Distributed Systems: General Services

1. Distributed Systems: General Services Distributed systems: general services

2. Overview of chapters • Introduction • Co-ordination models and languages • General services • Ch 11 Time Services, 11.1-11.3 • Ch 9 Name Services • Ch 8 Distributed File Systems • Ch 10 Peer-to-peer Systems • Distributed algorithms • Shared data Distributed systems: general services

3. This chapter: overview • Introduction • Time services • Name services • Distributed file systems • Peer-to-peer systems Distributed systems: general services

4. Introduction • Objectives • Study existing subsystems • Services offered • Architecture • Limitations • Learn about non-functional requirements • Techniques used • Results • Limitations Distributed systems: general services

5. This chapter: overview • Introduction • Time services • Name services • Distributed file systems • Peer-to-peer systems Distributed systems: general services

6. Time services: Overview • What & Why • Clock synchronization • Logical clocks (see chapter on algorithms) Distributed systems: general services

7. Time services • Definition of time • International Atomic Time (TAI)A second is defined as 9.192.631.770 periods of transition between two hyperfine levels of the ground state of Caesium-133 • Astronomical time Based on the rotation of the earth on its axis and its rotation about the sun. But due to the tidal friction the earth’s rotation is getting longer • Co-ordinated Universal Time (UTC)International standard based Atomic time, but a leap second is occasionally inserted or deleted to keep up with Astronomical time Distributed systems: general services

8. Time services (cont.) • Why is time important? • Accounting (e.g. logins, files,…) • Performance measurement • Resource management • some algorithms depend on it: • Transactions using timestamps • Authentication (e.g. Kerberos) • Example Make Distributed systems: general services

9. Time services (cont.) • An example: Unix tool “make” • large program: • multiple source files: graphics.c • multiple object files: graphics.o • change of one file  recompilation of all files • use of make to handle changes: • change of graphics.c at 3h45 • make examines creation time of graphics.o • e.g. 3h43: recompiles graphics.c to create an up-to-date graphics.o Distributed systems: general services

10. Graphics.o created Clock B 3h.. 47 48 49 50 time Clock A 3h.. 41 42 43 44 Graphics.c modified Equal times Time services (cont.) • Make in a distributed environment: • source file on system A • object file on system B Make graphics.o  no effect!! Distributed systems: general services

11. Time services (cont.) • Hardware clocks • each computer has a physical clock • H(t): value of hardware clock/counter • different clocks  different values • e.g. crystal-based clocks drift due to • different shapes, temperatures, … • Typical drift rates: • 1sec/sec 1 sec in 11.6 days • High precision quartz clocks: 0.1-0.01 sec/sec • Software clocks • C(t) =  H(t) +  • Ex. nanosecs elapsed since reference time Distributed systems: general services

12. Time services (cont.) • Changing local time • Hardware: influence H(t) • Software: change  and/or  • Forward: • Set clock forward • some clock ticks appear to have been missed • Backward: • Set backward? unacceptable • Let clock run slower for a short period of time Distributed systems: general services

13. Time services (cont.) • Properties • External synchronization to UTC (S) | S(t) - C(t) | < D • Internal synchronization | Ci(t) – Cj(t)| < D • Correctness Drift rate <  • Monotonicity t’ > t  C(t’) > C(t) Distributed systems: general services

14. Time services: Overview • What & Why • Synchronising Physical clocks • Synchronising with UTC • Christian’s algorithm • The Berkeley algorithm • Network Time Protocol • Logical clocks (see chapter on algorithms) Distributed systems: general services

15. Time servicessynchronising with UTC • Broadcast by Radio-stations • WWV (USA) (accuracy 0.1 - 10 msec.) • Broadcast by satellites • Geos (0.1 msec.) • GPS (1 msec) • Receivers to workstations Distributed systems: general services

16. P Ts Time servicesChristian’s algorithm • assumption: time server process Ts on a computer with UTC receiver • process P: request/reply with time server T-request reply (t) Distributed systems: general services

