11.4: Connecting to databases in three-tier applications

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Three-tier application structure is similar to the 7-layer OSI model of telecommunications, in that it breaks up the presentation, data, and application functions into each of their own levels. This segmentation provides for a more streamlined approach to scalability and ease of development, since the developer does not have to produce a new web technology for every instance. Instead, standards such as HTML, CSS, or JavaScript may be used.

When connecting to a three-tier based web application, the user will interact with the presentation tier. This layer is the visual wrapper between the functionality, database, and UI elements. This layer is displayed in a web browser, and served through the World Wide Web or a Local Area Network. Connecting to an IP address using a compatible browser will provide the web page to the user, being provided through a server which hosts the website and has access to the databases which it uses. The presentation tier is largely the visual aspect of a website or application and does not perform the computing or data storage/retrieval. This layer is important, however, because data entry and user intent must be clearly interpreted for the right API calls to be made by the presentation layer, to be sent to the application and data tiers.

By using a three-tier architecture, developers are able to concurrently work on each tier of the application as they are designing it. This parallel development allows for fewer limitations due to older design which cannot easily be changed. This is relevant to the connection to databases, due to the ability of the development teams to agree on which APIs and methods they wish to communicate with between each layer. For the database to be accessed, the user would make a request for a piece of information to be pulled from the database through the presentation layer. The presentation layer would process the request through the application layer (or logic layer) which would request the data from the database. All of this would happen through the use of API calls in a modern application. The database layer would be a version of SQL such as MySQL, Microsoft SQL Server, or another database type.

Once the request reaches the data layer, the database management system (DBMS) is what will actually do the work of finding what the request is looking for. This includes querying the database indexes to see where the information is stored, as databases are usually composed of one or more storage devices with many logical storage endpoints.