ClientServer Computing

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Client/Server Computing

• Basics

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Contents

• Welcome to Client/Server Computing
• What is client/server?
• Various client/server systems
• Fat servers or fat clients
• 2-tier versus 3-tier
• Client/Server Building Blocks
• A one-size-fits-all model
• Inside the building blocks
• Middleware

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Welcome to Client/Server Computing
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What Is Client/Server?(1/2)

• Client/Sever Computing
• The representative model of distributed computing
• Characteristics of Client/Server Systems
• Clean separation of services
• Server service provider
• Client service consumer
• Resources are shared by many clients
• Asymmetrical protocols
• Clients always initiate the dialog
• Servers are passively awaiting requests

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What Is Client/Server?(2/2)

• Transparency of server location
• Mix-and-match of HW or OS-independent SW
  platforms
• Message-based exchanges for client/server
  interaction
• Encapsulation of services
• Servers determine how to process clients
  requests
• Servers can be upgraded without affecting the
  clients
• Scalability
• Integrity by centrally managing the server code
  and data

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Various Client/Server Systems (1/8)

• File Server
• A very primitive form of data service
• Require many message exchanges over the network
• Useful for sharing files across a network

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Various Client/Server Systems (2/8)

• DB Servers
• The client passes SQL requests to the DB server
• The server uses its own computing power to find
  the requested data
• Useful for data warehousing

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Various Client/Server Systems (3/8)

• Transaction Servers
• The client invokes servers remote procedures,
  which execute a group of SQL statements
• A single request/reply message (vs. one
  request/reply message for each SQL statement in
  DB server)
• The client component includes a GUI
• The server component consists of SQL transactions
  called OLTP
• TP-Lite Based on the stored procedures provided
  by DB vendors
• TP-Heavy Based on the TP monitors provided by
  OLTP vendors

OnLine Transaction Processing
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Various Client/Server Systems (4/8)
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Various Client/Server Systems (5/8)

• Groupware Servers
• Manage semi-structures information (e.g., text,
  image, mail, bulletin boards, and the flow of
  work), so that people can contact directly with
  other people
• The communication middleware is typically
  vendor-specific
• Recently, the Internet is becoming the middleware
  platform as in Netscape and IBM/Lotus

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Various Client/Server Systems (6/8)

• Object Application Servers
• The client/server application is written as a set
  of communication objects using an Object Request
  Broker(ORB)
• The client invokes a method on a remote object
• The ORB locates an object instance, invoke the
  method and return the results
• Server objects support concurrency and sharing
• The ORB and CORBA Application Servers
• Examples of commercial ORBs complying OMGs CORBA
  standard Ionas Orbix, JavaSofts Java IDL,
  BEAs ObjectBroker, IBMs SOM
• CORBA application servers (Also called OTMs) like
  BEAs M3
• DCOM
• Distributed Component Object Model(DCOM) by MS
• COM is the latest name for this technology

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Various Client/Server Systems (7/8)
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Various Client/Server Systems(8/8)

• Web Application Servers
• Augment standard HTTP servers with server-side
  component frameworks
• Functionally, very similar to object servers

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Fat Servers or Fat Clients?(1/2)

• Client/Server Application
• Can be differentiated by how the distributed
  application is split between the client and the
  server

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Fat Servers or Fat Clients?(2/2)

• Fat Client Applications
• The more traditional form of client/server
• The bulk of the application runs on the client
  side
• The clients know how the data is organized and
  stored on the server side
• Examples file server and database servers
• Fat Server Applications
• Most of the code runs on the servers
• Easier to manage and deploy on the network
• Minimize network interchanges by creating more
  abstract levels of service by encapsulation
• The client provides the GUI and interacts through
  method invocation
• Example groupware, transaction, and Web servers

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2-tier versus 3-tier(1/7)

• 2-tier, 3-tier, and N-tier?
• The same basic idea with fat clients and fat
  servers
• All about how to split the client/server
  application into functional units
• The most typical functional units
• Presentation logic user interface
• Business(/Application) logic actual task
• Data logic shared data
• Many possible variations of multi-tier
  architecture depending on
• How to split the application
• The middleware to communicate between the tiers

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2-tier versus 3-tier (2/7)
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2-tier versus 3-tier (3/7)

