Introduction to Naming in Distributed Systems: Challenges and Solutions

1. Introduction to Naming CS-4513 Distributed SystemsHugh C. Lauer Slides include materials from Modern Operating Systems, 3rd ed., by Tannenbaum,Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne,Distributed Systems: Principles & Paradigms, 2nd ed. By Tanenbaum and Van Steen, and Distributed Systems: Concepts and Design, 4th ed., by Coulouris, et. al. Introduction to Naming

2. Naming • One of most difficult issues in all of computing • Most large, long-lived systems outgrow their naming subsystems • Scalability • Global distribution • Reliability • Consistency • … • No general solution Introduction to Naming

3. Naming (continued) • Applies to • Programs • Files • Machines (aka hosts) • Services (remote and local) • People • Places • Things (objects) • … Introduction to Naming

4. Resources • Tanenbaum (Modern Operating Systems) §8.4 • Naming Transparency • Other useful topics about Distributed Systems • Also, Coulouris, et al • Chapter 9 Introduction to Naming

5. Class discussion(laptops closed, please) What do we mean by “name”? Introduction to Naming

6. Fundamental Tricotomy • Names • (Abstract) strings, data structures, pictures, icons, sounds, etc., that refer to specific entities • Addresses • Identifiers (i.e., names) of places to find those entities • Routes • Sequences of names or addresses specifying steps to follow to get to those entities • Sometimes called paths Hoping that you end upat the right place! Introduction to Naming

7. Example — a Person • Name — Jonathan M. Smith • Address — 123 Park St., Andover, MA • Route — • From Andover center, go west 1.2 miles • Turn right, then take 3rd left • He lives at the 2nd house on the right Introduction to Naming

8. Example — a File • Name — index.html • Address — inode#54321 • Route (path) — • /csshare/cs4513/public_html/b10/index.html Introduction to Naming

9. Example — a computer • Name— ccc3.wpi.edu • Address — 130.215.36.150 • Route — • IP numbers are converted to routes by network layer of protocol stack • Route is a sequence of hops derived from routing tables at each node in network Introduction to Naming

10. Example — a Web Page • Name — CS-4513 Home Page • Address — inode#54321 • Route — • http://www.cs.wpi.edu/~cs4513/b10 Introduction to Naming

11. Names vs. Addresses • A name in one context may be an address in another • Example:– • “Park Street” is (part of) Jonathan Smith’s address • “Park Street” is a name of a road in the town Introduction to Naming

12. Definition • Resolve:–To find an object or entity, given its name … • …so that you (or your system) can • Access it • Communicate with it • Invoke its methods • … Introduction to Naming

13. Definition • Namespace – the set of possible names that can be resolved by a particular system or naming service • E.g., • Set of strings • Set of strings of a particular structure • Set of sounds • … Introduction to Naming

14. Names vs. Addresses (continued) • When you hand a name to a naming system to resolve, it must … • …look it up to find the corresponding object or entity • Or resolve it some other way • When you hand an address to a system, it already knows how to find it. • E.g., an IP address Introduction to Naming

15. Addresses vs. Paths • When you hand an address to a system, it already knows how to find it. • E.g., an IP address, an i-node of a file system • When you hand a path (i.e., a route) to a system, you are giving it a sequence of steps it knows how to follow • Iteratively or recursively • Hoping it eventually reaches the thing you want! Introduction to Naming

16. Naming Systems we all know • Unix/Linux/Windows file names • URL’s on the World Wide Web • Types and objects in a C++ or Java program • Computers attached to the Internet • … Introduction to Naming

17. Types of Names inDistributed Systems • Flat • All names are equivalent in namespace • Must be globally unique • Hierarchical • Names (usually) have structure • Unique only within immediately containing level • Each level resolved within context of next higher level Introduction to Naming

18. Flat Name Spaces • Need global directory • May be replicated • May be partitioned • Not (necessarily) tied to location … • But many challenges • Issues of scaling • Imagine 6  108 computers attached to Internet • (50,000 named “Frodo”) Introduction to Naming

19. Questions? Introduction to Naming

20. Some Naming Services • Directories of Files • DNS – the Domain Name Service • Grapevine • An e-mail system developed at Xerox PARC in the early 1980s. Introduction to Naming

21. Directories of Files • Maps file name to inode • Or metadata, extents, or file blocks • I.e., addresses as understood by file system • Scope • Limited to a single file system • Hierarchical • Root is a directory • Entries may point to files, other directories, or symbolic links • Locally owned and administered • Anyone (with permissions) may create and own a directory Introduction to Naming

22. Directories of Files (continued) • Names within a directory are unique • May be reused in other directories • Multiple names possible for objects • Unix and Linux • Some objects may have no names • Files in transition; files to be deleted upon close, etc. • No replication, not distributed • Except mounted directories Introduction to Naming

