Distributed Systems

Distributed Systems

1. Definition: A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages.

   • Teller machine
     • An automatic teller machine network enables bank customers to Banks and building societies maintain large networks of teller machines. Customer have high security, privacy and reliability requirements. Customers may want to withdraw cash from their account through a A front-end computer controls one or several tellers. It – transfers withdrawal requests to the computer of the account holder´s ○ Each bank has fault-tolerant systems to quickly recover from failures of their account holding computers. An example is the ´Hot standby´ computer which maintains a copy of the account database and can replace the main computer within seconds. 18 withdraw cash from their bank account. ○ ○ ○ ´foreign´ teller machine. ○ bank, – awaits the bank granting the request, and – therefore has to be interoperable with heterogeneous computer systems (Hang Seng Bank may have different account management systems than HongKong Bank and Bank of China).
   • DBMS
     • Different client applications want to access and update shared data in a database. ○ Client applications might be banking systems, real-estate agencies, airline-ticket reservation systems accessing data like balances of bank accounts, details of property that are for sale or to let, or airfares and aircraft reservation data. ○ The database is physically distributed over several processors to take advantage of local data accesses for increased performance of client applications. ○ Data may be replicated to reduce the impact of failures of a processor and/or the network. ○ Each processor runs a database monitor that implements the mapping between the database seen by clients and the physical database stored on the different processors. ○ Database monitors have to cooperate with each other to implement client accesses to remote data, updates of replicated data a
   • Internet
   • Mobile Computing
   • LAN
     • A local area network consists of a number of different computers. Workstations and Personal computers provide the front-end for network users. Different servers provide shared services. ○ One or several network file servers provide data storage services. Any workstation and PC may henceforth store files on disks maintained by these file servers. ○ A local name server maps machine names to IP addresses, user names to user ids and group names to group ids. Any machine can request a service to resolve a certain name. ○ Workstations and PCs have the server printing jobs for them. ○ ○ ○ ), user names (by user ids), and group name (by group ids). ○ 14 One or several print servers control the access to shared printers. Another component provides a gateway to the wide area network. As a user you need not be aware which machine provides which service. Name servers provide the information regarding machine (by IP address Local area networks can be connected together to form Intranets.

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