Monday, November 29, 1993

Distributed systems (chapter 12)
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Basic classification scheme for multi-computer systems
*tightly coupled system*: Processors share memory and a clock.  Also called
    multi-processor system.
*loosely coupled system*: Processors do not share memory or a clock.  Also
    called a distributed system.
Here we will consider distributed systems
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Why distributed systems?
- Resource sharing
  One site can use resources available at another
- Computation speedup
  Partition computation into independent subcomputations.  Load sharing to
  relieve site that is currently overloaded with jobs.
- Reliability
  Redundancy to permit system to continue operation even if some of its sites
  have failed.
- Communication
  Gives the users of separate systems the spportunity to exchange information,
  perhaps over great distances.
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Note that these examples have made an assumption about the characteristics of
the network (connectivity, speed of links, etc.)  .  Consider some of the
alternatives now in use:
- ARPAnet: 1st widely-used network, developed early 70's but still in
  use. Connected together large timesharing systems all over the country using
  leased phone lines. Provided mail, file transfer, remote login.
- Usenet: Developed late 70's, early 80's. Unix systems phone each other up to
  send mail and transfer files.
- Local area networks: Developed early 80's to hook together personal
  computers. Most popular interconnection for LANs is Ethernet. LANs are used
  very differently than wide-area networks.
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Network topology
Metrics:
- Basic cost (how many links)
- communication cost (how many hops to get message from A to B)
- reliability (how much failure before nodes cut off?)
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- Fully connected
  (all sites directly lined with all others)
  high cost because must have direct communication line between every two sites
  indeed links grow as square of number of sites
  advantage: very fast routing (only one link needed), very reliable (many
  links must fail to partition net)
  a net is *partitioned* if it has been split into two or more parts that lack
  connection between them
- partially conected
  some, but not all, nodes are linked
  message may have to pass through several links (slower communication)
  net may become partitioned more easily (not as reliable)
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Some examples of partially connected networks
note that multiple hops require nodes to store and forward messages
- Hierarchical network
  organized as a tree
  for example following corporate structure.
  note that failure of node partitions subtrees from eachother and from
  ancestors
- star network
  central hub
  at most two hops
  hub can become bottleneck so may need to be dedicated to switching
  applications
  failure of hub partitions net
- ring
  each node connected to exactly two other nodes
  unidirectional: messages flow in one direction only
  bidirectional: messages flow in either direction
  up to n-1 hops for unidirectional case
  up to n/2 for bidirectional case
  any failure partitions unidirectional net
  two failures partition bidirectional ring
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- multiaccess bus
  single shared link (the bus) with all sites connected to the bus
  sites communicate directly with one another via the bus
  comparatively low cost
  link can become bottleneck
  failure of nodes not important to partitioning but failure of link completely
  partitions network
  examples: Ethernet (linear bus configuration), satellite links
  issue is how to arrange for link to be shared by multiple nodes
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