Part six: Distributed systems

1. Part six: Distributed systems Natalia Shatokhina Ashwini Mundya

2. Outline • Reasons to build distributed system • Network OS vs Distributed OS • Data/process/communication migration in distributed OS • Communication Structure • Communication Protocols • Robustness • Design Issues • An Example: Networking

3. Distributed system Site A Site C server Network Resources client Site B

4. Why distributed systems? 4 major reasons for building distributed systems: • Resource Sharing • Computational speedup • Reliability • Communication

5. Reason 1: Resource sharing • User on one site may be able to use the resources available at another site • Using a printer at remote site • Access a file • Access a database

6. Reason 2: Computational Speedup • If particular computation can be split into subcomputations that can run in parallel, the distributed system allows to distribute computations among various sites • Speedup can be achieved when running computations in parallel • Speedup= “serial computation time”/”parallel computation time” • In addition, load sharing helps to achieve speedup: move the jobs from the site overloaded with jobs to other with lighter load • Automated load sharing is not yet common

7. Reason 3: Reliability • If one site fails in distributed system remaining sites can continue to operate and get the job done • The function of failed site can be taken over by another site To provide reliability: • The system must ensure the correct transfer of function • Failure of a site must be detected by the system and the services of that site should not be longer used • Mechanisms must be available to integrate the recovered site back to the system

8. Reason 4: Communication • Sites can communicate over a network by sending messages to one another. • The advantage of distributed system is that the messages can be carried over a great distance (two geographically distant sites). That provides for efficient collaboration.

9. Types of network-based operating systems • Network operating systems • Distributed operating systems • What is the difference?

10. Network operating systems (NOS) Definitions: • From book: A network operating system provides an environment in which users, who are aware of the multiplicity of machines, can access remote resources by either logging in to the appropriate machine or transferring data from the remote machine to their own machines. • From wikipedia: System designed to allow shared file and printer access among computers in a network, typically a local area network (LAN), a private network or Intranet.

11. CPU CPU Memory cache cache Memory Memory Memory CPU CPU CPU Hardware architectures (source: http://www.cs.jhu.edu/~yairamir/cs437/week7/sld005.htm): Parallel architecture • Multiprocessors • Tightly coupled • Shared memory • Picture that explains architecture • Types dynamic and static Distributed architecture • Multicomputers • Loosely coupled • Private memory • Autonomous

12. Network OS vs Distributed OS (source: http://www.cs.jhu.edu/~yairamir/cs437/week7/sld005.htm): • Network OS: Contains N copies of operating systems, communication between machines is via shared files. • Distributed OS: Contains N copies of operating systems, communication between nodes is via messages over a network. These messages pass the necessary parameters for the task and on completion messages return the results. Its as if the computers email each other with a request and an answer.

13. Network Operating systems Remote login: • Network OS (for example Unix) provides an Internet service that allows the user to log in remotely • telnet or ssh is used • After logging in user can compute on remote machine just as any local user. • Ssh is used to establish encrypted connection Remote file transfer (ftp): • Protocol allows to transfer a file from remote machine to local computer • User must know the commands and the exact location of the file he wants • Anonymous method provides access to files in directory tree “anonymous” to anyone • Since ftp is not secure, scp is currently used for file transfer

14. Distributed OS characteristics • In a distributed OS users access remote resources in the same way they access local resources • Data and process migration is under control of OS Abbreviated as NOS, an operating system that includes special functions for connecting computers and devices into a local-area network (LAN). Some operating systems, such as UNIX and the Mac OS, have networking functions built in. The term network operating system, however, is generally reserved for software that enhances a basic operating system by adding networking features. Novell Netware, Artisoft's LANtastic, Microsoft Windows Server, and Windows NT are examples of an NOS.

15. Examples of distributed systems(source: Wikipedia) • Network OS- UNIX; Windows 2000 Server; Netware. • Distributed OS • Mosix2 • implemented as an OS virtualization layer that provides to users and applications a single system image with the Linux run-time environment • Definition: single system image (SSI) cluster is a cluster of machines that appears to be one single system. • Plan 9 (developed for research purposes, planned as UNIX successor); • Amoeba (development is frozen). Project lead: Andrew Tannenbaum (MINIX author)

16. Examples of distributed systems • Clusters: • a group of linked computers often connected in LAN, working together closely thus in many respects forming a single computer • Implementations of cluster systems (commercial and free) exists for many OS • Categories of cluster systems: • high availability(Linux-HA, Lander Cluster), • load-balancing, • compute (Beowulf , OSCAR) • grid (Rocks). • Web

17. Rocks cluster architecture Cluster components • Front-end node • Compute node • Ethernet network • Application Message Passing Network (optional)

18. Data migration approaches in distributed OS • Transfer entire file (automated FTP system?). It was used in Andrew file system but found inefficient • Transfer only portions of the file that are necessary for immediate task. Another portion requires another transfer. Protocols that are using this method: NFS, newer versions of Andrew, SMB (running on top of TCP/IP or Microsoft NetBEUI) • The system may also perform data translation (if sites are using different character code representations)

19. Computation migration in distributed OS • Example: to obtain a summary of large files that reside on different sites. • Rather then copy files to one location and obtain a summary, it is more efficient to access the files at the sites where they reside and the return result to the initiating site • Process P wants to access file at site A. Access is initiated by RPC. Through UDP RPC executes a routine on remote site. P ivokes a pre-defined procedure at A, the results are returned to P • P sends a message to A. OS creates a new process Q that executes the task. When done Q sends the results back to P via the message system. This way P may execute concurrently with Q. Either method can be used to access files at different sites RPC might result in invocation of another RPC

