What is 3-tier in simple definition and advantages?

Question

I am working on converting my one page to 3 layers.

I believe advantages were that it was more organized.

I am getting confused as what's happening since I have 3 layers now and I realized I don't have a clear understanding of what 3-tier is.

This is what I know or think I know

Data Access Layer = Gets all the connections/values

Business Access Layer = Runs those connections and values from DAL. Not sure but does the error checking here?

Presentation layer = This is where I am more confused...This part calls out BAL? Why have 3 layers if this is true?

Answer 1

The presentation layer interacts with the user, gets their requests and shows them information. Let's use a banking application as an example. A user wants to log into their account, get their balance, and transfer some finds.

Presentation Layer: Gives the user a login prompt, gets the user's login information, tell them they logged in successfully, shows them their balance, offers them the option to transfer funds, gets the transfer details, tells the user the transfer was approved.

Business Access Layer: Validates the user's login information. Calculates the balance to show to the user. Decides that the transfer is allowed and approves it.

Data Access Layer: Stores the user's login information. Stores the account balance information, held funds, and so on.

Basically:

Data Access Layer stores information. Business Access Layer decides what information to retrieve, makes decisions based on it, passes on the results. Presentation Layer gets information from and gives information to the user.

Answer 2

This article here seems a good summary: http://en.wikipedia.org/wiki/Multitier_architecture#Three-tier_architecture

The main point of it that I would highlight is that you can run each tier on a different environment. You could run your presentation tier on a webserver with say asp.net. You could run your business access layer on another server (an application server) purhaps using a windows service, or WCF service etc. The last layer can sit on or near a database server.

The advantage of being able to potentially separate out your layers into different physical locations means you can increase scalability. An example could be that your webserver is struggling wiht lots of requests. You decide to have two webservers which call your business accesss layer.

Another advantage is maintability. It is easier to debug data access layer issues if you have all of your data access code next to each other. You have a project that will cut down the code by possibly a third so that finding what you are looking for is easier.

Yet another advantage could be swapability. Say you dont want to use asp.net anymore. You can swap in a php web layer and have that communicate to your .NET business logic more easily.

Essentially the advantages I have mentioned are pretty much modularity/granularity benefits.

Answer 3

First off, a "tier" is not synonymous with "layer".

A layer is a logical separation.

A tier is a potentially physical separation, usually at the machine or process level.

(see: http://pranshujain.wordpress.com/2006/09/15/layers-and-tiers/)

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That aside the reason for those three layers is to give you the ability to swap them out at will. For example, you might have multiple Presentation Layers (Web, Mobile, Desktop App). The purpose here is to separate presentation from everything else so that you can make business logic changes without having to redo all of your different presentation code.

A Business Object layer is there to encapsulate and enforce all of your regular logic. For example, you might say that a birthdates must be before today's date. A number of devs like to keep this separate from both storage and presentation.

The Data Access Layer is responsible for marshaling your data from the Business objects into your actual storage mechanism. With the idea being that you could decide to store things in SQL Server or Oracle or flat files and the rest of the application shouldn't care one way or the other.

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Are all of those layers necessary? No, but for a large number of applications they help when it comes time to change things.

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Now, concerning Tiers. The desktop, or browser, could be considered one Tier. Javascript code and HTML would execute here. If you have a desktop app, this is where that lives.

Another tier would be your Web Server or Web Services interface. This is responsible for serving up the html for rendering and usage client side. It usually houses both the Presentation rendering and the Business Object layer.

Yet another tier is your actual storage mechanism. For example SQL Server or Oracle.

It's entirely possible that you might have 5 or 6 different Tier's depending on how advanced (or sloppy...) your architecture is; however the number is usually 2 and no more than 3.

It's also entirely possible that you have only 1 layer or a dozen. However, the number here is usually 2 to 3. Again, depending on your architecture.