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CHAPTER 1 s DISTRIBUTED ARCHITECTURE
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This discussion illustrates why n-tier applications are viewed as relatively complex. There are a lot of factors, technical and non-technical, that must be taken into account. Unfortunately, it isn t possible to say definitively when n-tier does and doesn t fit. In the end, it s a judgment call that you, as an application architect, must make, based on the factors that affect your particular organization, environment, and development team.
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Relationship Between Logical and Physical Models
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Architectures such as Windows Distributed interNet Architecture (Windows DNA), represent a merger of logical and physical models. Such mergers seem attractive because they appear so simple and straightforward, but typically they aren t good in practice they can lead people to design applications using a logical or physical architecture that isn t best suited to their needs.
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s Note To be fair, Windows DNA didn t mandate that the logical and physical models be the same. Unfortunately, almost all of the printed material (even the mousepads) surrounding Windows DNA included diagrams and pictures that illustrated the proper Windows DNA implementation as an intertwined blur of physical and logical architecture. Although some experienced architects were able to separate the concepts, many more didn t, and created some horrendous results.
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The Logical Model
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When you re creating an application, it s important to start with a logical architecture that clarifies the roles of all components, separates functionality so that a team can work together effectively, and simplifies overall maintenance of the system. The logical architecture must also include enough layers so that you have flexibility in choosing a physical architecture later on. Traditionally, you would devise at least a 3-layer logical model that separates the interface, the business logic, and the data-management portions of the application. Today that s rarely sufficient, because the interface layer is often physically split into two parts (browser and web server), and the logic layer is often physically split between a client or web server and an application server. Additionally, there are various application models that have been used to break the traditional business logic layer into multiple parts model-view-controller and facade-data-logic being two of the most popular at the moment. This means that the logical layers are governed by the following rules: The logical architecture includes layers in order to organize components into discrete roles. The logical architecture must have at least as many layers as the anticipated physical deployment will have tiers. Following these rules, most modern applications have four to six logical layers. As you ll see, the architecture used in this book includes five logical layers.
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The Physical Model
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By ensuring that the logical model has enough layers to provide flexibility, you can configure your application into an appropriate physical architecture that will depend on your performance, scalability, fault tolerance, and security requirements. The more physical tiers included, the worse the performance will be; but there is the potential to increase scalability, security, and/or fault tolerance.
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Performance and Scalability
The more physical tiers there are, the worse the performance That doesn t sound right, but if you think it through, it makes perfect sense: performance is the speed at which an application responds to a user. This is different from scalability, which is a measure of how performance changes as load (such as increased users) is added to an application. To get optimal performance that is, the fastest possible response time for a given user the ideal solution is to put the client, the logic, and the data on the user s machine. This means no network hops, no network latency, and no contention with other users. If you decide that you need to support multiple users, you might consider putting application data on a central file server. (This is typical with Access and dBASE systems, for example.) However, this immediately affects performance because of contention on the data file. Furthermore, data access now takes place across the network, which means you ve introduced network latency and network contention, too. To overcome this problem, you could put the data into a managed environment such as SQL Server or Oracle. This will help to reduce data contention, but you re still stuck with the network latency and contention problems. Although improved, performance for a given user is still nowhere near what it was when everything ran directly on that user s computer. Even with a central database server, scalability is limited. Clients are still in contention for the resources of the server, with each client opening and closing connections, doing queries and updates, and constantly demanding the CPU, memory, and disk resources that are being used by other clients. You can reduce this load by shifting some of the work to another server. An application server, possibly running Enterprise Services or IIS, can provide database connection pooling to minimize the number of database connections that are opened and closed. It can also perform some data processing, filtering, and even caching to offload some work from the database server. These additional steps provide a dramatic boost to scalability, but again at the cost of performance. The user s request now has two network hops, potentially resulting in double the network latency and contention. For a single user, the system gets slower; but it is able to handle many times more users with acceptable performance levels. In the end, the application is constrained by the most limiting resource. This is typically the speed of transferring data across the network but if the database or application server is underpowered, it can become so slow that data transfer across the network isn t an issue. Likewise, if the application does extremely intense calculations and the client machines are slow, then the cost of transferring the data across the network to a relatively idle high-speed server can make sense.
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