Understanding Distributed Systems: An Overview

1. Introduction Chapter 1 Chapter 1  Introduction 1

2. Examples of Distributed Systems • DNS • Hierarchical distributed database • WWW • Origin servers and web caches • Distributed database • Cray T3E • 2048 tightly coupled homogeneous processors • Distributed/parallel computing • Condor/RES • Loosely coupled heterogeneous workstations • Parallel/distributed computing Chapter 1  Introduction 2

3. Other Distributed Systems • Email • Electronic banking • Airline reservation system • Peer-to-peer networks • Etc., etc., etc. Chapter 1  Introduction 3

4. Computer Revolution • Processing power • 50 years ago, $100M for 1 instr/sec • Today, $1K for 107 instructions/sec • Price/perform. improvement of 1012 • If cars had followed same path as computers… • “…a Rolls Royce would now cost 1 dollar and get a billion miles per gallon” • And it would “explode once a year, killing everyone inside” Chapter 1  Introduction 4

5. Computer Revolution • High speed networks • 30 years ago, networks were unknown • Today, Gigabit networks and the Internet • Before networks, centralized systems • Today, distributed systems • Computers in many locations work as one Chapter 1  Introduction 5

6. What is a Distributed System? • According to your textbook • “A collection of independent computers that appears to its users as a single coherent system” • Two parts to definition • Hardware machines are autonomous • Softwaremachines appear as one system • Implies that communication hidden from user • Implies that organization hidden from user Chapter 1  Introduction 6

7. What is a Distributed System? • According to dict.die.net • A collection of (probably heterogeneous) automata whose distribution is transparent to the user so that the system appears as one local machine • This is in contrast to a network, where the user is aware that there are several machines, and their location, storage replication, load balancing and functionality is not transparent • Crucial point is transparency Chapter 1  Introduction 7

8. How to Implement a Dist. System? • A distributed system is a collection of independent computers… • …that acts like a single system • How to accomplish this? • Middleware • Make distributed system as transparent as possible Chapter 1  Introduction 8

9. Role of Middleware • Distributed system as middleware • Middleware extends over multiple machines Chapter 1  Introduction 9

10. Goals • For a distributed system to be worthwhile authors believe it should • Easily connect users to resources • Hide fact that resources are distributed • Be open • Be scalable • First 2 of these about transparency • Transparent, open, scalable Chapter 1  Introduction 10

11. Transparency • Transparent system “acts” like one computer • Various aspects of transparency listed above Chapter 1  Introduction 11

12. Degree of Transparency • Cannot hide physical limitations • Time it takes to send packet • May be a tradeoff between transparency and performance • What to do if Web request times out? • Keeping replicated data current Chapter 1  Introduction 12

13. Openness • Open == standards-based • Provides • Interoperability • Portability • Ideally, flexible, i.e., extensible • But many useful systems follow the “American standard” • Do whatever you want Chapter 1  Introduction 13

14. Scalability • Scalability issues/limitations Chapter 1  Introduction 14

15. Scalability • Authors believe centralized is bad • Centralized server is source of congestion, single point of failure • Centralized data leads to congestion, lots of traffic • Centralized algorithm must collect all info and process it (e.g., routing algs) • Google? Napster? Chapter 1  Introduction 15

16. Scalability • Decentralized algorithms • No machine has complete system state • Decisions based on local info • Failure of one machine does not kill entire algorithm • No assumption of global clock • Examples? Chapter 1  Introduction 16

17. Geographic Scalability • Big difference between LAN and WAN • LANs have synchronous communication • Client can “block” until server responds • On LAN, global time may be possible (to within a few milliseconds) • WAN unreliable, point-to-point • WAN has different admin domains • A security nightmare Chapter 1  Introduction 17

18. Scaling Techniques • Scaling problems due to limited capacity of networks and servers • Three possible solutions • Hide latencies  do something useful while waiting (asynchronous comm.) • Distribution  DNS, for example • Replication  allows for load balancing • Replication creates consistency issues Chapter 1  Introduction 18

19. Scaling Techniques • Server or client check form as it’s filled out? • Having client do more, as in (b), may reduce latency (but may cause security problems) Chapter 1  Introduction 19

