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Additionally, these small, self-contained components can be considered Web services when they implement the workflows and standards associated with this environment. Web services can also breathe new life into older software. Legacy systems, often built with tools and technologies now considered outdated, can be adopted to participate in a service-oriented environment.

The Web-services model is designed to be indifferent to how any given computing task is accomplished. The job of a Web service lies in transmitting a request for a particular task to be accomplished and then delivering back the results. Although a new application will likely be specifically designed to operate as a Web service, legacy applications can be turned into Web services by simply creating the communications layers that enable the Web-service protocols.

By developing Web-service gateways, organizations can help preserve investments that went into developing previous generations of software applications. The internal-database architecture and content and business logic of a legacy system can continue to operate as before, with the application's input and output transmitted through the Web-service layers.

Web services hide the underlying hardware and software from view, and the Web-services model facilitates interoperability, neutralizing differences in hardware platforms, operating systems, and development tools. Although it's possible to run Web services on desktop systems, most reside on server-class hardware and operating systems. Web services can operate in a stateless mode, or they can be programmed to operate within specific, defined sessions. Many simple Web services work independently of any particular context or complex series of events. This type of Web service consists of a simple, discrete, request—response transaction.

In some circumstances, however, it is important to maintain a session among the participants in an operation of Web services so that multiple Web-service requests, i. In a distributed computing environment, no single computer application is likely to be entirely independent. It's often necessary for one computer component to rely on others to obtain specific items of information or to perform calculations or conversions.

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When a software application needs information to perform its function, Web services provide a mechanism for requesting that information, a way to move that information, and how to structure that information for transit. Web services provide a layer of communication technology that recognizes the interconnectedness of organizations and their related spheres of expertise, information, and services. It is a technology that allows organizations to better fulfill their roles as suppliers of services and provides opportunities to create value-added services based on enhancing services derived from others.

Web services can be considered a mature and well-established computing model. As a technology, Web services emerged in the late s and have been gaining ground since that time. The standards involved are well defined and broadly adopted, and a plethora of tools exist for developing a Web service.

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This is not a wait-and-see technology; Web services are currently in the mainstream and can be considered a safe investment for libraries. Although a number of technologies might be employed to implement a service-oriented architecture, the technology realm of Web services involves a particular set of standards and protocols. In this section, I'll delve into the acronym and abbreviation soup associated with Web services. The basic concepts of Web services focus on the roles and actions that software components play in the overall process:. The Web-services model assumes a particular flow of communications among applications.

Specific roles and actions apply.

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Figure 1 , below, illustrates the general flow. Let's step through the process. Keep in mind that all these steps happened almost instantly and unbeknownst to the person who invoked the information to be displayed on the Web page. The plumbing analogy is a good one to call upon in order to understand Web services; basically, Web services are just fancy plumbing for the Web. Although Web services operate as a set of technical mechanisms, they represent agreements and contracts among individuals or organizations. Agreements must be determined in advance—regarding which Web services will be offered by a provider, who may consume the Web services, and at what cost they are offered.

Some Web services may be free and publicly available to any requestor, but user authentication and security may need to be added to services that transmit restricted or sensitive content. In a complex environment, it is often necessary to perform some routing of the message before it reaches the service provider that can fulfill the service request. The final service provider is, therefore, often called the endpoint. In this context, Web services stand as an important piece of the overall architecture of the Web. The index page for its Web-service activities is located at www.

This page links to resources, which describe each of the standards, protocols, and practices operating within the arena of Web services. This Web site provides comprehensive and definitive information regarding the architecture of Web services and the individual components that comprise them. In typical W3C style, the information is presented in a straightforward but terse manner.

Those familiar with standards documents will find the content informative, though it makes for difficult reading for those not technically inclined. Now that I've established a general picture of the workflow, I can move on to identifying the components involved. Almost all applications that deal with data today use some form of XML as the structure and syntax to format data. XML is especially useful when moving information from a software application to another; many applications use XML internally as well.

