introduction to web services

J.E.D.I 1. Introduction to Web Services 1.1 Objectives Towards the end of the chapter, you should be able to learn the following: • Determine what web services are • Identify the characteristics of a web service and how it works • Point out the need for web services 1.2 Introduction The domain of web services is an emerging technology due to the growing demand of application-to-application communication and interoperability. The reason for its popularity is because of its flexible nature, since it exhibits itself as a standard interface which is not only platform-independent but technology independent as well. We'll talk more about this later. This chapter will cover the evolution of network computing, an overview of how the web service model came to be, the definition of web services, its properties and and what states or scenes are they applicable to. And lastly, the discussion of the current status of web services and where it is heading. 1.3 How did web services came to be? (A History of Distributed Computing) During the prehistoric era in software services, computer programs were written in assembly and are executed using single memory space (see Figure 1). The software's subroutines communicate digitally using the machine's registers. Programmers during this time find it hard to write, more so, maintain codes. Shortly after assembly, came procedural languages such as COBOL, C and FORTRAN, which allowed programs to be written more easily than their forerunner since they resemble more the human language. Moreover, these procedural languages allow programs to run on different machines. The software service during this time were functions that were directed by the use of control structures. Although this is a technological advancement in computing, it is no different with assembly in the sense that programs are executed in single memory space. 1 of 14 J.E.D.I Figure 1 Communication in single memory space in the same computer The next generation was the development of network computing, wherein, instead of communicating through magnetic tapes, the exchange of information became real-time. TCP/IP network protocol and the C programming language became the “in” thing back then. This was made possible by the invention of RPC (Remote Procedural Call) which allows software services to invoke each other the network (see Figure 2). This effectively broke the chain that holds back software services within the bounds of individual machines to collaborate in a larger scale. 2 of 14 J.E.D.I Figure 2 Functions communicating between memory spaces using RPC After procedural calls came the time of functional languages such as LISP, followed shortly by object-oriented languages such as C++ and of course, Java. The development of object oriented language allowed developers to create objects, which are real world counterparts. If you haven't noticed, objects are in truth software services. More sophisticated network protocols were discovered by this time. The most popular ones are CORBA (Common Object Request Broker Architecture) and DCOM (Distributed Common Object Model). These two will not be expounded as they are not within the scope of this course material, but suffice to say that they are network protocols that grants connection between software services. The limitation of using the said protocols was that the deployment of these technologies were homogeneous in nature. Homogeneous meaning, they are deployed in closed environments. The following was the case until the invention of the Internet. Thanks to the Internet, geographical barriers started to blur and businesses began to flourish by taking advantage of this wonderful innovation. Eventually, people began to see the power of distributed computing and the promise it shows. 1.3.1 Communication Patterns Two things sparked the start of web services the evolution of distributed computing and the revolution in the way we think about building large scale systems. As a recap from the previous courseware, web applications, we know that distributed computing 1 is as the definition below says, 1 http://en.wikipedia.org/wiki/Distributed_computing 3 of 14 J.E.D.I A very good example of a distributed system is the World Wide Web (WWW). As you are surfing the Internet, you might not realize it but, you are in reality, using distributed system. Your web browser is communicating with other web servers providing the web pages you visit. Your web browser talks to various web servers over the Internet via a system of routers, which are themselves part of a bigger distributed system. The aim of distributed computing is to find a way to connect users and resources in an open and scalable way. Before this goal was met, various computing obstacles were encountered and eventually as these impediments are overcome, the computing industry began to mature. The next section gives a blow-by-blow series of event leading to the design of web services. The illustration below (Figure 3) shows how distributed computing evolved from a simple client server model to what is expected of it in the future. Web services lies pretty much in the middle of it. Figure 3 2 2 “Web Services Overview.” Web Service Programming (with Passion!). Sang Shin. <http://www.javapassion.com/webservices/index.html#Web_Services_Overview> 4 of 14 Distributed computing is parallel computing using multiple independent computers communicating over a network to accomplish a common objective or task. The type of hardware, programming languages, operating systems and other resources may vary drastically. J.E.D.I Evoution of Communication Patterns In the beginning, there was the client-server model (see the first image from the left of Figure 3). If you would recall in the web programming module, in a client-server communication pattern, there are two entities involved, the client, which we could think as the person and his computer, requesting for a certain resource from the server (limited to the name of a document needed or a call to a server-side program), and the server, which returns the information requested. Figure 4 The consumer fetches information through HTML and a protocol Figure 4 is to look at the client-server model in finer detail. The graphical representation shows that clients or consumers requests information from the server, which in turn processes the requests and throws back a web page containing information requested by the client/consumer. Remember that this is done through the use of HTML and a network protocol such as HTTP. Although the client-server communication pattern made it easy for consumers to access web pages, still that is not enough to integerate business systems more flexibly. Thus, the introduction of the 3-tier communication pattern. In the web application courseware, you are taught about the MVC framework, which is in particular, the Model, View and Controller. It is a matter of fact a 3-tier communication pattern. A 3-tier communication pattern is a client-server model wherein the user-interface, a functional process logic, and a computer data storage are developed and maintained as autonomous modules. The illustration below shows the 3-tier communication pattern. 