94 Part I EXAM PREPARATION nique becomes overwhelming
94 Part I EXAM PREPARATION nique becomes overwhelming when the data is needed by multiple users sharing the same data. In this case, a two-tier with a central data store on a proper server is a better approach. A two-tier architecture places the user interface and data on separate machines. The client application sends queries across the network to be resolved by the data engine running on the server. The server resolves the query and sends the necessary data back across the network for the client application to display. There are two different implementations of a two-tier system: thin client and thick client, which is also known as fat client. In a thin approach the client application does little or no processing. A thin client just presents the data to the user and, when needed, communicates with the database engine on the server to work with the data. The thin client approach is best where the number of concurrent users accessing the data can be kept to a minimum. Because the server must perform the processing to validate the data as well as all other data manipulations, there is a lot of server overhead related to this approach. A thin client is a good approach for maintainability. If you need to upgrade software, for example, you do not have to do so on 1000 clients. It also works well for Internet applications. The thick client approach offloads some of the work needed to validate and process the data from the server machine to the client machine. In this approach the client may make some of the determinations as to whether data should be sent to the server based on validity checks coded in the client application. This approach enables many more users to access the same database concurrently. On the down side, though, application maintenance is more demanding and higher performance is required from the client. Although two-tier architectures allow for more flexibility and a larger number of users, it is still quite a limited architecture that can serve only small environments. When a larger number of concurrent user accesses is needed, a better choice of architecture is a multiple n-tier architecture or possibly an Internet architecture. Three- or More Tier Systems In a three-tier system, the client system presents the interface and
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