Distributed Systems in Advanced operating system
- 1. Lecture 2
- 2. Motivation Distributed system is collection of loosely coupled processors interconnected by a communications network Independent computers that appear to users of the system as a single computer Processors variously called nodes, computers, machines, hosts Site is location of the processor
- 3. Reasons for Distributed Systems Resource sharing sharing and printing files at remote sites processing information in a distributed database using remote specialized hardware devices Computation speedup – load sharing Reliability – detect and recover from site failure, function transfer, reintegrate failed site Communication – message passing
- 5. A Distributed System (contd.) A distributed system organized as middleware. Note that the middleware layer extends over multiple machines. 1.1
- 6. Types of Distributed Operating Systems Network Operating Systems Distributed Operating Systems System Description Main Goal DOS Tightly-coupled operating system for multi-processors and homogeneous multicomputers Hide and manage hardware resources NOS Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) Offer local services to remote clients
- 7. Network-Operating Systems Users are aware of multiplicity of machines. Access to resources of various machines is done explicitly by: Remote logging into the appropriate remote machine (telnet, ssh) Remote Desktop (Microsoft Windows) Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism
- 8. Network-Operating Systems Two clients and a server in a network operating system.
- 9. Distributed-Operating Systems Users not aware of multiplicity of machines Access to remote resources similar to access to local resources Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task Computation Migration – transfer the computation, rather than the data, across the system
- 10. Distributed-Operating Systems (Cont.) Process Migration – execute an entire process, or parts of it, at different sites Load balancing – distribute processes across network to even the workload Computation speedup – subprocesses can run concurrently on different sites Hardware preference – process execution may require specialized processor Software preference – required software may be available at only a particular site Data access – run process remotely, rather than transfer all data locally
- 11. Comparison between Systems Item Distributed OS Network OS Multiproc. Multicomp. Degree of transparency Very High High Low Same OS on all nodes Yes Yes No Number of copies of OS 1 N N Basis for communication Shared memory Messages Files Resource management Global, central Global, distributed Per node Scalability No Moderately Yes Openness Closed Closed Open
- 12. Network Structure Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed 10 – 100 megabits/second Broadcast is fast and cheap Nodes: usually workstations and/or personal computers a few (usually one or two) mainframes
- 13. Depiction of typical LAN
- 14. Network Types (Cont.) Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed 1.544 – 45 megbits/second Broadcast usually requires multiple messages Nodes: usually a high percentage of mainframes
- 15. Communication Processors in a Wide-Area Network
- 16. Network Topology Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: Basic cost - How expensive is it to link the various sites in the system? Communication cost - How long does it take to send a message from site A to site B? Reliability - If a link or a site in the system fails, can the remaining sites still communicate with each other? The various topologies are depicted as graphs whose nodes correspond to sites An edge from node A to node B corresponds to a direct connection between the two sites
- 17. Network Topology
- 18. Communication Structure Naming and name resolution - How do two processes locate each other to communicate? Routing strategies - How are messages sent through the network? Connection strategies - How do two processes send a sequence of messages? Contention - The network is a shared resource, so how do we resolve conflicting demands for its use? The design of a communication network must address four basic issues:
- 19. Naming and Name Resolution Name systems in the network Address messages with the process-id Identify processes on remote systems by <host-name, identifier> pair Domain name service (DNS) – specifies the naming structure of the hosts, as well as name to address resolution (Internet)
- 20. Routing Strategies Fixed routing - A path from A to B is specified in advance; path changes only if a hardware failure disables it Since the shortest path is usually chosen, communication costs are minimized Fixed routing cannot adapt to load changes Ensures that messages will be delivered in the order in which they were sent Virtual circuit - A path from A to B is fixed for the duration of one session. Different sessions involving messages from A to B may have different paths Partial remedy to adapting to load changes Ensures that messages will be delivered in the order in which they were sent
- 21. Routing Strategies (Cont.) Dynamic routing - The path used to send a message form site A to site B is chosen only when a message is sent Usually a site sends a message to another site on the link least used at that particular time Adapts to load changes by avoiding routing messages on heavily used path Messages may arrive out of order This problem can be remedied by appending a sequence number to each message
- 22. Connection Strategies Circuit switching - A permanent physical link is established for the duration of the communication (i.e., telephone system) Message switching - A temporary link is established for the duration of one message transfer (i.e., post-office mailing system) Packet switching - Messages of variable length are divided into fixed-length packets which are sent to the destination Each packet may take a different path through the network The packets must be reassembled into messages as they arrive
- 23. Connection Strategies (contd..) Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth Message and packet switching require less setup time, but incur more overhead per message
- 24. Contention CSMA/CD - Carrier sense with multiple access (CSMA); collision detection (CD) A site determines whether another message is currently being transmitted over that link. If two or more sites begin transmitting at exactly the same time, then they will register a CD and will stop transmitting When the system is very busy, many collisions may occur, and thus performance may be degraded CSMA/CD is used successfully in the Ethernet system, the most common network system Several sites may want to transmit information over a link simultaneously. Techniques to avoid repeated collisions include:
- 25. Contention (Cont.) Token passing - A unique message type, known as a token, continuously circulates in the system (usually a ring structure) A site that wants to transmit information must wait until the token arrives When the site completes its round of message passing, it retransmits the token A token-passing scheme is used by some IBM and HP/Apollo systems
- 26. Contention (Cont.) Message slots - A number of fixed-length message slots continuously circulate in the system (usually a ring structure) Since a slot can contain only fixed-sized messages, a single logical message may have to be broken down into a number of smaller packets, each of which is sent in a separate slot This scheme has been adopted in the experimental Cambridge Digital Communication Ring
- 27. Assignment 1 Compare the following topologies w. r. t. Basic Cost, Diameter and Reliability of the Communication Network: Ring Fully Connected Tree Cube/Hypercube