5 of 14 J.E.D.I Figure 5 The 3-tier communication pattern From 3-tier communication pattern, it evolved to the web application communication pattern (Figure 6). The web application communication model (J2EE architecture) is comprised of a web server, application server and a database server. On the other hand, J2SE and J2ME is responsible for the client tier. I wouldn't linger more on this topic since it was disucssed in detail in the previous courseware. As a matter of fact, the web service course assumes that you have knowledge about web applications before moving on further. Moving on, an improvement to the web application communication pattern is the web services communication model (Figure 7), which is what this course is all about. The picture shows the technolgies in which a lot of business organizations and industries could leverage on. 6 of 14 J.E.D.I Figure 6 3 Web application as communication pattern 3 “Web Services Overview.” Web Service Programming (with Passion!). Sang Shin. <http://www.javapassion.com/webservices/index.html#Web_Services_Overview> 7 of 14 J.E.D.I Figure 7 4 Web Services as Communication Pattern Due to the attributes stated above, various software services are not confined to interacting with their kind, such as the what is shown in Figure 8. Figure 8 Software service interaction through the use of web service The web service model above shows that a web service exhibits the application logic, also known as the application function of one communication participant (can be thought of as the server) to multiple or heterogeneous client base (such as cellphone, PDA, and other types of clients) independent of its object model implementation, programming language or runtime environment. This functionality is what distinguishes web services from other remote access mechanisms like Remote Method Invocation (RMI), and CORBA. Having said the these, the web service infrastructure can be view as though each of the participants can act as both user and provider of service. This means that, information can be retrieved from a PDA by a cellphone, a laptop to a swing client and vice versa. Having said all these, let us go to the formal definition of what a web service is. 1.4 What is a web service? When you look around the Internet and various reading materials, you must have concluded that there's a lot of hype, and marketing talk about it. Not much of these materials can be of use to developers. And even today, it is hard to grasp a consistent definition for what it really is. The most reasonable definition I came up with my research is that from the World Wide 4 “Web Services Overview.” Web Service Programming (with Passion!). Sang Shin. <http://www.javapassion.com/webservices/index.html#Web_Services_Overview> 8 of 14 J.E.D.I Web Consortium (W3C) 5 . The said governing body defined web service as follows: Don't worry if you find the said definition a little overwhelming. You'll be able to understand and appreciate it as we go along the rest of the chapters. For now, think of web service as a software application available over the web that is accessed by clients using XML- based protocols. For starters, you can look at the site http://www.xmethods.com. The said site contains a handful of web services which are implemented using different technologies. At the time of my visit, I found the following listing: Figure 9 5 http://www.w3c.org/TR/ws-arch 9 of 14 “ a software system identified by a URL, whose public interface and bindings are defined and described using XML. Its definition can be discovered by other software systems. These systems then interact with the web service in a manner prescribed by its definition, using XML based messages conveyed by Internet protocols.” J.E.D.I Sample Web service listing The listing above won't exactly bring you to the service itself. But you will be sent to a page saying where to get the service that you want and who to contact for more information. To be able to connect to the service being offered on the page, you need to create a client application, which we will be discussing in the succeeding chapters. Suffice to say that these services are free, and you only need a web browser and an Internet connection. 1.5 Why Web Services? You must be thinking, what's so special about web services that HTML, JSP and other technologies cannot accomplish? An example presented in the book, Java Web Services in a Nutshell (A quick desktop references) 6 , presents the situation where a certain publisher wants to keep track of the sale of books. He has three options to go about this: 1. Use the site's search facility to locate a few titles, read the review. To go about this, each stage entails rendering HTML, and answering or clicking a form. Obviously, this option is too tedious to do, bordering insanity. 2. Write a code that extracts information from HTML. This means that the application will strip the HTML and get the raw data. This idea is better than the first one since it has some level of automation into it. Although still not cost efficient since, you only need the figure of the sales ranking of the book, to bother with the program. Apparently, this is still not the best solution since the layout of the HTML can change, thereby rendering the program invalid. 