A distributed system is a

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• A distributed
  system is a
• collection of
  machines
• sharing files and
  inde-
• pendent storage
  resources
• Service is the specification of what the file
  system offers to its clients
• server service software running on a single
  machine
• client a process that can invoke a service
  using its client interface

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• A distributed file
  system
• (DFS) should hide
  the
• multiplicity of its
  servers
• The most important measurement of a DFS is the
  amount of time needed to satisfy service requests
  (disk-access time, small amount of CPU-processing
  time the time to deliver the request and to get
  the response)

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• Naming is a mapping between logical and physical
  objects.
• Files are named by some combination of their host
  name and local name, so they will have unique
  name
• Users deal with logical data objects represented
  by file names. The system manipulates physical
  blocks of data stored on disk tracks

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• Remote file access
• Remote service mechanism
• Caching (holding the most recently used files in
  the servers main memory)
• File replication
• Caching and replication are used to improve
• performance

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• Process synchronization for
• distributed systems
• A distributed system has no
• common memory or clock, so it is
• impossible to decide which of two events
• occurred first
• The solution is define happen-before relation,
  set timestamp to each process and define within
  each process a logical clock. The timestamp for
  an event is the value of the logical clock for
  that event

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