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

2. 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. 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. 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. 5 Consistent and inconsistent cuts

6. 6 Causal history

7. 7 The snapshot protocol (Chandy&Lamport)

8. 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. 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. 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. 11 Collective communication

12. 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. 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. 14 Broadcast primitives and their relationships

15. 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

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17. 17 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

18. 18 Performance models When utilization tends to 1  time in system becomes unbounded. Network congestion.

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21. 21 Scheduling policies/algorithms Static/Dynamic algorithms Centralized/Distributed Policies: FCFS LCFS Priority Round-Robin Weighted Fair Queuing

22. 22 Service policies for the server with vacation model Exhaustive Gated Semi-gated K-limitted

23. 23 Scheduling on a grid Resources under the control of different administrative authorities. Resource reservations. Market-based scheduling algorithms.

24. 24 Scheduling on a grid

25. 25 Security models Problems and solutions: Confidentiality  encription Authentication  authentication services Authorization (controlled access to system resources)  access control

26. 26 Secret key and public key cryptography

27. 27 Major challenges in distributed systems Concurrency Mobility

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