20. Process migration in distributed system • Migration: the process is not get executed at the site where it was initiated • Reasons: • For load balancing • For speedup: if a process may be divided into subprocesses • Hardware preference: process is more suitable for execution on some specialized processor • Software preference: process requires software that is available only at particular site • Data access: the same as in computation migration - for big chunks of data • If load balancing is needed then a system itself can move processes • when the process must satisfy hardware -software preference then the user has to specify how process should migrate • Examples: Web server/database, Java Applets

21. Networks for distributed systems Local area networks (LAN) : • Composed of processors distributed over small area (single building) • Emerged in 1970 • More economical then large mainframe computer • Data sharing in a single enterprise • Communication link: high quality cable • Communication speed: 1Mbit/sec - 1 Gbit/sec • Typical LAN consist of computers, shared peripheral devices (printers, tapes) and one or more gateways that provide access to other networks • Ethernet scheme is used to construct LAN • Wireless LAN networks can be built with only one wireless router • Compared to Ethernet systems WiFi networks run at slower speed

22. Networks for distributed systems Wide area networks (WAN): • Composed of autonomous processors distributed over a large area (such as US territory) • Emerged in 1960s as academic research project • First WAN - Arpanet - a cluster of four computers • Communication link: phone lines, microwave links, satellite channels

23. Networks topology Configurations used for distributed system network: • Partially connected network • Fully connected network • Tree-structured network • Star network • Ring network • Criteria to compare the configurations: • Installation cost • Communication cost • Availability

24. Network communication structure Communication network design issues: • Naming and name resolution • Routing strategies • Packet strategies • Connection strategies • Contention

25. Naming and name resolution • How could processes at different sites specify each other to communicate? • Process identifier (PID) is used for processes within computer system, but since networked hosts share no memory different approach is needed • Pair <host name, identifier> is used for process identification • Host name is alphanumeric • Host name is human readable, but computers use host-id instead. There is a mechanism to resolve host name into host-id - Domain name system • Hosts in Internet are logically addressed with with multipart names (IP addresses)

26. Naming and name resolution • Each component in IP name has name server • A system that needs to use the DNS is configured with the known addresses of the root servers • Example: bob.cs.brown.ed How to get IP address for IP name bob.cs.brown.edu ? • Kernel of system A issues a request to the root server for edu domain asking for the address of the name server for brown.edu • The edu root server returns the address of the host on which brown.edu name server resides • The kernel on system A then queries the name server at this address and asks about cs.brown.edu • An address is returned ; and a request to that address returns host-id 128.148.31.100 • Java provides API for mapping IP names to IP addresses • Generally, OS is responsible for accepting a message from its processes for <host name, identifier> and transferring it to appropriate host • Kernel of destination host transfers the message to the process named by identifier

27. Naming and name resolution

28. Routing strategies • When a process from site A wants to communicate with process at site B how is a message sent? • Use physical path from A->B • If multiple paths exists then use Routing table • May include info about speed & cost of communication paths

29. Routing strategies comparison • Routing schemes • Fixed routing • Path is specified in advance (shortest path) and doesn’t change • Virtual routing • Path is fixed for the duration of one session. Examples: file transfer, remote login period etc • Dynamic routing • Path is chosen when message is sent. The message is sent over the link that is the least used at that particular time • Tradeoffs • Order of messages • Fixed & virtual scheme deliver messages in order • Environment • Dynamic scheme performs better in complicated environment Usually the combination of these schemes is used - for example: within a site each host has a static route to the gateway, but the gataway uses dynamic routing to reach any host on the network.

30. Communication Structure (Cont.)‏

31. Packet Strategies • Variable length message divided into fixed-length message called packets/frames • Packets transferred to destination using 2 kinds of services : • Reliable service • Unreliable service

32. 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

33. Connection Strategies(Cont.)‏ • 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

34. Contention • Several sites may want to transmit information over a link simultaneously. Techniques to avoid repeated collision include • CSMA/CD • Token Passing

35. Contention (Cont.)‏ • 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

36. 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

37. Communication Protocols

38. Communication Protocols • Simplify the design problem by partitioning the problem into multiple layers. • The 2 types of reference models • ISO's (International Standard Organization) OSI (Open System Interconnection) Model • TCP/IP Model

39. OSI Reference Model • Defines 7 network layers

40. Physical Layer • Handles the mechanical and electrical details of the physical transmission of a bit stream • This layer is implemented in the hardware of the networking device

41. Data Link Layer • This layer ensures that messages are delivered to the proper device on LAN using hardware address • Handles the frames, or fixed-length parts of packets, including any error detection and recovery that occurred in the physical layer

42. Network Layer • Responsible for tracking the location of devices in the network and determine the best way to move the data • Routers work at this level • Check if the packet belongs to that network else forward the packet to the destination address using routing table

43. Hop-to-hop delivery

44. Transport Layer • Segments and reassembles the data into a data stream • Provides the end-to-end transport services – using TCP/UDP • Also called as Reliable Networking – acknowledgment, maintaining packet order (sequencing) and flow control

45. Session Layer • Responsible for setting up, managing and tearing down the sessions between presentation layer entities • Performs message synchronization

46. Presentation Layer • Presents the data to the application layer • Translator – provides coding and conversion functions (Before transmit the data it has to be in standard format. Computers are configured to receive this data and convert onto native format). • By providing the translation service presentation layer ensures that data sent from the Application layer of one system is read by the Application layers of another one.

47. Presentation Layer (Cont.)‏

48. Application Layer • Provides services to the end user • Examples – File Transfer, E-Mail, Remote Login

49. OSI Reference Model

50. An OSI network message