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72 Integrating Airport Information Systems With these drawbacks, why are legacy systems still in use? Some reasons not to replace a legacy system are as follows: The cost of replacing the system is too high. No one knows exactly how the system works or everything it does, so replacing it is risky. The system was built to be highly available and cannot be taken down. Integration Strategies and Technologies In this Handbook, strategies refers to a specific software systems integration approach. Each strategy has different strengths and weaknesses and is appropriate for different situations. Under- standing how these strategies work is key to determining the most effective strategy for a partic- ular airport's situation. Technology refers to the tools used to build and integrate software. Integration technologies include the following: Standards used to format and store data, such as Structured Query Language (SQL) and XML; Software techniques such as relational databases and online analytical processing (OLAP); and RP, such as CUPPS. The use of a specific technology does not imply the use of any particular strategy. For example, XML is a technology that is widely relied on to integrate software systems, and it can be used to implement all of the strategies described in this Handbook. Integration Strategies This section describes popular software systems integration strategies (i.e., data warehousing, enterprise information integration, and enterprise application integration) and compares their strengths and weaknesses. Data Warehousing This strategy gathers data from different software systems and puts the data in a central location called a data warehouse. The warehouse uses software to scrub the data--apply preset business rules and analyze the data--to use the data in preset calculations to provide needed information. Other systems then go to this central location to get the data as input for their calculations, or in the case of a large data warehouse, data is first distributed to departmental data marts, such as a financial data mart or operations data mart. Think of the data in a data warehouse as the items sold by a large retail chain. All of the items (data) are received from the different manufacturers (software systems) in the central warehouse (data warehouse). The employees (data scrubbing routines) of the central warehouse ensure that the items received meet the standards of the retail chain. The items are organized and distributed to the retail stores (data marts) based on which stores need what items. When IT people discuss data warehousing, they often mention an associated strategy called Extract, Transform, and Load (ETL). This strategy typically uses technologies like open databases, flat files, and XML to extract the data from various sources before transforming or scrubbing the data so that it can be loaded into the data warehouse. The data warehouse and data marts are usually made up of one or more databases. These databases identify relationships between data and provide for open communication of data between databases. The data in a data warehouse or data mart are read-only, so they cannot be modified. Therefore, this strategy is used only for viewing and reporting on data; it does not allow software systems to interact with each other.

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Architecture, Strategies, Technologies, and Contracts 73 Figure 6-1 shows how a typical airport's information process would look using a data warehouse strategy. The strengths and weaknesses of data warehousing are as follows: Strengths. Good for analyzing historical data, analyzing trends, and finding hidden correla- tions in data. Weaknesses. To the extent that making real-time decisions is important, this strategy may not be as appropriate. Enterprise Information Integration This strategy leaves data in the various software systems and a central EII software program gets data from those systems when data are needed. The information in these various systems is termed distributed data. The EII software presents a single, unified model of the distributed data so that queries and reports can be written against this central model, regardless of in which system individual data elements reside. Think of an EII system as a multi-restaurant delivery service. The customer (a report or query) calls the delivery service (EII software) and orders Chinese food (accounting data), pizza (main- tenance data), and an apple pie for dessert (flight data). The delivery service picks up the food from three different restaurants (software systems) and delivers it to the customer. In discussing EII software, people often mention technologies like Open Database Connectiv- ity (ODBC), Java Database Connectivity (JDBC), and Web Services. These technologies are used to access data from the various distributed sources. Some of the strengths and weaknesses of EII are as follows: Strengths Because data are left in the original software system, reports are always up to date. Users can access data from multiple systems in an integrated manner. Figure 6-1. Schema for data warehousing strategy.