Web Services and Contemporary SOA (Part II: Advanced Messaging, Metadata, and Security)
In Chapter 6 we established a series of composition and activity management concepts, each with a different scope and purpose, but all somewhat related within the context of composable SOA. Those initial concepts are complemented by additional WS-* extensions that govern specific areas of the SOAP messaging framework, the creation and exchange of metadata, and the introduction of message-level security. (Figure 7.1 introduces the individual concepts and shows how they typically inter-relate.)
As we explore the various extensions in this chapter, it becomes increasingly clear that SOAP messaging is the lifeblood of contemporary service-oriented architecture. It realizes not only the delivery of application data, but also the composable nature of SOA. The innovation of SOAP headers accounts for almost all of the features covered in Chapters 6 and 7.
To demonstrate common concepts, this chapter borrows terms provided by the following current Web services specifications:
WS-Policy Framework (including WS-PolicyAttachments and WS-PolicyAssertions)
WS-Security (including XML-Encryption, XML-Signature, and SAML)
WS-Notification Framework (including WS-BaseNotification, WS-Topics, and WS-BrokeredNotification)
As with Chapter 6, we only explore concepts related to WS-* extensions in this chapter. Language element descriptions and examples for the first five specifications in the preceding list are provided in Chapter 17.
Figure 7.1 Specifications and concepts covered in this chapter.
What addressing brings to SOAP messaging is much like what a waybill brings to the shipping process. Regardless of which ports, warehouses, or delivery stations a package passes through en route to its ultimate destination, with a waybill attached to it, everyone it comes into contact with knows:
where it’s coming from
the address of where it’s supposed to go
the specific person at the address who is supposed to receive it
where it should go if it can’t be delivered as planned
The WS-Addressing specification implements these addressing features (Figure 7.2) by providing two types of SOAP headers (explained shortly). Though relatively simple in nature, these addressing extensions are integral to SOA’s underlying messaging mechanics. Many other WS-* specifications implicitly rely on the use of WS-Addressing.
Figure 7.2 Addressing turns messages into autonomous units of communication.
7.1.1 Endpoint references
Early on in this book we established that the loosely coupled nature of SOA was implemented through the use of service descriptions. In other words, all that is required for a service requestor to contact a service provider is the provider’s WSDL definition. This document, among other things, supplies the requestor with an address at which the provider can be contacted. What if, though, the service requestor needs to send a message to a specific instance of a service provider? In this case, the address provided by the WSDL is not sufficient.
Traditional Web applications had different ways of managing and communicating session identifiers. The most common approach was to append the identifier as a query string parameter to the end of a URL. While easy to develop, this technique resulted in application designs that lacked security and were non-standardized.
The concept of addressing introduces the endpoint reference, an extension used primarily to provide identifiers that pinpoint a particular instance of a service (as well as supplementary service metadata). The endpoint reference is expected to be almost always dynamically generated and can contain a set of supplementary properties.
Figure 7.3 A SOAP message containing a reference to the instance of the service that sent it.
An endpoint reference consists of the following parts:
address—The URL of the Web service.
reference properties—A set of property values associated with the Web service instance. (In our previous In Plain English example, the "attention" line used in the first scenario is representative of the reference ID property.)
reference parameters—A set of parameter values that can be used to further interact with a specific service instance.
service port type and port type—Specific service interface information giving the recipient of the message the exact location of service description details required for a reply.
policy—A WS-Policy compliant policy that provides rules and behavior information relevant to the current service interaction (policies are explained later in this chapter).
Additional parts exist, which mostly identify corresponding WSDL information. With the exception of the address, all parts are optional.
7.1.2 Message information headers
In the previous chapter we covered the various primitive message exchange patterns of which complex activities are comprised. These MEPs have predictable characteristics that can ease the manner in which Web services are designed but also can limit the service interaction scenarios within which they participate.
In sophisticated service-oriented solutions, services often require the flexibility to break a fixed pattern. For example, they may want to dynamically determine the nature of a message exchange. The extensions provided by WS-Addressing were broadened to include new SOAP headers that establish message exchange-related characteristics within the messages themselves. This collection of standardized headers is known as the message information (or MI) headers (Figure 7.4).
Figure 7.4 A SOAP message with message information headers specifying exactly how the recipient service should respond to its arrival.
The MI headers provided by WS-Addressing include:
destination—The address to which the message is being sent.
source endpoint—An endpoint reference to the Web service that generated the message.
reply endpoint—This important header allows a message to dictate to which address its reply should be sent.
fault endpoint—Further extending the messaging flexibility is this header, which gives a message the ability to set the address to which a fault notification should be sent.
message id—A value that uniquely identifies the message or the retransmission of the message (this header is required when using the reply endpoint header).
relationship—Most commonly used in request-response scenarios, this header contains the message id of the related message to which a message is replying (this header also is required within the reply message).
action—A URI value that indicates the message’s overall purpose (the equivalent of the standard SOAP HTTP action value).
(Also of interest is the fact that the WS-Addressing specification provides an anonymous URI that allows MI headers to intentionally contain an invalid address.)
Outfitting a SOAP message with these headers further increases its position as an independent unit of communication. Using MI headers, SOAP messages now can contain detailed information that defines the messaging interaction behavior of the service in receipt of the message. The net result is standardized support for the use of unpredictable and highly flexible message exchanges, dynamically creatable and therefore adaptive and responsive to runtime conditions.
7.1.3 Addressing and transport protocol independence
Historically, many of the details pertaining to how a unit of communication arrives at point B after it is transmitted from point A was left up to the individual protocols that controlled the transportation layer. While this level of technology-based abstraction is convenient for developers, it also leads to restrictions as to how communication between two units of processing logic can be achieved.
The standardized SOAP headers introduced by WS-Addressing remove much of this protocol-level dependence. These headers put the SOAP message itself in charge of its own destiny by further increasing its ability to act as a standalone unit of communication.
7.1.4 Addressing and SOA
Addressing achieves an important low-level, transport standardization within SOA, further promoting open standards that establish a level of transport technology independence (Figure 7.5). The use of endpoint references and MI headers deepens the intelligence embedded into SOAP messages, increasing message-level autonomy.
Empowering a message with the ability to self-direct its payload, as well as the ability to dictate how services receiving the message should behave, significantly increases the potential for Web services to be intrinsically interoperable. It places the task-specific logic into the message and promotes a highly reusable and generic service design standard that also facilitates the discovery of additional service metadata.
Figure 7.5 Addressing relating to other parts of SOA.
Further, the use of MI headers increases the range of interaction logic within complex activities and even encourages this logic to be dynamically determined. This, however, can be a double-edged sword. Even though MI headers can further increase the sophistication of service-oriented applications, their misuse (or overuse) can lead to some wildly creative and complex service activities.
Finally, by supporting the referencing of service instances, SOAs can be scaled in a standardized manner, without the need to resort to custom or proprietary application designs (scalability is a key QoS contribution). Having stated that, it should be pointed out that by providing functionality that enables communication with service instances, WS-Addressing indirectly supports the creation of stateful services. This runs contrary to the common service-orientation principle of statelessness (as explained in Chapter 8) and emphasizes the need for this feature to be applied in moderation.
SUMMARY OF KEY POINTS
Addressing extensions, as implemented by the WS-Addressing specification, introduce two important concepts: endpoint references and message information headers.
Endpoint references provide a standardized means of identifying a specific instance of a Web service.
Message information headers add message exchange properties to a specific message, conveying interaction semantics to recipient services.
Though simple in comparison to other WS-* specifications, WS-Addressing inserts a powerful layer of messaging autonomy within a service-oriented architecture.