Access Transparency
Enables local and remote information objects to be accessed using identical operations.
• Example: File system operations in NFS.
• Example: Navigation in the Web.
• Example: SQL Queries
Location Transparency
Enables information objects to be accessed without knowledge of their location.
• Example: File system operations in NFS
• Example: Pages in the Web
• Example: Tables in distributed databases
Concurrency Transparency
Enables several processes to operate concurrently using shared information objects without interference between them.
• Example: NFS
• Example: Automatic teller machine network
• Example: Database management system
Replication Transparency
Enables multiple instances of information objects to be used to increase reliability and performance without knowledge of the replicas by users or application programs
• Example: Distributed DBMS
• Example: Mirroring Web Pages.
Failure Transparency
• Enables the concealment of faults
• Allows users and applications to complete their tasks despite the failure of other components.
• Example: Database Management System (DBMS)
Migration Transparency
Allows the movement of information objects within a system without affecting the operations of users or application programs
• Example: NFS, Web Pages
Performance Transparency
Allows the system to be reconfigured to improve performance as loads vary.
• Example: Distributed make.

Scaling Transparency
Allows the system and applications to expand in scale without change to the system structure or the application algorithms.
• Example: World-Wide-Web, Distributed Database
Distributed Systems: Hardware Concepts
• Multiprocessors – multiprocessor system has more than one processor, with common memory shared between processors
• Multicomputer – multicomputer system has more than one processor, with each processor having local memory
In either case, processors maybe on a common bus(close coupled), or distributed on a network(loosely couples)
Networks of Computers
Multiprocessors and Multicomputer
Distinguishing features:
• Private versus shared memory
• Bus versus switched interconnection

High degree of node heterogeneity:
• High-performance parallel systems (multiprocessors as well as multicomputers)
• High-end PCs and workstations (servers)
• Simple network computers (offer users only network access)
• Mobile computers (palmtops, laptops)
• Multimedia workstations
High degree of network heterogeneity:
• Local-area gigabit networks
• Wireless connections
• Long-haul, high-latency connections
• Wide-area switched megabit connections

Distributed Systems: Software Concepts

Distributed operating system (DOS): A distributed operating system is a software over a collection of independent, networked, communicating, and physically separate computational nodes. They handle jobs which are serviced by multiple CPUs. Each individual node holds a specific software subset of the global aggregate operating system

Network operating system: is the software that allows multiple computers to communicate, share files and hardware devices with one another. Some examples of network operating systems include Novell NetWare, Microsoft Windows NT, Microsoft Windows 2000, Microsoft Windows XP, Sun Solaris, Linux, etc

Middleware: Middleware in the context of distributed applications is software that provides services beyond those provided by the operating system to enable the various components of a distributed system to communicate and manage data. Middleware supports and simplifies complex distributed applications

Distributed Operating System
Some characteristics:
_ OS on each computer knows about the other computers
_ OS on different computers generally the same
_ Services are generally (transparently) distributed across computers

Network Operating System
Some characteristics:
_ Each computer has its own operating system with networking facilities
_ Computers work independently (i.e., they may even have different operating systems)
_ Services are tied to individual nodes (ftp, telnet, WWW)
_ Highly file oriented (basically, processors share only files)

Distributed System (Middleware) Some characteristics:
_ OS on each computer need not know about the other computers
_ OS on different computers need not generally be the same
_ Services are generally (transparently) distributed across computers


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