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Chapter: XML and Web Services : Building XML-Based Applications : Architecting Web Services

Technical Motivations for Web Services

The technical motivations for Web Services are far more complex than the business moti-vations. Fundamentally, technologists are looking for the simplicity and flexibility promised, but never delivered, by RPC architectures and object-oriented technologies.

Technical Motivations for Web Services


The technical motivations for Web Services are far more complex than the business moti-vations. Fundamentally, technologists are looking for the simplicity and flexibility promised, but never delivered, by RPC architectures and object-oriented technologies.


Limitations of CORBA and DCOM


Programming has been performed on a computer-by-computer basis for much of the his-tory of computing. Programs were discrete chunks of computer code that ran on individ-ual computers. Even object-oriented programming originated in a single-computer environment. This isolated computer mindset has been around so long that it pervades all thinking about software.


Then along came networks, and technologists looked for ways to break up program func-tionality onto multiple computers. Early communication protocols, such as the Network File System for Unix and Microsoft’s Distributed Computing Environment, focused on the network layer. These protocols, in turn, led to the development of wire protocols for distributed computing—in particular, the Object Remote Procedure Call (ORPC) proto-col for Microsoft’s DCOM and the Object Management Group’s Internet Inter-ORB Protocol (IIOP) that underlies CORBA.


RPC architectures such as DCOM and CORBA enabled programs to be broken into dif-ferent pieces running on different computers. Object-oriented techniques were particu-larly suited to this distributed environment for a few reasons. First, objects maintained their own discrete identities. Second, the code that handles the communication between objects could be encapsulated into its own set of classes so that programmers working in a distributed environment needn’t worry about how this communication worked.


However, programmers still had that isolated computer mindset, which colored both DCOM’s and CORBA’s approach: Write your programs so that the remote computer appears to be a part of your own computer. RPC architectures all involved marshalling a piece of a program on one computer and shipping it to another system.

Unfortunately, both DCOM and CORBA share many of the same problems. DCOM is expressly a Microsoft-only architecture, and although CORBA is intended to provide cross-platform interoperability, in reality it is too complex and semantically ambiguous to provide any level of interoperability without a large amount of manual integration work. In addition, the specter of marshalling executable code and shipping it over the Internet opens up a Pandora’s box of security concerns, such as viruses and worms.


Furthermore, each of these technologies handles key functionality in its own, proprietary way. CORBA’s payload parameter value format is the Common Data Representation (CDR) format, whereas DCOM uses the incompatible Network Data Representation (NDR) format (Web Services use XML). Likewise, CORBA uses Interoperable Object References (IORs) for endpoint naming, whereas DCOM uses OBJREFs (Web Services use URIs, which are generalized URLs).

By representing business concepts with systems of business components, business modelers seek to achieve the following objectives:


   Limit complexity and costs by developing coarse-grained software units.

   Support high levels of reuse of business components.

   Speed up the development cycle by combining preexisting business components and continuous integration.


   Deliver systems that can easily evolve.


   Allow different vendors to provide competing business components that serve the same purpose, leading to a market in business components.


Unfortunately, large-scale business modeling has not widely achieved any of these objectives, for several reasons, including the following:


   Business components in reality typically have complex, nonstandard interfaces, which makes reuse and substitutability difficult to achieve.


   As systems of business components evolve into increasingly complex, compre-hensive systems, it becomes very difficult to maintain the encapsulation of the components. Ideally, each component is a black box that can be plugged into the underlying framework; in reality, developers must spend time tweaking the internal operations of the components.


   The business drivers behind the development of the business components lead to custom development, which makes each component unique and custom in its own right. Every company handles its business models differently, so every business component is different.


The Web Services model can be thought of as the next step in the evolution of business components. Whereas business components are large, recursively defined collections of objects, Web Services should be relatively small, self-organizing components with well-defined, dynamic interfaces.


Problems with Vendor Dependence


Early leaders in every nascent industry find that they must integrate their companies ver-tically. For example, Standard Oil drilled the wells, transported the petroleum, refined it, distributed it, and then ran the gas stations that sold it. It had to follow this business model, because there were no other companies that could provide each of these services at a low-enough cost or with adequate quality.


The same is true of the software industry. ERP systems were essential to companies’ operations, because the only cost-effective way to get all the operational components that make up ERP systems to work together was to get them from the same company. If you tried to cobble together accounting and manufacturing software back in 1995, you would have found large variations in quality and extremely high integration costs.

Simply put, the single main advantage to single-vendor distributed software solutions is that they work. When the cost of integration is high, going with a single vendor will save money. However, there are also several disadvantages to obtaining software from a single vendor. The disadvantages are as follows:


   As the market matures, other vendors will offer individual packages that are of a higher quality than the single vendor, making a “best-of-breed” approach more attractive.


   The purchasing company’s business grows to depend on the business strategy of the vendor. Shifts in strategic direction or business problems at the vendor can filter down to the vendor’s customers (the “all-the-eggs-in-one-basket” problem).


   It is very difficult to integrate a “one-stop shop” vendor’s product with other ven-dors’ products at other companies. As a result, a single vendor approach limits the potential of e-business.


Taking a vendor-independent software strategy solves the problems of vendor depen-dence but is only cost effective when certain conditions are met:


   A “best-of-breed” approach makes sense because the market is mature enough to offer competing packages of sufficient quality.


   There is a broadly accepted integration framework that allows for inexpensive inte-gration of different packages, both within companies and between companies.


The Web Services model has the potential to meet both of these conditions. In particular, Web Services’ loose coupling is the key to flexible, inexpensive integration capabilities.


Reuse and Integration Goals


Software reuse has been a primary goal of object-oriented architectures but, like the Holy Grail, has always been just out of reach. Creating objects and components to be reusable takes more development time and design skill, and therefore more money up front.


However, conventional wisdom says that coding for reusability saves money in the long run, so why isn’t coding for reusability more prevalent?


The problem is that the goal of software reuse presupposes a world with stable business requirements, and such a world just doesn’t exist. Building a component so that it can handle future situations different from the current ones tends to be wasted work, because the future always brings surprises. Instead, it usually makes more sense to take an agile approach to components and include only the functionality you need right now. Such an approach keeps costs down and is more likely to meet the business requirements, but the resulting component is rarely reusable.


Simple integration of software applications is likewise just out of reach. This problem is especially onerous in the area of legacy integration. Today’s approach to integrating legacy systems into component architectures is to create a “wrapper” for the legacy system so that it will expose a standard interface that all the other components know how to work with. What ends up happening is that getting that wrapper to work becomes the major expense and takes the most time. Maybe 90 percent of your software is easy to integrate, but the remaining 10 percent takes up most of your budget.c

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