XML provides the syntax for structuring data, and all XML implementations follow the same rules regarding basic syntax. Consistent rules apply to how tags surround data elements, how tags are nested, and how documents begin and end. Although XML is designed to be processed by computers, it is readable by humans. In previous eras of computing, great efforts were made to store data in the most compact form possible, therefore, data and record formats were often expressed in binary representations that, although compact, could not be easily interpreted by humans.

In the library arena, MARC serves as an example of this approach. The leader, tags, fixed fields, variable fields, and sub-field indicators structure bibliographic data in a way that conserves space but is difficult to read. XML, by contrast, appears quite verbose. Instead of cryptic numerical tags, XML employs descriptive tag names. Today's low-cost digital storage and expanded bandwidth remove many of the previously existing motivations to compress data to formats that only computers can decipher. Although XML offers great flexibility, it demands exact precision in the way that documents are constructed.

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Yet most Web browsers can perform operations in spite of errors; they are programmed to estimate the author's intentions and then continue to process the page. XML parsers, on the other hand, will reject a document if even a single error is present. The tags available for use in any given XML document are defined according to the needs of the application, and through agreements, are established among communities of users.

The members of an academic discipline, a business sector, or other groups of individuals or organizations that deal with similar types of information have a common interest in following the same XML conventions. In the Web-service arena, great attention must be paid to the namespaces and schema that apply to any given XML document. Because Web services involve exchanging data among diverse participants, it is essential to ensure that the data structures used are compatible.

Designing my Server Architecture for Scalable Web Applications

The code in figure 2 below illustrates a very simple XML document. Notice that the document begins with a header that describes the version of XML it uses and the character set, or encoding , used for all data within the document. One of the trickiest aspects of XML involves making sure that all data adheres to the encoding rules established. Special characters and punctuation marks often need to be converted to an acceptable form, lest the XML document be invalid and be rejected by the parser. In figure 2 , you can see a number of XML characteristics from this simple example.

Each XML document must have a root element—a tag that marks the beginning and end, encompassing all the other tags. Figure 2 also illustrates the fundamental syntax rule that all tags have matching opening and closing elements. Any unclosed tags will cause the document to be rejected.

Those who plan to develop applications based on XML will need to become intimately familiar with all its nuances. As has been explained, Web services involve sending a request from a service consumer to a service provider and receiving a response. SOAP functions as one of the main mechanisms for transmitting the messages—between the service consumer and the service provider—involved in a Web service. SOAP is a technology developed under the oversight of the W3C; the documents that describe the current version of the protocol reside on the W3C Web site www. But because the protocol really isn't that simple, and it deals with data structures in many ways as objects as well as many others , the purveyors of the latest version of the standard chose to simply refer to it as SOAP—it is no longer considered an acronym.

It uses XML technologies to define an extensible messaging framework providing a message construct that can be exchanged over a variety of underlying protocols. The framework has been designed to be independent of any particular programming model and other implementation specific semantics. The current version, SOAP 1. The crucial role for SOAP lies in wrapping the requests and responses according to the specification. The example illustrates the components that comprise a SOAP message.

The envelope provides the locations of the schemas that support this instance of SOAP. The SOAP body carries the methods and data that represent the request or response. Once properly formed or formatted, SOAP messages then can be transported to the proper destination by the Web-service application. The WSDL consists of an XML document that can be used by a service consumer to automatically configure itself to invoke a Web service from a service provider.

Web Services

WDSL can also serve as documentation to a programmer that wants to construct a static Web-service client. It is possible to access a Web service based on advanced knowledge of how it works. If the same developer, for example, creates both the Web service and the client that accesses it, the formality of a WSDL may not be necessary. For more complex environments that include many services, WSDL is more of a necessity. The WSDL will include seven sets of definitions:. Universal Description, Discovery, and Integration, referred to as the abbreviation UDDI , is a protocol that functions to find Web services within a domain.

