Distributed Systems: Client/Server Computing

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Distributed Systems Client/Server Computing

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Client/Server Computing

• Major trend in information systems, replacing
  both the centralized approaches and alternative
  forms of distributed computing
• Main concepts
• Clients
• Single-user PCs or workstations with
  user-friendly, usually graphical, interfaces
• Servers
• Provide a set of shared user services to clients
• Example Database server, controlling a
  relational database
• Allows many clients to share access to the same
  database
• Can use a high-performance computer system to
  manage the database
• Network
• Clients and servers are connected to a LAN or WAN
  or by an internet of networks

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Client/Server Computing (cont.)
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Client/Server Computing (cont.)

• Client/server architecture
• Platforms and the OSs may be different
• Communications protocols and the applications
  supported must be common
• Allocation of application-level tasks between
  clients and servers is central to the
  architecture. It must
• Optimize platform and network resources
• Optimize the ability of users to perform various
  tasks and to cooperate with one another
• Presentation services in client must provide a
  user interface (e.g., GUI) that is easy to use,
  powerful, and flexible

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Client/Server Computing (cont.)

• Relational database applications
• Server is responsible for maintaining the
  database using a database management system
  software module
• Client has applications that make use of the
  database
• Client/server interactions are in the form of
  transactions in which the client makes a database
  request and receives a database response, e.g.,
  the Structured Query Language (SQL)
• Not all the application logic (e.g., data
  analysis) has to reside in the client

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Client/Server Computing (cont.)

• Example Online access for record lookup
• Server is maintaining a database of 1 million
  records
• User wants to perform a search using a certain
  search criteria (e.g., records older than 1999)
  and expects to find at most a few records
• First query yields a server response of 100,000
  records
• User sends another query adding new qualifiers
• Response returns 1,000 records
• Client issues third request with additional
  qualifiers and receives a single record

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Client/Server Computing (cont.)

• Example 2 Online access for record lookup
• Server is maintaining a database of 1 million
  records
• Client would like to compute the grant total of a
  certain field across many records
• The query results in 300,000 records transmitted,
  which would be a misuse of the client/server
  architecture
• Solution move part of the application logic to
  the server

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Client/Server Computing (cont.)

• Classes of client/server applications
• Wide spectrum of implementations in partitioning
  work between client and server
• Host-based processing
• All processing done on a central host
• Users station is limited to the role of a
  terminal emulator, even if it employs a
  microcomputer
• Traditional mainframe computing, not a true
  client/server architecture

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Client/Server Computing (cont.)

• Classes of client/server applications (cont.)
• Server-based processing
• Most basic class of client/server configuration,
  early implementations (thin client model)
• Client provides a graphical user interface
• Server provides all processing
• This configuration does not provide significant
  improvements in the use of resources

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Client/Server Computing (cont.)

• Classes of client/server applications (cont.)
• Client-based processing
• All application processing is done by the client,
  except for the data validation routines
• Advantage allows users to employ applications
  best suited for local needs
• Most common client-server approach in use today
• Fat client model

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Client/Server Computing (cont.)

• Classes of client/server applications (cont.)
• Cooperative processing
• Distribution of application processing is
  optimized, taking advantage of the strengths of
  both client and server machines and of the best
  distribution of data
• More difficult to set up and maintain
• Can lead to greater network efficiency and
  highest productivity gains

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Client/Server Computing (cont.)

• Threetier client/server architecture
• Application software is distributed between three
  types of machines a (thin) client, a middle-tier
  server, and a backend server
• The middle-tier server provides a gateway
  function between the clients and the different
  backend servers
• Convert protocols and map from one type of
  database query to another
• Can merge/integrate results from different
  sources
• Can serve as a gateway between the new desktop
  applications and the backend legacy applications
• The middle-tier server is both a server and a
  client server for the thin client and client for
  the backend server

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Client/Server Computing (cont.)

• File cache consistency
• To reduce the performance penalty of accessing
  remote files, individual systems can use file
  caches
• Typical distribution mechanism for caching files
  among network workstations a file access request
  in the client results in (1) request to client
  cache (file traffic), then to local disk (disk
  traffic), then to file server (server
  traffic), cache and then disk
• Problem caches can become inconsistent when the
  remote data are changed and local cache is
  obsolete
• Solutions (a) File locking techniques to prevent
  simultaneous file access by more than one client,
  and (b) Sprite approach on a write by a client,
  all other clients that opened the file are
  notified

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Client/Server Computing (cont.)

• Middleware
• The rapid development and deployment of
  client/server products has outpaced the standards
  efforts difficult to implement an integrated,
  multi-vendor client/server solution
• Developers need a set of tools that provide a
  uniform means and style of access to system
  resources across all platforms
• Middleware
• A set of standard programming interfaces and
  protocols that sit between the application above
  and the communications software and operating
  system below
• Hide the complexities and disparities of
  different network protocols and operating systems
• Client and server vendors provide a number of
  middleware packages as options
• The user selects a middleware strategy and then
  assembles equipment from different vendors that
  support that strategy

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Client/Server Computing (cont.)

• Middleware architecture
• The role of the middleware depends on the style
  of client/server computing used
• There is both a client and a server component of
  middleware
• Goal allow an application or user at the client
  to access different services on servers, even
  though the servers implementations may be
  different
• Many relational databases, even though they
  support Structured Query Language (SQL), they
  also add proprietary extensions to SQL
• Example A distributed system supporting a
  personnel department is using both Gupta and
  Oracle databases

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Client/Server Computing (cont.)

• A logical view of a distributed system using
  middleware
• The entire distributed system can be viewed as a
  set of applications and resources available to
  its users
• Applications run over a uniform Applications
  Programming Interface (API)
• The middleware operates over all client and
  server platforms and is responsible for routing
  client requests to the appropriate servers

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Client/Server Computing (cont.)

• An example of the use of middleware to integrate
  different products