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CHAPTER 1 OVERVIEW OF .NET APPLICATION ARCHITECTURE
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individual tiers and the individual layers, the wrong choices for marshaling can impair your scalability on the one hand, and/or impair your dependability and availability on the other. The perils and importance of these decisions provide a fine segue into Microsoft s next generation messaging stack: Windows Communication Foundation (formerly known as Indigo). Windows Communication Foundation s design goal is to unify the call stack that s used whenever out-of-process communication occurs, be it across application domains, inter-process, across machine boundaries, or across the world. In Part 3, we move to the final critical tier of our application: the data access layer. This is where all the real action resides, where all the bits people actually want to see are housed, where even the nontechnical business users in your enterprise know the heart of the business lies. This is your critical resource in a distributed application, as it is the transactional nerve center of the enterprise. This is also the layer of a distributed application that tends to get the most reuse. Once you write something down, people tend to want to get to it. The decisions you make in this part of your architecture will make or break your suite of applications. ADO.NET exposes the data access object model in .NET. We ll examine the ins and outs of using these managed providers of data access, how to pick the right tool for the job, and how to employ best practices when using them. We ll also survey other data access services in .NET, and get a sneak peak at the next generation database server: SQL Server 2005.
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Nonfunctional Requirements
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Architectural requirements are defined by nonfunctional requirements, or quality attributes. These are the requirements of the application the business or functional requirements do not describe. It is your job to capture these, and to define a technical infrastructure that meets the captured requirements. A key deliverable you need to provide is a definition of the different atomic pieces of the application that will be used, and justification for using them by explaining how they meet different nonfunctional requirements. You also need to define how these elements will interact. You need to address how type information will be discovered by a calling processes; how the information will be marshaled to and from the service; the constraints of this interaction; the platforms that must be supported in this communication; as well as which pieces are public knowledge (part of the interface) and which are internal (hidden details of the implementation). You ll need to answer all of these questions in order to design the technical infrastructure. Many things can fall into the realm of nonfunctional requirements. While these requirements can be considered separately, it s their interactions that become the critical influence on the design: Many of them work against one another, creating a tension between them, and a balance must be struck. Let s look at a few.
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Availability
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Availability concerns system failure. This brings to question: What are the points of failure in the system How often do they become unavailable And how does someone know when a system is unavailable Further, when there s a failure, how much time passes before the system becomes available again Also, what percentage of the time does the application need to be available In an environment where availability is a high priority, this is usually expressed in the ninety nine and n nines form. In these environments, it s usually a given that a system has to be available more than 99 percent of the time. A system that s down for three minutes
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CHAPTER 1 OVERVIEW OF .NET APPLICATION ARCHITECTURE
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every five hours will likely be problematic. That s almost fifteen minutes per day. This measure doesn t usually include planned downtime, for backups and routine maintenance, for example. On the other hand, some environments don t need to be highly available. In many environments, as long as an application is available during business hours, folks are happy. However, as applications become more connected, more global, and more automated, requirements for availability will increase. Failure is inevitable. Hardware fails. Disk drives crash. Networks go down. Accepting this is a given; you provide for availability by adding redundancy to the system. You must add redundancy at each point of failure. Having redundancy at every point of failure is frequently called n+1 reliability. N is a measure of the amount of resources needed to do the job. The plus one provides the availability when failure occurs. Given n+1 reliability isn t cheap; it s only put in place for mission-critical applications. If a company s entire revenue stream is web based, an unavailable website means zero dollars in the revenue stream. Suddenly the expense of n+1 reliability doesn t seem like so much money, after all. ISPs that host the big websites typically have four T4 lines running to the building, one from each direction on the compass. They may have several diesel generators in case the power fails, and then in case a generator (or two) fails. They may also be fortified, to guard against sabotage, like a bank. The company adds capacity for the servers automatically when a spike in traffic occurs. All of this redundancy allows them to guarantee the sites they host will be available. Availability of static pieces of the system can be attained by scaling out, which is a fancy term for throwing more servers at the problem. Since one web server can go down, another is added to the mix, and a load balancer is put in front of them. A single IP now maps to more than one machine. Failover is provided by the load balancer, which will send all traffic to the live machine when one dies. If these machines are not physically colocated, some type of persistent connection needs to be maintained between them. This can be a factor when a failover strategy also needs to account for a disaster recovery scenario. These requirements and the decisions made to meet them can affect the design of the software systems that will be hosted on this physical infrastructure. The introduction of a load balancer means anything stored in the memory of the web server can be in the memory of more than one physical machine. This may be fine for read-only information, where more than one copy is acceptable. But for mutable, or user-specific, information, this situation introduces a definite problem. The web server s memory becomes an unsuitable location to store this information. It must be marshaled out-of-process, and stored in a central location. Failure to account for this means failure of the application when a web server blows a gasket. This problem is accounted for with out-of-process session state available in ASP .NET. State information can be stored either in an out-of-process state server (no redundancy), or SQL Server, which can be made redundant with clustering. This introduces a definite performance hit, but architecture is frequently about trade-offs. You must find a balance. Maybe this hit is not acceptable, and the application will be designed not to use session information at all. It depends. Clustering will actually need to be present in a highly available system, regardless of how session state is being dealt with. Scaling out will not work on the database tier of the application for the same reason it doesn t work with user-specific session information. It changes. There can t be n copies of it, because these copies would start to deviate from one another. Clustering maintains a hot backup of a single server and a mirrored copy of the information written to disk that the system is dependent upon. Drive crashes Switch over to the mirror. Server crashes Switch to the backup. This is also called scaling up. We can scale out at the
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