The UDDI registry plays a role only in helping a service consumer locate a suitable service provider. Once a service consumer receives the location of the service, UDDI binds the service with the service provider directly. In many cases the service consumer will learn about a service via other means. Most public developers of Web services such as those that developed the popular Amazon Web Service will discover the services available through documentation provided by the service provider.

Larger-scale implementations of Web services, with a complex matrix of service consumers and service providers, will rely on UDDI to enable applications to operate without human intervention. Domains that have a large number of Web services will find UDDI a useful tool, making the environment more manageable. When a developer makes a change to the service or adds a new service, registering the service in the UDDI will eliminate the need to make programming changes in Web-service clients. A UDDI registry can be either private or public.

A private registry operates within a closed domain, such as within the enterprise network of a company. A public registry is available to all users on the Internet. The latest version, UDDI v3. This version can be considered as completely interoperable among both private and public registries. UDDI is a technically complex specification. Although the concept of the registry is simple, the API application programming interface used to submit documents and make queries is one of the more challenging components to implement.

The messages involved in an operation of Web services rely on standard network protocols. Web services that operate over HTTP can take advantage of the same Web-server software—usually Apache—that delivers standard Web pages. It is also possible to create Web services that operate over TCP sockets. Such a Web service would provide its own daemon a program that runs unattended to perform continuous or system-wide functions that monitors the network for incoming requests. This approach pays off in an environment with a full-fledged service-oriented architecture in place, which supports a complex set of business applications.

It is up to the service consumer to parse the XML stream and make use of the results. Although a RESTful service can often be tested by using a Web browser, in actual operation, the URL that invokes the service is sent by some other software application. The advantages of REST include simplicity and speed. Though RSS finds many uses, the primary appeal lies in its ability to disseminate newly created information—called a feed —data that tells interested readers new items have been posted on a blog, news service, or Web site. Many libraries use RSS to disseminate information about library events, new acquisitions, or to deliver search results.

The feed on this site can be invoked with the URL www. The realm of Web services has all the same security concerns as other networked systems. It is essential that a Web service provides information only to the intended recipients and that sensitive information is not exposed in such a way that it can be intercepted by an unauthorized third party.

All of the authorization and authentication mechanisms available for general network and Web operations can be applied to Web services. It is common for Web services involving business transactions, which carry sensitive data e. In addition to these fundamentals, a number of other specifications have emerged, which enhance and extend Web services in business transactions and that demand special considerations not adequately addressed in the basic protocols.

These second-generation specifications are in an early phase of development and adoption and are not likely to be found in the current generation of library implementations of Web services.

Some of these later standards include:. As it has evolved, and continues to evolve, the realm of Web services—and the protocols, specifications, and standards I've highlighted thus far in this report are but just a few—has continued and will continue in its ability to accommodate complex business transactions.

The nuances of functionality taking place in these specifications each represent important issues in transaction-oriented business and financial systems. But as library-automation systems evolve to use Web services in order to integrate with the financial systems of their parent institutions, they may well need to be more aware and knowledgeable of the specifications that relate to complex business transactions. Flexibility abounds when it comes to the development of Web services. All of the major programming languages and development environments provide some method for creating Web services.

The following is just a sample of some of the development tools and environments commonly used for building Web services. Microsoft Corporation has embraced Web services as a key strategic technology, and it has been actively involved in the development of protocols, standards, and specifications related to Web services. The company calls its Web-service strategy.

In the end, there will be a set of standards which allow us to do things we could not do before, but in the mean time different people and companies approach them from different positions, and with different expectations. In , Web Services have also been a buzzword used repeatedly and claimed to be one of the hot new technologies. The common themes are The Web in Web Services is, from the first point, a misuse: the term Internet Services would be more appropriate.

The Web comes from the second point - the use of the HTTP and XML is already in use as a well-understood and well-debugged set of protocols which support the Web, and so it makes sense to reuse them in providing remote operations and those things connected with them. The third point is what makes web service requirements so different from a local RPC system. The fact that data is exchanged for business purposes and between different social entities means that accountability is required, rather than just reliable transmission.

