Other models:

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Lecture 9

• Other models
• Monitoring models
• Reliability and fault-tolerance models
• Performance models. Scheduling policies.
• Security models

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Student presentations and midterm

• I expect a progress report the week after the
  Spring break (March 18 24).
• The final project report is due the week before
  last.
• Midterm two weeks from today
• Material Chapters 1,2, and 3 up to the last
  lecture.
• Open book.
• 3 questions 30 minutes

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Monitoring models

• A monitor could be a process responsible to
  establish the global state of a System.
• Intrusion Heissenbers uncertainty for quantum
  processes.
• Run a total ordering of all events in the global
  history of a process.
• Cut a subset of the local history of all
  processes.
• Frontier of a cut the last event of every
  process in the cut.

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Consistent and inconsistent cuts

• Consistent cut a cut that agrees with causality.
  
• Inconsistent cut violates causality.
• Causal history of an event the smallest cut
  including the event.
• The snapshot algorithm of Chandy and Lamport.
• Checkpointing in parallel and distributed
  computing.

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Consistent and inconsistent cuts
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Causal history
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The snapshot protocol (ChandyLamport)
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Reliability and fault-tolerance models

• A failure at time t is un undesirable event
  characterized by its
• Manifestation incorrect timing or value of
  variables
• Consistency the system may fail in a consistent
  or in an inconsistent state.
• Effects benign/ malign
• Occurrence mode singular or repeated

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Failure modes for processes P and for
communication channels C

• Crash - PC
• FailStop - P
• Send Omissions - P
• Receive omissions - P
• General omissions PC
• Byzantine PC
• Arbitrary with message authentication - P
• Timing P

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Collective communication

• Broadcast and multicast.
• Applications
• Routing in mobile ad hoc networks.
• Routing in the Internet to disseminate
  topological information flooding algorithms.
• Used to achieve consensus.
• Multicasting of audio and video streams to reduce
  the bandwidth.
• Parallel algorithms barrier synchronization.

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Collective communication
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Properties of a broadcast algorithm (I)

• Validity if a correct cc-process broadcasts a
  message m all correct cc-processes eventually
  deliver m.
• Agreement - if a correct cc-process delivers
  message m all correct cc-processes eventually
  deliver m.
• Integrity every correct cc-process delivers m
  once and only once and only if the message was
  broadcast by a cc-process

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Properties of a broadcast algorithm (II)

• FIFO order if a correct cc-process broadcasts a
  message m before m then no correct cc-processes
  delivers m unless it has previously delivered m.
• Causal order - if a correct cc-process broadcasts
  m that causally precedes m then no correct
  cc-processes delivers m unless it has previously
  delivered m.
• Total order if two correct cc-processes p and q
  both deliver messages m and m then p delivers m
  before m if and only if q delivers m before m.

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Broadcast primitives and their relationships
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Performance models

• Resource sharing!!!
• Arrival process distribution of inter-arrival
  times or arrival rates.
• Service process distribution of service times
  or inter-departure times.
• Number of servers
• Quantities of interest
• Time in system, T
• Waiting time W
• Number in system, N
• Littles law N ? T

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Performance models

• Types of systems
• Deterministic D/D/1
• Markov arrival, Markov service - M/M/1
• Markov arrival, general service M/G/1
• Batch arrival.
• Server utilization ? ratio of arrival rate to
  service rate.
• Stability ?lt 1 necessary but not sufficient
• Time in system is finite
• Number in system is finite

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Performance models

• When utilization tends to 1 ? time in system
  becomes unbounded.
• Network congestion.

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Scheduling policies/algorithms

• Static/Dynamic algorithms
• Centralized/Distributed
• Policies
• FCFS
• LCFS
• Priority
• Round-Robin
• Weighted Fair Queuing

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Service policies for the server with vacation
model

• Exhaustive
• Gated
• Semi-gated
• K-limitted

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Scheduling on a grid

• Resources under the control of different
  administrative authorities.
• Resource reservations.
• Market-based scheduling algorithms.

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Scheduling on a grid
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Security models

• Problems and solutions
• Confidentiality ? encription
• Authentication ? authentication services
• Authorization (controlled access to system
  resources) ? access control

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Secret key and public key cryptography
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Major challenges in distributed systems

• Concurrency
• Mobility

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