17. Time servicesChristian’s algorithm • how to set clock at P? • time t inserted in Ts • time for P: t + Ttrans • Ttrans = min + x • min = minimum transmission time • x? depends on • network delay • process scheduling Distributed systems: general services

18. Time servicesChristian’s algorithm • In a synchronous system • Upper bound on delay: max • P sets time to t + (max – min) / 2 • Accuracy: (max – min) / 2 • In an asynchronous system? Distributed systems: general services

19. Time servicesChristian’s algorithm • practical approach • P measures round-trip time: Tround • P sets time to t + Tround/2 • accuracy? • time at Ts when message arrives at P is in range: [t + min, t + Tround - min] • accuracy = ((Tround/2) - min) • problems: • single point of failure • impostor time server Distributed systems: general services

20. Time servicesThe Berkeley algorithm • Used on computers running Berkeley Unix • active master elected among computers whose clocks have to be synchronised • no central time server • upon failure of master: re-election Distributed systems: general services

21. Time servicesThe Berkeley algorithm • algorithm of master: • periodically polls the clock value of other computers • estimates its local clock time (based on round-trip delays) • averages the obtained values • returns the amount by which each individual’s slave clock needs adjustment • fault-tolerant average (strange values excluded) may be used Distributed systems: general services

22. Time services Network Time Protocol • Standard for the Internet • Design aims and features: • Accurate synchronization despite variable message delays • Statistical techniques to filter timing data from different servers • Reliable service that can survive losses of connectivity • Redundant servers • hierarchy of servers in synchronised subnets Distributed systems: general services

23. Time servicesNetwork Time Protocol Stratum1 have UTC receiver 1 Level = stratum 2 2 3 3 3 synchronisation Distributed systems: general services

24. Time servicesNetwork Time Protocol • Three working modes for NTP servers • Multicast mode • used on high speed LAN • slave sets clock assuming small delay • Procedure-call mode • similar to Christian’s algorithm • Symmetric Mode • used by master servers and higher levels • association between servers Distributed systems: general services

25. T i-2 T i-1 Server B m m’ time Server A T i-3 T i Time servicesNetwork Time Protocol • Procedure-call and Symmetric mode • each message bears timestamps of recent message events Distributed systems: general services

26. d i = a - b o i = (a + b) / 2 o i - d i / 2  o  o i + d i / 2 estimates Time servicesNetwork Time Protocol • Compute approximations of offset and delay T i-2 T i-1 Server B m m’ o true offset Transmission times t t’ Server A T i-3 T i d delay total transmission time T i-2 = T i-3 + t + o T i = T i-1 + t’ - o a = T i-2 - T i-3= t + o b = T i-1 - T i= o – t’ Distributed systems: general services

27. Time servicesNetwork Time Protocol • Procedure-call and Symmetric mode • data filtering applied to successive pairs • NTP has complex peer-selection algorithm; favoured are peers with : • a lower stratum number • a lower synchronisation dispersion • Accuracy: • 10 msec on Internet • 1 msec on LAN Distributed systems: general services

28. This chapter: overview • Introduction • Time services • Name services • Distributed file systems • Peer-to-peer systems Distributed systems: general services

29. Name services • Overview • Introduction • Basic service • Case study: Domain name system • Directory services Distributed systems: general services

30. Name servicesIntroduction • Types of name: • users • files, devices • service names (e.g. RPC) • port, process, group identifiers • Attributes associated with names • Email address, login name, password • disk, block number • network address of server • ... Distributed systems: general services

31. Name servicesIntroduction • Goal: • binding between name and attributes • History • originally: • quite simple problem • scope: a single LAN • naïve solution: all names + attributes in a single file • now: • interconnected networks • different administrative domains in a single name space Distributed systems: general services

32. Name servicesIntroduction • naming service separate from other services: • unification • general naming scheme for different objects • e.g. local + remote files in same scheme • e.g. files + devices+ pipes in same scheme • e.g. URL • integration • scope of sharing difficult to predict • merge different administrative domains • requires extensible name space Distributed systems: general services

