Multi-Tier Architecture

Episode Breakdown

18:20 Presentation Tier

The user interface is stored in the client or browser. This gives the user access to the application and presents data to the user. It can permit manipulation of that data or entry of new data.

The interface can be traditional such as native apps on desktop or mobile or web-based applications. The client-side stores and presents data in tables, graphs, and charts.

It interacts only with the application tier where it sends requests and receives responses. This keeps the user interface separate from the logic of the application and the data storage.

22:59 Application Tier

The application tier is also known as mid tier, logic, or business tier. It exists on the server and assists in resource sharing between the user interface and the data source.

The functionality of the app exists in the application tier and consists of business and data rules such as business algorithms, legal regulations, and consistency of data structures. It is used to solve critical business problems and is written in one of the “typical” programming languages such as C#, C++, Ruby, Python, etc.

It interacts with both the presentation tier and the data tier serving as a buffer between client and database. The application tier receives requests from the presentation tier and sends responses back. It also sends requests to the data tier and receives responses.

30:04 Data Tier

The database servers store permanent data. They are used for information storage and retrieval. Most are written in SQL.

The data tier serves to keep data separate from the application and business logic. It receives requests from the application tier and returns responses.

40:25 Rules of Three Tier Architecture

This section of the episode was paraphrased from Tony Marston’s material.

The code for each layer must be contained with separate files which can be maintained separately. Each layer may only contain code which belongs in that layer.

The Presentation layer can only receive requests from, and return responses to, an outside agent. The Presentation layer can only send requests to, and receive responses from, the Business layer. The Business layer can only receive requests from, and return responses to, the Presentation layer. The Business layer can only send requests to, and receive responses from, the Data Access layer. The Data Access layer can only receive requests from, and return responses to, the Business layer.

Each layer should be totally unaware of the inner workings of the other layers. The Business layer must be database-agnostic and not know or care about the inner workings of the Data Access object. The presentation layer may take the data and construct an HTML document, a PDF document, a CSV file, or process it in some other way, but that should be totally irrelevant to the Business layer.

50:13 Benefits and Drawbacks

The advantages of using a three tier approach include reusability or the ablity to share and reuse components distributing them as necessary. Also the flexibility to deploy and redeploy components, keep up with changes, and grow the application. It allows for scalability by dividing large projects into simpler ones so the workload may be distributed among developers. Finally the mid tier ensures that only valid data is allowed providing better security and no direct access to the database.

The disadvantages include an increased complexity with more points of communication requiring a larger team. The physical separation of the three tiers may affect performance. Finally the system can be difficult to build and maintain.