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CHAPTER 1 DIS TRIBUTED ARC HITE CTURE
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In short, because the client workstations are outside the domain of trust, you should assume that they re compromised and potentially malicious. You should assume that any code running on those clients will run incorrectly or not at all; in other words, the client input must be completely validated as it enters the domain of trust, even if the client includes code to do the validation.
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I ve had people tell me that this is an overly paranoid attitude, but I ve been burned too many times: any time an interface is exposed (Windows, web, XML, and so on) so that clients outside your control can use it, you should assume that the interface will be misused. Often, this misuse is unintentional for example, someone may write a buggy macro to automate data entry. That s no different than if they made a typo while entering the data by hand, but user-entered data is always validated before being accepted by an application. The same must be true for automated data entry as well, or your application will fail.
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This scenario occurs in three main architectures: Windows smart clients, rich Internet applications (RIAs), and SOA systems. If you deploy a WPF or Windows Forms client application to external workstations, you should design it as a stand-alone application that calls your server application through services. 21 shows how you can do this with the object-oriented concepts in this book. You may create an RIA with Asynchronous JavaScript and XML (Ajax) technologies or newer technologies such as Silverlight. In either case, the RIA often validates data or otherwise provides a richer experience for the user, but your server code should assume that the RIA didn t do anything it was supposed to. It is far too easy for a user to subvert your client-side JavaScript or otherwise bypass client-side processing as such, nothing running in an RIA can be trusted. The code running in the browser should be viewed as a separate application that is not trusted by the server application. Service-oriented systems imply that there s one or more (potentially unknown) applications out there consuming your services. The very nature of SOA means that you have no control over those applications, so it would be foolish to assume they ll provide valid input to your services. A healthy dose of paranoia is critical when building any service for an SOA system. As you ll see, you can use the object-oriented concepts and techniques shown in this book to create smart client applications that call services on your servers. You can use the same concepts to create the services themselves. You can also use them to create web applications ranging from simple Web Forms to Ajax to Silverlight.
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Fault Tolerance
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You can achieve fault tolerance by identifying points of failure and providing redundancy. Typically, applications have numerous points of failure. Some of the most obvious are as follows: The network feed to your user s buildings The power feed to your user s buildings The network feed and power feed to your data center The primary DNS host servicing your domain Your firewall, routers, switches, etc. Your web server Your application server Your database server Your internal LAN
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C HAPTE R 1 DISTRIBUTED A RCHITEC TURE
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In order to achieve high levels of fault tolerance, you need to ensure that if any one of these fails, some system will instantly kick in and fill the void. If the data center power goes out, a generator will kick in. If a bulldozer cuts your network feed, you ll need to have a second network feed coming in from the other side of the building, and so forth. Considering some of the larger and more well-known outages of major websites in the past couple of years, it s worth noting that most of them occurred due to construction work cutting network or power feeds, or because their ISP or external DNS provider went down or was attacked. That said, there are plenty of examples of websites going down due to local equipment failure. The reason why the high-profile failures are seldom due to this type of problem is because large sites make sure to provide redundancy in these areas. Clearly, adding redundant power, network, ISP, DNS, or LAN hardware will have little impact on application architecture. Adding redundant servers, on the other hand, will affect the n-tier application architecture or at least the application design. Each time you add a physical tier, you need to ensure that you add redundancy to the servers in that tier. Thus, adding a fault-tolerant physical tier always means adding at least two servers to the infrastructure. The more physical tiers, the more redundant servers there are to configure and maintain. This is why fault tolerance is typically expensive to achieve. Not only that, but to achieve fault tolerance through redundancy, all servers in a tier must also be logically identical at all times. For example, at no time can a user be tied to a specific server, so no single server can ever maintain any user-specific information. As soon as a user is tied to a specific server, that server becomes a point of failure for that user. The result is that the user loses fault tolerance. Achieving a high degree of fault tolerance isn t easy. It requires a great deal of thought and effort to locate all points of failure and make them redundant. Having fewer physical tiers in an architecture can assist in this process by reducing the number of tiers that must be made redundant. To summarize, the number of physical tiers in an architecture is a trade-off between performance, scalability, security, and fault tolerance. Furthermore, the optimal configuration for a web application isn t the same as the one for an intranet application with smart client machines. If an application framework is to have any hope of broad appeal, it needs flexibility in the physical architecture so that it can support web and smart clients effectively, as well as provide both with optimal performance and scalability. Beyond that, it needs to work well in a service-oriented environment to create both client and server applications that interact through message-based communication.
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