction, using a simple atom
userTransaction.begin(com.hp.mw.xts.TxTypes.ATOM);
// now invoke the business logic
restaurant.bookSeats(3);
theatre.bookSeats(3, 2);
taxi.bookTaxi();
// prepare the transaction (first phase of two phase
coordination)
userTransaction.prepare();
// confirm the transaction (second phase of two phase
coordination)
userTransaction.confirm();
Listing 2: A BTP Begin Message
<?xml version="1.0" encoding="UTF-8" ?>
<SOAP:Envelope
SOAP:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP:Body>
<btp:begin transaction-type="atom"
xmlns:btp="urn:oasis:names:tc:BTP:1.0:core" />
</SOAP:Body>
</SOAP:Envelope>
Listing 3: An Application Message with BTP Context
<?xml version="1.0" encoding="UTF-8" ?>
<SOAP:Envelope
SOAP:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP:Header>
<btp:messages xmlns:btp="urn:oasis:names:tc:BTP:1.0:core">
<btp:context>
<btp:superior-address>
<btp:binding-name>soap-http-1</btp:binding-name>
<btp:binding-address>
http://mybusiness.com/btpservice
</btp:binding-address>
</btp:superior-address>
<btp:superior-identifier>
12fa6de4ea3ec
</btp:superior-identifier>
<btp:superior-type>atom</btp:superior-type>
</btp:context>
</btp:messages>
</SOAP:Header>
<SOAP:Body>
<ns:booking xmlns:ns="http://btp.restaurant.org/">
<seats xsi:type="xsd:int">99</seats>
</ns:booking>
</SOAP:Body>
</SOAP:Envelope>
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