Other articles have dealt with the fundamental architectural difference between remote operations and the architecture of the information space, and the mappings between the two. However, the essential part of Web services is the Interact relationship between a Service provider and Service requestor. This is the Web Service.

Discovery agencies need not be used - they will in some cases but not in others. The discovery agencies are well represented as a cloud, rather than being a well-defined module in the web services architecture. They will become interface to a huge world of data and query services which provide data about web services as well as many other things. The Interact between between requestor and provider is the essential defining element to web services.

As we shall see, the metadata about web services. The management layer is a supervisory layer allowing the conrol of the many agents involved in a web services-based operation. The "Application semantics" layer indicates the necessity, for any useful interoperability, to have. The design work of web services is divided between the run time protocols and the descriptions of services. There is a bifurcation in the design at this point, as SOAP operates basically in two modes. In this mode, XML is used as the marshalling style, but the system is a distributed using remote procedure call in a fairly traditional way.

Interfaces between software modules have well-defined functions, which in turn have well-defined and typed input and output parameters. Stubs dummy routines which similate the remote procedure by a local one which communcates with the remote one can be generated directly from the WSDl definition.

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This is less rigid than the RPC style. The communication is more apparent to the application writer, who deals with the document object model DOM of the recived message, rather than having parameters unmkarshalled automatically. The document mode of SOAP seems to be getting the most traction in the ecommerce stack. This is not an accident. It is easier in principle to extend an XML-based message system to include more information as a system grows. In fact, RDF is especially powerful in this area, as new information can be parsed into an entity-relationship form by old agents, and it becomes logically clear which parts can be ignored by those who do not understand them.

Functionality which has been mentioned as required above the basic layer at runtime includes:. Higher layers in the description above WSDL are known variously as coordination, orchestration, choreography, composition. Composability of web services refers to the building, from a set of web services, of something at a higher level, typically itself exposed as a larger web service. Choreography refers more abstractly the part of the description of web services which defines a way, or the ways, in which a acyual invokations to various web services work together.

Peltz uses Choregraphy when it involves multiple parties, and Orchestration when it is internal to one party. Thus the former crosses application boundaries, the latter also crosses organization boundaries. There is so small amount overlap here, which has led to some confusion. To be general, one might say, for example, that a flight confirmation must involve an already reserved flight. This the actual constraint. One can describe a particular choregraphy a particular dance, if you like in which a flight query service is called, and produced a list of flights, and then a reservation service is called to reserve the flight, that is successful, and the resulting reservation is passed to the confirmation service.

It may be that there are other ways -- other choreographies -- in which one could have achived a reserved flight. The engineer has the choice of modelling the many possible ways all in one choreogrpahy, or of making several choreographies. Web services can be combined in such as way that messages are passed around in a very random fashion.

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However, a particular design techniqe is for a master process to delegate to other services in a recusive tree-like manner, as has been de rigueur in programming languages since Pascal. For example, if the consumer asks the travel agent and the the travel agent books a hotel, the hotel will reply to the travel agent, not to the consumer.

This makes everything orderly. This is a programming language approach with. As neither is intended to do the actual calculations or business rules, it would be closer to compare themm with scripting shells such as bash which handle concurrency and synchronization but actually call programs or rather web services to do the real work. Different attitudes - top down program design, or bottom-up agent design, bottom up document design. The Paper Trail concept is that the state of a mult-agent multi-process system can be looked at, sometimes rather effectively, as a function of the documents which have been transmitted.

The process-oriented attitude to a bank-customer relationship may be "In parallel, the customer writes checks, merchants pay in checks, credit card transactions happen, all month. Then, the charges, interest are assessed and a bank statement sent from the bank to the customer". The document-, or message-oriented one is more like "Every month a bank balance lists valid transaction dated that month. A cleared incoming check in a valid transaction.