20. Scaling Techniques • DNS name space divided into zones • Goto server in Z1 to find server Z2 and so on • Like a binary search for correct server Chapter 1  Introduction 20

21. Hardware Issues • For our purposes, 2 kinds of machines • Multiprocessor • Different processors share same memory • Multicomputer • Each processor has it’s own memory • Each of these could use either bus or switched architecture Chapter 1  Introduction 21

22. Hardware Issues multiprocessor multicomputer Chapter 1  Introduction 22

23. Multiprocessors • A bus-based multiprocessor • Cache coherence is an issue Chapter 1  Introduction 23

24. Multiprocessors • A crossbar switch • Omega switching network Chapter 1  Introduction 24

25. Homogeneous Multicomputer Grid Hypercube Chapter 1  Introduction 25

26. Software Concepts • DOS  Distributed Operating Systems • NOS  Network Operating Systems • Middleware  self-explanatory Chapter 1  Introduction 26

27. Uniprocessor OSs • Separate apps from OS code via microkernel Chapter 1  Introduction 27

28. Multiprocessor OSs • How to protect count from concurrent access? monitor Counter { private: int count = 0; public: int value() { return count;} void incr () { count = count + 1;} void decr() { count = count – 1;} } Chapter 1  Introduction 28

29. Multiprocessor OSs monitor Counter { private: int count = 0; int blocked_procs = 0; condition unblocked; public: int value () { return count;} void incr () { if (blocked_procs == 0) count = count + 1; else signal (unblocked); } void decr() { if (count ==0) { blocked_procs = blocked_procs + 1; wait (unblocked); blocked_procs = blocked_procs – 1; } else count = count – 1; } } • Protect count from concurrent access • Using blocking Chapter 1  Introduction 29

30. Multicomputer OSs • Multicomputer OS Chapter 1  Introduction 30

31. Multicomputer OSs • ??? Chapter 1  Introduction 31

32. Multicomputer OSs • Huh? Chapter 1  Introduction 32

33. Programming Issues • Programming multicomputers much harder than multiprocessors • Why? • Message passing • Buffering, blocking, reliable comm., etc. • One option is to emulate shared memory on multicomputer • Large “virtual” address space Chapter 1  Introduction 33

34. Distributed Shared Memory • Pages of address space distributed among 4 machines • After CPU 1 references pg 10 • If page 10 read only and replication used Chapter 1  Introduction 34

35. Distributed Shared Memory • False sharing of page between two processes • Two independent processors share same page Chapter 1  Introduction 35

36. Network OS • Network OS • Each processor has its own OS Chapter 1  Introduction 36

37. Network OS • Clients and server in a network OS • Global shared file system Chapter 1  Introduction 37

38. Distributed System • Distributed OS not a distributed system by our definition • Network OS not a distributed system by our definition • What we need is middleware… Chapter 1  Introduction 38

39. Positioning Middleware • A distributed system as middleware • Individual node managed by local OS • Middleware hides heterogeneity of underlying systems Chapter 1  Introduction 39

40. Middleware and Openness • Open middleware-based system • Middleware layer should • Use the same protocols • Provide same interfaces to apps Chapter 1  Introduction 40

41. Comparison of Systems • Middleware rocks! Chapter 1  Introduction 41

42. Middleware Services • Main goal is access transparency • Hides low level message passing • Naming • Like yellow pages or URL • Persistence • For example, a distributed file system • Distributed transactions • Read and writes are atomic • Security Chapter 1  Introduction 42

43. Client Server Model • Read this section Chapter 1  Introduction 43

44. Clients and Servers • Interaction between client and server Chapter 1  Introduction 44

45. Example Client and Server • header.h • Used by client • And by server Chapter 1  Introduction 45

46. Example Client and Server • A sample server Chapter 1  Introduction 46

47. Example Client and Server • Client using server to copy a file Chapter 1  Introduction 47

48. Processing Level • Internet search engine as 3 layers Chapter 1  Introduction 48

49. Multitiered Architectures • Alternative client-server organizations Chapter 1  Introduction 49

50. Multitiered Architectures • A server acting as client Chapter 1  Introduction 50