1. Lecture 4: Sun: 23/4/1435 Distributed Operating Systems Lecturer/ Kawther Abas k.albasheir@sau.edu.sa CS- 492 : Distributed system & Parallel Processing

2. Consists of several computers that do not share a memory or a clock; The computers communicate with each other by exchanging messages over a communication network. Each computer has its own memory and runs its own operating system. Distributed System is used to describe a system with the following characteristics:

3. Architecture of Distributed OS

4. Definition of Distributed OS It extends the concepts of resource management and user friendly interface for shared memory computers a step further, encompassing a distributed computing system consisting of several autonomous computers connected by a communicating network.

5. Issues in Distributed OS Global Knowledge Naming Scalability Compatibility Process Synchronization Resource Management Security Structuring

6. Network Operating Systems Distributed Operating Systems Type of Distributed Operating Systems

7. Network Topology they are compared with respect to the following criteria: –Basic cost - –Communication cost - –Reliability -

8. Network Topology

9. Communication Structure Naming and name resolution Routing strategies Connection strategies Contention The design of a communication network must address four basic issues:

10. Communication Processors in a Wide-Area Network

11. Communication Protocol Physical layer Data-link layer Network layer Transport layer Session layer Presentation layer Application layer The communication network is partitioned into the following multiple layers:

12. Communication Via ISO Network Model

13. Robustness Failure detection Reconfiguration

14. Failure Detection Detecting hardware failure is difficult To detect a link failure, a handshaking protocol can be used Assume Site A and Site B have established a link If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost Site A can now send an Are-you-up? message to Site B If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B

15. Failure Detection (cont) If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred Types of failures: - Site B is down - The direct link between A and B is down - The alternate link from A to B is down - The message has been lost However, Site A cannot determine exactly why the failure has occurred

16. Reconfiguration When Site A determines a failure has occurred, it must reconfigure the system: 1. If the link from A to B has failed, this must be broadcast to every site in the system 2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available When the link or the site becomes available again, this information must again be broadcast to all other sites