Transcript Model View Controller

Architectures
MVC and other n-tier Architectures
Traditional Host Systems
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A Central Processing System (Mainframe)
provides all processing.
Local Terminals are responsible for display
and keyboard for user input and viewing
capabilities. Local Terminals do not contain
any intelligent processing capabilities.
Traditional Host Systems
Distributed Systems
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Distributed System
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Both data and transaction processing are divided
between one or more computers connected by a
network, each computer playing a specific role in
the system.
Replication
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Ensures data at all sites in a distributed system
reflects any changes made anywhere in the
system.
Distributed Systems
Client/Server 2-Tier Architecture
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Two-tier client/server architectures have 2 essential components
 A Client PC and
 A Database Server
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2-Tier Considerations:
 Client program accesses database directly
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Requires a code change to port to a different database
Potential bottleneck for data requests
High volume of traffic due to data shipping
Client program executes application logic
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Limited by processing capability of client workstation (memory, CPU)
Requires application code to be distributed to each client workstation
Advantages
Disadvantages
Development Issues:
• Simple structure
• Easy to setup and maintain
Development Issues:
• Complex application rules difficult to
implement in database server – requires
more code for the client
• Complex application rules difficult to
implement in client and have poor
performance
• Changes to business logic not
automatically enforced by a server –
changes require new client side software to
be distributed and installed
• Not portable to other database server
platforms
Performance:
• Adequate performance for low to medium
volume environments
• Business logic and database are
physically close, which provides higher
performance.
Performance:
• Inadequate performance for medium to
high volume environments, since database
server is required to perform business
logic. This slows down database operations
on database server.
3-Tier Client/Server Architecture
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3-Tier client-server architectures have 3 essential components:
 A Client PC
 An Application Server
 A Database Server
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3-Tier Architecture Considerations:
 Client program contains presentation logic only
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Less resources needed for client workstation
No client modification if database location changes
Less code to distribute to client workstations
One server handles many client requests
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More resources available for server program
Reduces data traffic on the network
3-Tier Client/Server Architecture
Advantages
Disadvantages
Development Issues:
Development Issues:
• Complex application rules easy to
• More complex structure
implement in application server
• More difficult to setup and maintain.
• Business logic off-loaded from database
server and client, which improves
performance
• Changes to business logic automatically
enforced by server – changes require only
new application server software to be
installed
• Application server logic is portable to
other database server platforms by virtue of
the application software
Performance:
• Superior performance for medium to high
volume environments
Performance:
• The physical separation of application
servers containing business logic functions
and database servers containing databases
may moderately affect performance.
3-Tier Architectures
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A 3-tier architecture is one which has a client tier,
a middle tier, and a database tier.
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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
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
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The thin-client 3-tier model has these tiers:
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The database management system (DBMS)
The main application software
A web browser
Examples
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http://cse.unl.edu/~sscott/teach
http://contests.unl.edu
http://ebay.com*
http://amazon.com*
Lotus Notes Web Client*
(*These may actually be n-tier)
Thick-Client 3-Tier Models
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The thick-client 3-tier model has these tiers:
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The database management system (DBMS)
The main application software
Some sort of interface software which must be installed
on each client machine
Examples:
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Lotus Notes
Desktop applets that display weather, etc.
RealPlayer and other applications that download CD
information from the Web
Another 3-Tier Model
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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)
n-Tier Models
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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
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The following are important questions one must
ask when thinking about n-tier architectures
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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
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The purpose of these notes is not to
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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
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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
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There are many popular database management
systems (DBMSs), including
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IBM DB2
Oracle
Microsoft SQL Server
Microsoft Access
MySQL
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, most
of what you learn is transferable to the others
Middle Tier Choices
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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
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Thin clients are generally web browsers, so the important
choice was made in the middle tier
For thick clients, we might use
 Java applications
 C++ applications with GUI provided by
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MFC
Tcl/Tk
GTK
Qt
(Microsoft Foundation Classes)
(Tool command language)
(Gimp ToolKit)
MVC Architecture
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It is common to think of an application as
having three main layers:
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presentation (UI),
application logic, and
resource management.
In MVC, the presentation layer is split into
controller and view. The most important
separation is between presentation and
application logic.
MVC Controller
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Model
 The domain-specific representation of the information on which
the application operates. The model is another name for the
application logic layer. Application (or domain) logic adds
meaning to raw data (e.g., calculating if today is the user’s
birthday, or the totals, taxes and shipping charges for shopping
cart items). Many applications use a persistent storage
mechanism (such as a database) to store data.
View
 Renders the model into a form suitable for interaction, typically a
user interface element. MVC is often seen in web applications,
where the view is the HTML page and the code which gathers
dynamic data for the page.
Controller
 Processes and responds to events, typically user actions, and
may invoke changes on the model and view.
MVC
MVC Control Flow
The control flow generally works as follows:
 The user interacts with the user interface in some way (e.g., user
presses a button)
 A controller handles the input event from the user interface, often via a
registered handler or callback.
 The controller accesses the model, possibly updating it in a way
appropriate to the user’s action (e.g., controller updates user’s
shopping cart). Complex controllers are often structured using the
command pattern to encapsulate actions and simplify extension.
 A view uses the model to generate an appropriate user interface (e.g.,
view produces a screen listing the shopping cart contents). The view
gets its own data from the model. The model has no direct knowledge
of the view. (However, the observer pattern can be used to allow the
model to indirectly notify interested parties, potentially including views,
of a change.)
 The user interface waits for further user interactions, which begins the
cycle a new.
MVC and Java EE