11-10-2012, 05:45 PM
Distributed System Structures
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Motivation
Distributed system is collection of loosely coupled processors
interconnected by a communications network
Processors variously called nodes, computers, machines, hosts
Site is location of the processor
Reasons for distributed systems
Resource sharing
sharing and printing files at remote sites
processing information in a distributed database
using remote specialized hardware devices
Computation speedup – load sharing
Reliability – detect and recover from site failure, function transfer,
reintegrate failed site
Communication – message passing
Network-Operating Systems
Users are aware of multiplicity of machines. Access to resources of
various machines is done explicitly by:
Remote logging into the appropriate remote machine (telnet,
ssh)
Remote Desktop (Microsoft Windows)
Transferring data from remote machines to local machines, via
the File Transfer Protocol (FTP) mechanism
Network Structure
Local-Area Network (LAN) – designed to cover small geographical area.
Multiaccess bus, ring, or star network
Speed ≈ 10 – 100 megabits/second
Broadcast is fast and cheap
Nodes:
usually workstations and/or personal computers
a few (usually one or two) mainframes
Distributed-Operating Systems
Users not aware of multiplicity of machines
Access to remote resources similar to access to local
resources
Data Migration – transfer data by transferring entire file, or
transferring only those portions of the file necessary for the
immediate task
Computation Migration – transfer the computation, rather than the
data, across the system
Communication Structure
Naming and name resolution - How do two processes
locate each other to communicate?
Routing strategies - How are messages sent through the
network?
Connection strategies - How do two processes send a
sequence of messages?
Contention - The network is a shared resource, so how do
we resolve conflicting demands for its use?
Network Topology
Sites in the system can be physically connected in a variety of ways; they
are compared with respect to the following criteria:
Basic cost - How expensive is it to link the various sites in the system?
Communication cost - How long does it take to send a message from
site A to site B?
Reliability - If a link or a site in the system fails, can the remaining
sites still communicate with each other?