3. The obvious choice is the use of web services. Amazon can provide a web service interface and expose the appropriate information in XML form. The publisher only need to write a client program to retrieve information from the web service. Through web services, the publisher won't have to deal with markup, but instead go straight to data he/she needs. As a matter of fact, Amazon really did that in the middle of 2002. Another advantage of using web services is that with the retrieved information from the service, the publisher can customize how the data is displayed to his/her own format and not conform with that of Amazon's web page. In addition to the advantages presented by the scenario, here are the other reasons why web services is very appealing: • XML-based – using XML rids any networking, operanting system, or platform binding dependency that a protocol may have. As a data representation layer, XML allows for iteroperability at the core level. We will talk about XML in detail in the next lesson. • Loosely-coupled – the attribute of beind loosely-coupled could only be equated with versatility, in the sense that a consumer of a web service is not bound to that web service directly. This means that, say the service interface changes, the client need not adopt to the changes made. Being loosely-coupled makes software systems more manageable. • Coarse-grained – perhaps the best way to explain this attribute is to first show what the opposite of it – fine-grained, means. A Java program has methods, and these methods have specific tasks. These methods carrying-out specific jobs are fine-grained methods. So, when we build a Java program from scratch, we are therefore making a lot of fine- 6 Topley, Kim. Java Web Services In a Nutshell. O'Reilly & Associates, Inc. June 2003 10 of 14 [...]... the gist of the web service life cycle 1.9 Where is web services, and where it is heading? Currently, the various technologies (SOAP, WSDL, UDDI) that holds web services together are still evolving and releasing new versions to support more functionalities and aid for bug fixes Not to mention, the emerging additional standards that are currently being defined to enable web services to realize their...J.E.D.I grained methods, which in turn will be used to create coarse-grained services Businesses and interfaces exposed by the service should be coarse-grained and web services provides a natural way to accomplish this end • Ability to be synchronous or asynchronous – synchronicity, in general, refers to the binding of the client to service execution Web services allows for either synchronous or asynchronous... world how and what the other service use to implement its program The communicating servers need not be in the same network location, and still the software service can still talk to one another Just goes to show why more and more businesses are using web services 1.6 How can I use web services? This question is more like, “How can web service benefit me?” The answer to this actually depends on your specific... book from Amazon that it is able to sell AkingAklat.com gets its book listings from Amazon by providing a link leading to Amazon But what the former wants is to be able to display the book listing in accordance to its web site's format This is made possible through the use of web services The entire process is reflected in the figure below: 12 of 14 J.E.D.I Figure 10 Web service exchange between AkingAklat.com... succeeding chapters) to Amazon.com's web service interface The web service client will invoke the specific method from the interface which generates the list of books The values from this is wrapped to an XML data and is returned to the web service client through SOAP The web service client in turn, translated this data, and uses JSP to render it as HTML The HTML is then returned to AkingAklat.com's... waits for the service to complete its operation before continuing ■ Asynchronous – allows the client to call on a service, doesn't necessarily have to wait for it to finish before it could execute another function since the client can recover the result at a later time This is the key factor to a loosely coupled system • Support for Remote Procedural Calls (RPC) – web services allows to access procudures,... is to make two legacy systems one running in C++ and the other is a portal written in Java, communicate with one another You can take advantage of web services' cross language and cross-platform features by not rewriting the program running in C or to Java and instead use web services This obviously saves you time and money by not going to the trouble of reimplementing existing code 2 Business -to- business... want to connect to Ebay, Yahoo or Amazon, or some other sites as long as they would expose the same interface, you can reuse the same client program You don't need to know the nitty-gritty of each supplier's system, what's important is that you are able to transact with them End of story 11 of 14 J.E.D.I 3 Service-based software An example of which are small shell clients connecting to various web services. .. The companies that hosts the web service can charge you each time you access their application For example, a company that tells the exchange rate from a given currency to another, can charge users who want to get their hands on this kind of information 1.7 What comprises web services? There are 5 major elements in this great scheme To shed more light into what comprises a web service, let us tackle... extended to provide enhanced web services support Having said this, you can say that web service is far from being perfect and has still a long way to go Although this is the case, more and more businesses are adopting it because of its many strong points as mentioned in the previous section However, for this technology to fully satisfy the business industry, there are points for improvement that needs to . Introduction to Web Services 1.1 Objectives Towards the end of the chapter, you should be able to learn the following: • Determine what web services are • Identify the characteristics of a web. 14 J.E.D.I Figure 6 3 Web application as communication pattern 3 Web Services Overview.” Web Service Programming (with Passion!). Sang Shin. <http://www.javapassion.com/webservices/index.html #Web_ Services_ Overview> 7. the web service model came to be, the definition of web services, its properties and and what states or scenes are they applicable to. And lastly, the discussion of the current status of web services