23. DNS — Domain Name Service • Maps names of the form www.cs.wpi.edu to IP addresses • Hierarchical • Structured names denote hierarchy • DNS hierarchy versus file hierarchy • In file hierarchy, we often • Cache part of path name • Set up working directory • Use relative names • In DNS hierarchy, we usually • Use fully qualified names • Cache levels only forperformance Introduction to Naming

24. DNS (continued) • Designed to be scalable to large numbers of names and objects • Usable world-wide • Distributed and replicated • Robust against failures of individual servers • Inconsistency is tolerable • Infrequent changes • Local administration of levels within hierarchy Introduction to Naming

25. Common DNS Queries • Host name resolution • E.g, www.dcs.qmul.ac.uk IP address • Multiple IP addresses possible for one name • Mail host location • E.g, tom@dcs.qmul.ac.uk  mail server name • Multiple mail servers possible for one domain • Multiple domains for one mail server Introduction to Naming

26. Other DNS Queries • Aliases • Reverse resolution • IP address  host name • Host information • Not used so often • “Well-known” services • FTP, telnet, etc. • Other attributes Introduction to Naming

27. DNS Naming Hierarchy Introduction to Naming

28. Resolving DNS names to IP addresses • Two approaches:– • Iterative • Recursive Introduction to Naming

29. Iterative Resolution of Names E.g., ftp.cs.vu.nl Introduction to Naming

30. Recursive Resolution of Names E.g., ftp.cs.vu.nl Introduction to Naming

31. Hybrid Resolution Possible • E.g., global server may not be willing to recursively resolve ftp.cs.vu.nl • Returns name server for nl only • nl server may not be willing to recursively resolve ftp.cs.vu • Returns name server for vu only • vu server resolves ftp.csrecursively within vu.nl domain Introduction to Naming

32. Text file containing records Each record is {Name, Type, value(s)} DNS Domain Registry Database Introduction to Naming

33. Example Introduction to Naming

34. DNS Implementation • One master copy per domain or subdomain • Edited manually by system administrator • Using text editor or GUI tool • Multiple slave copies • Automatically copied / updated periodically from master • Stored in file on slave server, reloaded up restart • Caching in DNS clients • Lots and lots of caching • Entries include TTL (time-to-live) specification Master and slave copies areauthoritative I.e., the truth! Cached copies are notauthoritative Pointer to authoritative copyis sometimes provided Introduction to Naming

35. Implementation in Linux/Unix • BIND — Berkeley Internet Name Domain • http://www.bind9.net/ • named — the Name Daemon • Implements local DNS service • Multiple databases • Primary or secondary • Secondary database points back to primary • Pointer to “higher level” service • For resolving names not in own database Introduction to Naming

36. Example • Want to find www.cs.wpi.edu • My DNS contacts DNS server 68.87.71.226 • A Comcast server specified in my DHCP lease • Comcast DNS service • Almost certainly has root (global) domain in cache • Probably has many .edu entries in cache (very large) • Possibly has .wpi.edu in cache (many local users) • May have .cs.wpi.edu • Consults cache or official server for IP address Introduction to Naming

37. Example (continued) C:\>nslookup cs.wpi.edu Server: cns.chelmsfdrdc2.ma.boston.comcast.net Address: 68.87.71.226 Non-authoritative answer: Name: cs.wpi.edu Address: 130.215.28.181 Introduction to Naming

38. Some Special Cases • Google • Yahoo • MSN • Need to distribute names geographically • Need to distribute different addresses for same name • Special handling of replicated databases • Implementation:– • Beyond the scope of this course Introduction to Naming

39. Naming Privacy • Problem:– corporations need to have own domains • www.merl.com • Some public hosts – mail server, web server, etc. • Does not want to expose names of internal hosts to outside world • E.g., proprietary stuff • But wants to make them visible internally • hotspur.merl.com Introduction to Naming

40. Solution • Two name services for same domain name! • Internal • External • External — visible to Internet (DMZ) • Database contains only a few names • Points to other internet DNS’s for resolution of internet names • Internal — seen only by internal hosts • Database contains all internal names • Points to external version for resolution of internet names Introduction to Naming

41. Result • Internal names can be resolved internally, not externally • hotspur.merl.com • Internal names and IP addresses are invisible on Internet • All external names can be resolved internally • Two levels of indirection Introduction to Naming

42. Comments – Domain Name Service • Root servers are highly replicated • Maintained by internet authorities • Change infrequently • First level domain servers are highly replicated • Maintained by internet or national authorities • Domains come and go; most are static • Very large – .com, .net, .edu • Everyone caches root and first level domain names Introduction to Naming

43. Summary – Domain Name Service • Hierarchical • Distributed and replicated • Decentralized below top-level and first-level domains • Scalable • Supports 108 to 109 nodes • Reading • Tanenbaum, §8.4 Introduction to Naming

44. Another Example Grapevine Registry System Introduction to Naming

45. Questions? Introduction to Naming