• 2-tier
• Good for creating departmental applications
• Quick and simple application development with
  visual builder tool
• Example small-scale groupware, simple Web
  publishing application
• The business logic is embedded either inside the
  user interface or within the DB on the server (or
  both)
• Intrinsic problems of 2-tier C/S
• Not scalable
• Unmanageable when being deployed beyond the
  departmental LAN

2-tier hits a brick wall
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2-tier versus 3-tier (4/7)

• 3-tier to the Rescue
• The business logic is in the middle-tier
• managed and deployed separately from the GUI and
  the DB
• Meet the requirements of large-scale Internet and
  intranet client/server applications
• Integrate data from multiple sources
• Easier to manage and deploy on the network
  especially with Java applets and beans
• Minimize network interchanges by creating
  abstract levels of services
• Provide better security by not exposing the DB
  schema to the client

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2-tier versus 3-tier (5/7)

• When Should We Use 3-tier?

2-tier and 3-tier compared (source GantnerGroup)
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2-tier versus 3-tier(6/7)

• Should use 3-tier if an application has any of
  following characteristics (according to
  GatnerGroup)
• Many application services or classes ( gt 50)
• Applications programmed in different languages or
  written by different organizations
• Two or more heterogeneous data sources
• An application life that is longer than 3 years
  especially with many modifications or additions
• A high-volume workload (transactions/day gt 50,000
  or concurrent users on the same system accessing
  the same DB gt 300 )
• Significant inter-application or inter-enterprise
  communication like EDI

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2-tier versus 3-tier(7/7)

• The Middle-tier in Most 3-tier Applications
• is not implemented as a monolithic program
• Instead, is implemented as a collection of
  components
• So, most of the time 3-tier is really N-tier
• Component
• Automates a relatively small business function
• Call other components to help it implement a
  request
• Clients frequently combine several middle-tier
  components within a single business transaction
• Some components may act as gateway that
  encapsulate legacy applications running on
  mainframe

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Client/Server Building Block
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A One-Size-Fits-All Model

• Client/Server Model Has a Wide Spectrum from the
  Tiny to the Intergalactic
• Three Basic Building Blocks of Client/Server
  Model
• Client
• Server
• Middleware

Server
Client
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Inside the Building Blocks(1/2)

• Inside the Client Building Block
• GUI, OOUI(Object Oriented UI)
• Thin clients require a Web browser to download
  JavaBeans and applet on demand
• The OS most often pass the non-local services to
  the middleware building block
• The client also runs a component of the
  DSM(Distributed System Management) element
• Inside the Server Building Block
• The five server platform
• SQL DB servers, TP monitors, groupware servers,
  object servers, Web
• The server also runs a DSM component

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Inside the Building Blocks(2/2)

• Inside the Middleware Building Block
• Run on both the client and server sides of an
  application
• 3 categories Transport stack, NOSs,
  Service-specific MW

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Middleware(1/4)

• Definition
• All the distributed software needed to support
  client/server interactions
• The glue that lets a client obtain a service from
  a server
• Scope
• Start with the API set on the client side
• Cover the transmission of the request and the
  resulting response
• Does not include the software that provides the
  actual service nor the database
• Does not include the user interface

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Middleware(2/4)

• The Role of Middleware in N-tier World
• As a platform and pipes
• Pipes
• Provide the inter-component (and
  inter-application) communication services
• Example RPCs, MOMs, and ORBs
• Platforms
• The application servers that run the server-side
  components
• Used across multiple OSs to provide a unified
  view of the distributed environment
• Example TP Monitors, Object Transaction
  Monitors, Web Application Servers

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Middleware (3/4)

• Types of pipes
• General pipes
• Include the communication stacks, distributed
  directories, authentication services, network
  time, RPCs, distributed file and print services
• Commercial products DCE, TCP/IP, IPX/SPX,
  NetBIOS
• Service-specific pipes
• Provide a particular client/server type of
  service
• DB-specific middleware ODBC, JDBC, Oracle
  SQLNet
• OLTP-specific middleware Tuxedos ATMI,
  X/Opens TxRPC, CORBAs OTS, MSs DTC and TIP
• Groupware-specific middleware MAPI, SMTP, POP3,
  IMAP, Lotus Notes calls

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Middleware (4/4)

• Object-specific middleware OMGs CORBA/IIOP,
  MSs COM, and JavaSofts RMI-over-IIOP
• Internet-specific middleware HTTP, CGI, XML
• System management-specific middleware SNMP,
  CMIP, RMON, and ORBs