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Distributed Systems

Distributed Systems CS 654 Lecture 3 September 11 th , 2006 Basic Categories Uniprocessor Operating Systems 1.11 Microkernel design What does a monolithic design look like? What does a SMP OS look like? Multicomputer Operating Systems 1.14 Is the middle layer in kernel mode?

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Distributed Systems

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  1. Distributed Systems CS 654Lecture 3September 11th, 2006

  2. Basic Categories

  3. Uniprocessor Operating Systems 1.11 • Microkernel design • What does a monolithic design look like? • What does a SMP OS look like?

  4. Multicomputer Operating Systems 1.14 • Is the middle layer in kernel mode?

  5. Communications • Fundamentally, how is communication done on SMP vs. a cluster (multicomputer)? • From the programming point of view, what are the two fundamental ways to communicate? • Is it possible to communicate using the network within an SMP? • Is it possible to provide a shared memory model on cluster?

  6. DSM • What is virtual memory? • Where are the page frames stored when paged (swapped) out?

  7. Distributed Shared Memory Systems (1) • Pages of address space distributed among four machines • Situation after CPU 1 references page 10 • Situation if page 10 is read only and replication is used • What kinds of bad things can happen? • Using to communicate would be bad.

  8. Distributed Shared Memory Systems (2) 1.18 • False sharing of a page between two independent processes.

  9. Examples of DOS? • DSM can be considered a failure. • MPI is the norm.

  10. Network Operating System (1) • General structure of a network operating system. • Generally, services are not in the kernel. • Services are remote login, file access, etc.

  11. Network Operating System (2) 1-20 • Two clients and a server in a network operating system.

  12. Network Operating System (3) 1.21 • Different clients may mount the servers in different places. • Examples of NOS?

  13. Middleware • What is middleware? • What is its function? • Where is it located? • Why does it exist?

  14. Positioning Middleware • General structure of a distributed system as middleware.

  15. Figure 2.1Software and hardware service layers in distributed systems Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  16. Middleware and Openness • In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications. • Why should they be open? Why should they not be open? 1.23

  17. Typical Middleware Services • Communication • Naming • Persistence • Distributed transactions • Security

  18. Middleware Models • Distributed files • Examples? • Remote procedure call • Examples? • Distributed objects • Examples? • Distributed documents • Examples? • Others? • Message-oriented middleware (MOM) • Service oriented architecture (SOA) • Document-oriented

  19. DOS vs. NOS vs. Middleware Discussion • What is good/bad about DOS? • Transparency • Other issues have reduced success. • Problems are often socio-technological. • What is good/bad about NOS? • Simple. • Decoupled, easy to add/remove. • Lack of transparency. • What is good/bad about middleware? • Easy to make multiplatform. • Easy to start something new. • But this can also be bad.

  20. Comparison between Systems

  21. Example • A user wants to access a file on a cluster. How should we design this? • Issues? • Transparency • Fault tolerance • Possibilities: • SAN • Shared? • GFS • NFS • Middleware? • What should we do differently if it is on a WAN?

  22. Example • A process is started on a cluster via the fork system call. How should we design this? • Issues • Load balancing • Transparency, networking • Possibilities • Do nothing. • User picks machine, uses ssh. • System call. • Disadvantages? • Library function. • Transparency?

  23. Discussion • Have NOS been successful? • Have DOS been successful? Why or why not? • An abstraction that is too leaky? • Has middleware been successful?

  24. “Leaky” Abstractions • What is an abstraction? • What is encapsulation? • How well does this work in practice? • High-level languages • TCP/IP, reliable byte sequence • Packet size? • Ignore packet loss? • Reliability?

  25. Example Systems

  26. intranet % % ISP % % backbone satellite link desktop computer: server: network link: Figure 1.1A typical portion of the Internet Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  27. Figure 1.2A typical intranet Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  28. Figure 1.3Portable and handheld devices in a distributed system Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  29. What are typical latencies and bandwidths? • WAN? • Millisecs • Anywhere from Mbps to Gbps • LAN? • Microsecs • Anywhere from Mbps to Gbps • Mobile? • Microsecs to days • Anywhere from Mbps to Kbps

  30. http://www.google.comlsearch?q=kindberg www.google.com Browsers Web servers Internet www.cdk3.net http://www.cdk3.net/ www.w3c.org http://www.w3c.org/Protocols/Activity.html File system of Protocols www.w3c.org Activity.html Figure 1.4Web servers and web browsers Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  31. Computers Date Web servers 188 0 1979, Dec. 1989, July 130,000 0 1999, July 56,218,000 5,560,866 2003, Jan. 171,638,297 35,424,956 Figure 1.5Computers in the Internet Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  32. Figure 1.6Computers vs. Web servers in the Internet Date Computers Web servers Percentage 1,776,000 130 0.008 1993, July 1995, July 6,642,000 23,500 0.4 1997, July 19,540,000 1,203,096 6 1999, July 56,218,000 6,598,697 12 2001, July 125,888,197 31,299,592 25 42,298,371 Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  33. Why Did the Web Succeed? • Timing • Need a critical mass of • Internet • Users with PCs • Technology • Provided enough functionality, without providing too much • What would have been the wrong design? • How good is HTTP? • Connections are expensive to set-up/tear-down. • No pipelining • No chunking

  34. Architectural Models

  35. Client-Server • What is a client? • What is a server? • What would be the alternative? • Can something be both client and server?

  36. Figure 2.2Clients invoke individual servers Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  37. Clients and Servers • General interaction between a client and a server.

  38. An Example Client and Server (1) • The header.h file used by the client and server.

  39. An Example Client and Server (2) • A sample server.

  40. An Example Client and Server (3) 1-27 b • A client using the server to copy a file.

  41. Layering • Why layer? • Flexible • You can add functionality without changing underlaying layers. • Reuse • Many applications can use Java jars, for example. • Helps you solve the problem. • Too hard to hold everything in your head at once.

  42. Buying an airline ticket • How would you design the system? • A terminal on one end, write a single program on the other end. • A single program at the agent end. All things are broadcast to everyone.

  43. 3-Tier Architectures • Interface • (Business) logic • Database

  44. Processing Level • The general organization of an Internet search engine into three different layers 1-28

  45. Multitiered Architectures (2) • An example of a server acting as a client. 1-30

  46. Peer-to-Peer • How does it differ from previous? • Can all apps be done as P2P?

  47. Figure 2.3A distributed application based on peer processes Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

  48. Modern Architectures • An example of horizontal distribution of a Web service. 1-31

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