1. Lecture 1: Multi-tier Architecture Overview
IS333: MULTI-TIER APPLICATION DEVELOPMENT

2. Objectives
To explain the concept of multi-tier design.
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3. What is a “Tier”? A "tier" can be referred to as a "layer".
A wedding cake is said to have tiers while a chocolate cake is said to have layers, but they mean the same thing.
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4. Tiers/ Layers Characteristics - in Software World
Each tier/layer should be able to be constructed separately, possibly by different teams of people with different skills.
Several tiers/layers should be able to be joined together to make a whole "something".
Each tier/layer should contribute something different to the whole. A chocolate layer cake, for example, has layers of chocolate cake and cream.
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5. Tiers/ Layers Characteristics - in Software World (cont.)
There must also be some sort of boundary between one tier and another.
Each tier/layer should not be able to operate independently without interaction with other tiers/layers.
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6. Basic Idea of Multi-tier Design
The basic idea of a multi-tier design is that all of the logical functions of the application are (or at least have the potential to be) separated on provides output to other layers.
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7. The Standard Logical Layers
Presentation logic: The user interface (UI) which displays data to the user and accepts input from the user. In a web application, this is the part which receives the HTTP request and returns the HTML response.
Business logic: Handles data validation, business rules and task-specific behavior.
Data Access logic: Communicates with the database by constructing SQL queries and executing them via the relevant API.
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8. Example of Business Logic
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9. Tiers vs. Layers in Software
Tier is a physical unit, where the code / process runs. E.g.: client, application server, database server
Layer is a logical unit, how to organize the code. E.g.: presentation (view), controller, models, repository, data access
A three-layer solution could easily be deployed on a single tier, such as a personal workstation
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10. 1-Tier Architecture All 3 layers are on the same machine
All code and processing kept on a single machine
Presentation, Logic, Data layers are tightly connected
Scalability: Single processor means hard to increase volume of processing
Portability: Moving to a new machine may mean rewriting everything
Maintenance: Changing the layer requires changing other layers
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11. 1-Tier Architecture
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12. 1-Tier Architecture (blurry boundary)
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13. 2-Tier Architecture Database runs on Server
Separated from client
Easy to switch to a different database
Presentation and logic layers still tightly connected
Heavy load on server
Potential congestion on network
Presentation still tied to business logic
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14. 2-Tier Architecture
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15. 3-Tier Architecture
Each layer can potentially run on a different machine
Presentation, logic, data layers disconnected
Presentation Layer
contains
Presentation logic
Business Layer
Business logic
Data
Layer
Data Access logic
Database
Client
Server
DB Server
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16. 3-Tier Architecture
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17. The Rules of the Multi-tier Architecture
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. Thus business logic can only reside in the Business layer, presentation logic in the Presentation layer, and data access logic in the Data Access layer.
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18. Requests and Responses in the 3 Tier Architecture
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19. The Rules of the Multi-tier Architecture (Cont.)
The Presentation layer can only receive requests from, and return responses to, an outside agent. This is usually a person, but may be another piece of software.
The Presentation layer can only send requests to, and receive responses from, the Business layer. It cannot have direct access to either the database or the Data Access layer.
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20. The Rules of the Multi-tier Architecture (Cont.)
The Business layer can only receive requests from, and return response to, the Presentation layer.
The Business layer can only send requests to, and receive responses from, the Data Access layer. It cannot access the database directly.
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21. The Rules of the Multi-tier Architecture (Cont.)
The Data Access layer can only receive requests from, and return responses to, the Business layer. It cannot issue requests to anything other than the DBMS which it supports.
Each layer should be totally unaware of the inner workings of the other layers.
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22. The 3-Tier Architecture for Web Application
Presentation Layer
Static or dynamically generated content rendered by the browser, e.g., HTML, CSS, Javascript (front-end)
Logic Layer
A dynamic content processing and generation level application server, e.g., Java EE, ASP.NET, PHP, ColdFusion platform (middleware)
Data Layer
A database, comprising both data sets and the database management system or RDBMS software that manages and provides access to the data (back-end)
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23. Web services or external HTML pages
N-Tier Architecture
Elsewhere Layer
Web services or external HTML pages
Requester/ Consumer Layer
Presentation Layer
Business-Logic Layer
Accessor Layer
Persistance Layer
Web browser/ XML parser or other client
Validation/ Processing/ Content Providing modules
Database
Get Update/ Delete modules
Display modules
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24. N-Tier Architecture (cont.)
This architecture differs from standard (3-Tier) in three respects:
It explicitly states that the persistence layer (the database and its database management system) and the accessor layer, which feeds SQL queries to the persistence layer, are separated.
It explicitly lists the requester/consumer layer as part of the application, even though the software running there is usually not under the control of the developer building the rest of the application (in the case of a browser, it’s from the Mozilla team or Microsoft or whomever).
It acknowledges that remote resources (the elsewhere layer) can be an integral part of a multi-tier application.
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25. N-Tier Architecture (cont.)
Persistence Layer: The database or other storage mechanism that keeps data in an organized way.
Accessor Layer: The programs that pass queries to the persistence layer (in other words, the programs in which structured query language (SQL) statements are hard- coded). These programs expose “get” and “set” functions.
Business-Logic Layer: The programs that process user input and stored data and come up with a useful result.
Presentation Layer: The programs that format the results of the logic layer as hypertext markup language (HTML), extensible markup language (XML), or whatever else the user requires.
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26. N-Tier Architecture (cont.)
Requester/consumer Layer: The Web browser, XML parser, or other client that makes requests of the application and receives its output.
Elsewhere Layer: The external Web services, HTML pages, and other resources to which applications make reference.
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27. Advantages of Multi-Tier Architecture
Flexibility - By separating the business logic of an application from its presentation logic, a Multi-Tier architecture makes the application much more flexible to changes.
Maintainability - Changes to the components in one layer should have no effect on any others layers. Also, if different layers require different skills (such as HTML/CSS in the presentation layer, PHP/Java in the business layer, SQL in the data access layer) then these can be managed by independent teams with skills in those specific areas.
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28. Advantages of Multi-Tier Architecture
Reusability - Separating the application into multiple layers makes it easier to implement re-usable components. A single component in the business layer, for example, may be accessed by multiple components in the presentation layer, or even by several different presentation layers (such as desktop and the web) at the same time.
Scalability - A Multi-Tier architecture allows distribution of application components across multiple servers thus making the system much more scalable.
Reliability - A Multi-Tier architecture, if deployed on multiple servers, makes it easier to increase reliability of a system by implementing multiple levels of redundancy.
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29. References
Multi-tier Application Programming with PHP, Practical Guide for Architect and Programmer, David Wall, 1st Edition
architecture.html
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