Architectures

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Architectures

• MVC and other n-tier Architectures

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Traditional Host Systems

• 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.

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Traditional Host Systems
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Distributed Systems

• Distributed System
• 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
• Ensures data at all sites in a distributed system
  reflects any changes made anywhere in the system.

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Distributed Systems
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Client/Server 2-Tier Architecture

• Two-tier client/server architectures have 2
  essential components
• A Client PC and
• A Database Server
• 2-Tier Considerations
• Client program accesses database directly
• 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
• Limited by processing capability of client
  workstation (memory, CPU)
• Requires application code to be distributed to
  each client workstation

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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.
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3-Tier Client/Server Architecture

• 3-Tier client-server architectures have 3
  essential components
• A Client PC
• An Application Server
• A Database Server
• 3-Tier Architecture Considerations
• Client program contains presentation logic only
• 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
• More resources available for server program
• Reduces data traffic on the network

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3-Tier Client/Server Architecture
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Advantages Disadvantages
Development Issues Complex application rules easy to implement in application server 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 Development Issues More complex structure More difficult to setup and maintain.
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.
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3-Tier Architectures

• 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
• An n-tier architecture is one which has n tiers,
  usually including a database tier, a client tier,
  and n-2 tiers in between.

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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//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)

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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
• Lotus Notes
• Desktop applets that display weather, etc.
• RealPlayer and other applications that download
  CD information from the Web

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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)

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n-Tier Models

• In general an n-tier model will have
• The database management system (DBMS)
• (n-2) application layers
• A GUI (thin or thick)

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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?

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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

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Database Choices

• There are many popular database management
  systems (DBMSs), including
• 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

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Middle Tier Choices

• Almost anything is possible, with some common
  choices being

• Java
• JSP
• PHP
• C
• Perl

• Visual Basic
• C
• .NET
• ASP

• 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

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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 applications
• C applications with GUI provided by
• MFC (Microsoft Foundation Classes)
• Tcl/Tk (Tool command language)
• GTK (Gimp ToolKit)
• Qt

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MVC Architecture

• It is common to think of an application as having
  three main layers
• 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.

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MVC Controller

• 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 users 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.

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MVC
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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 users
  action (e.g., controller updates users 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.

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MVC and Java EE