Multi-Tier Multi-tier development can be defined as a
Multi-Tier Multi-tier development can be defined as a development process where applications are built by components from different layers. Each layer provides services to the other layers, meaning each layer can abstract a particular aspect of the application. This abstraction leads to very maintainable applications since changes to one particular layer can be done without modifying the others. PHP has a lot of features and capabilities that allow the implementation of a multi-tier oriented methodology to programming. The common layers that can be identified with a web application are: . Content layer . Logic layer . Presentation layer Each layer encapsulates a specific part of the application: The Content Layer The content layer consists of components that provide access routes to the application’s data. All the program components built on this layer must go through it to access the application’s data. The most important entity in this layer is the data model. The data model defines how you store your data and how you should manipulate it. It is best to select a data modeling strategy for your content before you actually start adding to the content. The most common data modeling alternatives are: . Plain files model . Relational database model . XML model . Hybrid models These types should be abstracted to the high-level program code using common objects. These are called Data Access Components (or Data Access Objects, simply DAOs). Plain Files Model The idea behind this model is quite simple: you can use binary or text files to store data. The model consists of textual descriptions of the data structure and names to be used for the data files. There are some applications where this type of a data model would be highly useful. For example, massive search engines using indices or hashing with plain files run much faster and have easier maintenance than gigantic data stores. This model can use a very wide range of file structures, from plain sequential files to b-trees, b* trees, b+ trees, hash tables, binomial heaps, union-find structures, and many others. Let’s consider building a web-based poll application. Here we have different polls with a number of options each, we have votes and might even have comments about polls. We could design a data model based on plain files like this: Polls file: Binary file, fixed length registers. Pollname: 40 bytes. Question: 250 bytes. Options file: Binary file, fixed length registers. Pollname: 40 bytes. Option: 80 bytes. Page 529
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