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Lecture 9

Lecture 9. Other models: Monitoring models Reliability and fault-tolerance models Performance models. Scheduling policies. Security models. Student presentations and midterm. I expect a progress report the week after the Spring break (March 18 – 24).

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Lecture 9

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  1. Lecture 9 • Other models: • Monitoring models • Reliability and fault-tolerance models • Performance models. Scheduling policies. • Security models

  2. 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

  3. Monitoring models • A monitor could be a process responsible to establish the global state of a System. • Intrusion – Heissenber’s 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.

  4. 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.

  5. Consistent and inconsistent cuts

  6. Causal history

  7. The snapshot protocol (Chandy&Lamport)

  8. 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

  9. Failure modes for processes [P] and for communication channels [C] • Crash - [P&C] • FailStop - [P] • Send Omissions - [P] • Receive omissions - [P] • General omissions – [P&C] • Byzantine – [P&C] • Arbitrary with message authentication - [P] • Timing – [P]

  10. 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.

  11. Collective communication

  12. 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

  13. 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’.

  14. Broadcast primitives and their relationships

  15. 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 • Little’s law: N =  T

  16. 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: <= 1 necessary but not sufficient • Time in system is finite • Number in system is finite

  17. Performance models • When utilization tends to 1  time in system becomes unbounded. • Network congestion.

  18. Scheduling policies/algorithms • Static/Dynamic algorithms • Centralized/Distributed • Policies: • FCFS • LCFS • Priority • Round-Robin • Weighted Fair Queuing

  19. Service policies for the server with vacation model • Exhaustive • Gated • Semi-gated • K-limitted

  20. Scheduling on a grid • Resources under the control of different administrative authorities. • Resource reservations. • Market-based scheduling algorithms.

  21. Scheduling on a grid

  22. Security models • Problems and solutions: • Confidentiality  encription • Authentication  authentication services • Authorization (controlled access to system resources)  access control

  23. Secret key and public key cryptography

  24. Major challenges in distributed systems • Concurrency • Mobility

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