3-Tier Architectures

(or 3-Tier Applications)

Chuck Cusack
Spring 2004
 Simple Programs
• The programs you have written so far have
probably had the following properties:
– Input/output was text-based, GUI, and/or with files
– Ran locally—that is, they had to be downloaded to the
machine on which you (or your T.A.) ran them
– The data had to be on the same machine as your
program
– Were not able to be run simultaneously by more than
one person.
– Had no interaction with other programs
• In the real world, most software is much more
complicated than this
 Databases and the Web
• Many real applications are different than what you have
programmed in two important ways
– They make extensive use of databases to store data
– They are accessible from anywhere by multiple people
simultaneously via the World Wide Web
• Sites like amazon.com and ebay.com are entirely database
driven—that is, each time you go to the site, what you see
is determined by data in their database
• As we will see, applications that use databases and are web
accessible can be implemented using a 3-tier architectural
model
 3-Tier Architectures
• Definition: A 3-tier architecture is one which has
a client tier, a middle tier, and a database tier.
– The database tier manages the database
– The middle tier contains most of the logic and
communicates between the other tiers
– The client tier is the interface between the user and the
system
• Definition: An n-tier architecture is one which
has n tiers, usually including a database tier, a
client tier, and n-2 tiers in between.
 Thin-Client 3-Tier Models
• The thin-client 3-tier model has these tiers:
– The database management system (DBMS)
– The main application software
– A web browser
• Examples
– http://www.cs.hope.edu/~cusack/Teaching
– http://ebay.com*
– http://amazon.com*
– Many other websites.
(*These may actually be n-tier)
 Thick-Client 3-Tier Models
• The thick-client 3-tier model has these tiers:
– The database management system (DBMS)
– The main application software
– Some sort of interface software which must be installed
on each client machine
• Examples:
– BD4 (http://bd4.amristar.com.au/)
– World of Warcraft
– Desktop applets that display weather, etc.
– RealPlayer and other applications that download CD
information from the Web
 Another 3-Tier Model
• Another common model has these tiers:
– The database management system (DBMS) and a persistence
manager which controls all data flow into and out of the database
– The main application software
– A GUI (thin or thick)
• The main difference here is that the main application
software is not allowed to interact directly with the
database
• You could also think of this as a 4-tier architecture:
– The database management system (DBMS)
– A persistence manager
– The main application software
– A GUI (thin or thick)
 2- and n-Tier Models
• The 2-tier model is more simple, but more limited, than a
3-tier model, and often includes
– The database management system (DBMS)
– The main application software, including GUI
• Here, the entire application is generally run on the client
machine (Certainly a thick-client)
• In some contexts, the 2-tier model is also know as the
client-server model, where the server can be something
other than a database
• In general an n-tier model will have
– The database management system (DBMS)
– (n-2) application layers
– A GUI (thin or thick)
 n-Tier Questions
• The following are important questions one must
ask when thinking about n-tier architectures
– How many tiers should be used?
– What tasks should be done by each tier? In other words,
how exactly should the layers be divided?
– Should I use thin or thick clients?
– Should the application be web-accessible?
– How should connections to the database be managed?
– What database management system (DBMS) should be
used?
– What languages(s), platform(s), and software should
the system use?
 n-Tier Answers
• The purpose of these notes is not to
– Present clear answers to all of the questions on the
previous slide
– Be the authoritative source for information about n-tier
architectures
– Make you an expert in n-tier architectures
• Rather, the purpose is to
– Introduce you to the concept of n-tier architectures
– Get you to start thinking about the issues involved
– Give you partial answers to some of the question
 Database Choices
• There are many popular database management systems
(DBMSs), including
– IBM DB2
– Oracle
– Microsoft SQL Server
– Microsoft Access
– MySQL
– PostgreSQL
• Which one you should use depends on many factors,
including number of expected users, size of the application
and/or the database, budget, etc.
• Fortunately, the interfaces to these DBMSs have a lot in
common, so if you learn to use one, much of what you
learn is transferable to the others
 Middle Tier Choices
• Almost anything is possible, with some common choices
being
– Java – Visual Basic
– JSP – C#
– PHP – .NET
– C++ – ASP
– Perl
• However, whether or not the client will be thin or thick
will influence this choice
• For a thin client, the obvious middle tier choices are
– Java applets, JSP, PHP, ASP, and Perl
– Of course with all of these, HTML is involved as well
 Client Choices
• Thin clients are generally web browsers, so the important
choice was made in the middle tier
• For thick clients, we might use
– Java application
– C++ application with GUI provided by
• MFC (Microsoft Foundation Classes)
• Tcl/Tk (Tool command language)
• GTK (Gimp ToolKit)
• Qt
– Visual Basic (I suppose…)
 Examples
• You could build a 3-tier application which has
– Oracle DBMS, C++ middle, and C++/MFC client
– MySQL DBMS, PHP middle, and web browser client
(http://www.cs.hope.edu/~cusack/Teaching uses these choices)
– IBM DB2 DBMS, JSP middle, and Java applet client
– MySQL DBMS, Java Applet middle, and web browser client
(Algoraph uses these choices)
– MySQL DBMS, JSP middle, and web browser client
(http://graph.computinggames.org uses these choices)
– Microsoft SQL Server DBMS with ASP access control, ASP
middle, and web browser client
 Common Choices
• The following is a very common way to make very
powerful websites:
– Thin-client 3-tier architecture model
– Database tier: MySQL
– Middle tier: PHP/HTML and Java applets
– Client tier: Your favorite web browser
• It is important that you realize that
– Thin clients are not always the best choice
– Three is not always the best number of tiers
– These database and language choices are not the best or only
choices, but are reasonable for many applications
– The approach we take to design and implement 3-tier applications
is not the only, and not necessarily the best way
 References

• Hugh E. Williams & David Lane, Web Database