33. Name servicesIntroduction • General requirements: • scalable • long lifetime • high availability • fault isolation • tolerance of mistrust Distributed systems: general services

34. Name services • Overview • Introduction • Basic service • Domain name system • Directory services Distributed systems: general services

35. Name servicesBasic service • Operations: identifier lookup (name, context) register (name, context, identifier) delete (name, context) • From a single server to multiple servers: • hierarchical names • efficiency • availability Distributed systems: general services

36. Name servicesBasic service • Hierarchical name space • introduction of directories or subdomains • /users/pv/edu/distri/services.ppt • cs.kuleuven.ac.be • easier administration • delegation of authority • less name conflicts: unique per directory • unit of distribution: server per directory + single root server Distributed systems: general services

37. Name servicesBasic service • Hierarchical name space • navigation • process of locating naming data from more than one server • iterative or recursive • result of lookup: • attributes • references to another name server Distributed systems: general services

38. Lookup(“/users/pv/edu/…”,…) Ref to server-users Name servicesBasic service P root users src sys pv ann Distributed systems: general services

39. root Lookup(“pv/edu/…”,…) Ref to server-pv users src sys pv ann Name servicesBasic service P Distributed systems: general services

40. Name servicesBasic service • Efficiency  caching • hierarchy introduces multiple servers more communication overhead more processing capacity • caching at client side: • <name, attributes> are cached • fewer request  lookup performance higher • inconsistency? • Data is rarely changed • limit on validity Distributed systems: general services

41. Name servicesBasic service • Availability  replication • at least two failure independent servers • primary server: accepts updates • secondary server(s): • get copy of data of primary • period check for updates at primary • level of consistency? Distributed systems: general services

42. Name services • Overview • Introduction • Basic service • Case study: Domain name system • Directory services Distributed systems: general services

43. Name servicesDomain name system • DNS = Internet name service • originally: an ftp-able file • not scalable • centralised administration • a general name service for computers and domains • partitioning • delegation • replication and caching Distributed systems: general services

44. Name servicesDomain name system • Domain name • hierarchy nix.cs.kuleuven.ac.be country (België) univ. (academic) K.U.Leuven dept. computer science name of computersystem Distributed systems: general services

45. Name servicesDomain name system • 3 kinds of Top Level Domains (TLD) • 2-letter country codes (ISO 3166) • generic names (similar organisations) • com commercial organisations • org non-profit organisations (bv. vzw) • int international organisations (nato, EU, …) • net network providers • USA oriented names • edu universities • gov American government • mil American army • new generic names • biz, info, name, aero, museum, ... Distributed systems: general services

46. Name servicesDomain name system • For each TLD: • administrator (assign names within domain) • “be”: DNS BE vzw (previous: dept. computer science) • Each organisation with a name : • responsible for new (sub)names • e.g. cs.kuleuven.ac.be • Hierarchical naming + delegation  workable structure Distributed systems: general services

47. Name servicesDomain name system • Name servers • root name server + • server per (group of) subdomains+ replication  high availability+ caching  acceptable performance • time-to-live to limit inconsistencies Distributed systems: general services

48. Systems/subdomains type IP-adresof cs.kuleuven.ac.be nix A 134.58.42.36 idefix A 134.58.41.7 droopy A 134.58.41.10 stevin A 134.58.41.16 ... Name servicesDomain name system Name server cs.kuleuven.ac.be A = Address Distributed systems: general services

49. Name servicesDomain name system Name server kuleuven.ac.be Machines/subdomeinen type IP-adresvan kuleuven.ac.be cs NS 134.58.39.1 esat NS … www A … ... NS = NameServer Distributed systems: general services

50. www. cs.vu.nl 130.37.24.11 Root-NS Lokale NS (cs.kuleuven.ac.be) NS (nl) NS (vu.nl) 130.37.24.11 NS (cs.vu.nl) Name servicesDomain name system Example : www.cs.vu.nl Distributed systems: general services