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:
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.
For an overview of the WS-Addressing language, see the WS-Addressing language basics section in Chapter 17.
In Plain English
As our car washing company grows, so do the administration duties. Every week Chuck reviews the mail and takes care of necessary paperwork. This week he receives two letters: one from our insurance company and the other from the tax office.
The first letter includes our renewed insurance policy statement, along with an invoice for another year of coverage. The "from" address on this letter is simply the name and location of the insurance company's head office. The enclosed statement contains a letter written by our account representative, outlining some of the changes in this year's policy and requesting that we mail our check directly to him. Chuck therefore encloses our payment in an envelope with a "to" address that includes an "attention" line stating that this letter should be delivered directly to the account representative.
The next letter contains another bill. This time, it's a tax statement accompanied by a letter of instruction and two return envelopes. According to the instructions, we are to use the first envelope (addressed to the A/R office) to mail a check if we are paying the full amount owing. If we cannot make a full payment, we need to use the second envelope (addressed to the collections department) to send whatever funds we have.
These scenarios, in their own crude way, demonstrate the fundamental concepts of endpoint references and message information headers, which are explained in the following sections.
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:
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:
(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.
Figure 7.5. Addressing relating to other parts of SOA.
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.
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.
In the previous chapter we provided several examples that explained the steps behind the vendor invoice submission process. One of these steps required that, upon receiving an invoice from a vendor, the TLS Accounts Payable Service interacts with the TLS Vendor Profile Service to have the received invoice validated against vendor account information already on file.
Due to the volume of invoice submissions received by TLS, there can be, at any given time, multiple active instances of the Accounts Payable Service. Therefore, as part of the message issued by the Accounts Payable Service to the Vendor Profile Service, a SOAP header providing a reference id is included. This identifier represents the current instance of the Accounts Payable Service and is used by the Vendor Profile Service to locate this instance when it is ready to respond with the validation information (Figure 7.6).
Figure 7.6. Separate service instances communicating using endpoint references and MI headers across two pools of Web services within TLS.
SUMMARY OF KEY POINTS