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Chapter 5: Naming and Name Service

Chapter 5: Naming and Name Service. Name: a unique (abstract) identifier to refer an entity in a system. Naming space: a closed/open space in which all names are meaningful and no ambiguity. Name resolution: a process to map a name to its identified entity. Commonly used naming methods.

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Chapter 5: Naming and Name Service

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  1. Chapter 5: Naming and Name Service Name: a unique (abstract) identifier to refer an entity in a system. • Naming space: a closed/open space in which all names are meaningful and no ambiguity. • Name resolution: a process to map a name to its identified entity. Distributed Systems

  2. Commonly used naming methods • Address: an access point entity:address  1:1 or 1:M address could be static or dynamic • Identifier: an integer or a binary string an ID refers to at most one entity each entity is referred to by at most one ID An ID always refers to the same entity • Textual name: a human-friendly symbolic string location independent many different forms: file names, DNS names Distributed Systems

  3. Name Spaces A general naming graph with a single root node. Distributed Systems

  4. root devices etc nfs server1 u1 book Chapter5 Path names Path: a sequence of directed links(labels) UNIX absolute path: /nfs/server1/u1/book/Chapter5/ UNIX relative path：book/Chapter5 DOS absolute path：C:\u1\book\Chapter\ MacOS relative path：u1:book:Chapeter5 Distributed Systems

  5. r (a) Tree with shared leaf nodes r (b) Directed graph without rings r (c) Directed graph Possible topologies of naming space • Global name: denotes the same entity, no matter where the name is used ( absolute path) • Local name: interpretation depends on where the name is used (relative path) • Root: the name whose node has no incoming edges Distributed Systems

  6. File: from name to entity Boot block: automatically loaded to install OS into main memory Super block: contains information on the entire file system Index node: contains exact information about its associated file Disk block: contains portion of file data Distributed Systems

  7. root output file personal file A B C D root others users Student faculty root network file clients bob tom john ken Mounting point Mounting point Server 1 Client Server 2 Linking and Mounting Linking: allow multiple absolute path names to refer to the same node Mounting: allow a node to refer to a node from a different name space Distributed Systems

  8. path /nfs/server1/u1/book/Chapter5 port PORT:9 ID: 23456 server file Network address：2:60:8c:5b:7a NFS: from name to entity NFS: Network File System name resolution process maps an absolute path name to a physical file over the network Distributed Systems

  9. NFS: access remote name space Distributed Systems

  10. Client Server Client process Virtual file system Virtual file system Local File Sys. NFS Client module NFS RPC NFS server module Local File Sys. NFS: software architecture NFS RPC: open/close, read/write, rename, set attributes, etc. Distributed Systems

  11. Freq. used directories： DI：622 (world) #543=#622/Asia #642=#622/N. America Asia N. Am DI：543 DI：642 China DI：288 DI：457 Organization of the DEC GNS USA DEC: Digital Equipment Co. GNS: Global Name Service Distributed Systems

  12. DEC GNS: Merge name spaces Merging two name space Distributed Systems

  13. com edu gov mil cn fr uk ca ……. sun yale ……. ……. uoguelph ……. cs eng Admin. layer cis math ……. ……. snowhite Managerial layer li draco ……. DNS: Domain Name System Global Layer Distributed Systems

  14. DNS: logical layers • Global layer: formed by highest-level nodes, such as com, edu, gov, int, cn, ca, us, …; directory tables are rarely changed, nodes represent organizations, or groups. • Administrational layer: formed by nodes within a single organization, such as universities, companies, …; directory tables are relatively stable. • Managerial layer: formed by nodes that may typically change regularly, such as users, student accounts, email accounts, … Distributed Systems

  15. DNS: A comparison of logical layers • Zone: a part of name space that is implemented by a separate name server • Domain: a sub-tree of DNS name space • Domain Name: a path name in DNS name space (either absolute or relative) Distributed Systems

  16. Client request：li.snowhite.cis.uoguelph.ca 1 li.snowhite.cis.uoguelph.ca client name resolver Root name server 2 #<ca> li.snowhite.cis.uoguelph. 3 li.snowhite.cis.uoguelph ca name server 4 #<uoguelph> li.snowhite.cis 5 li.snowhite.cis uoguelph name server 6 #<snowhite> li 7 li snowhite name server 8 #<li> #<ca ,uoguelph, cis, snowhite, li> DNS: name resolution (iterative) Distributed Systems

  17. Client request：li.snowhite.cis.uoguelph.ca 1 li.snowhite.cis.uoguelph.ca client name resolver Root name server 8 #<ca, uoguelph, cis, snowhite, li> 2 li.snowhite.cis.uoguelph 7 #<uoguelph, cis, snowhite, li> ca name server 3 li.snowhite.cis 6 #<snowhite,li> uoguelph name server 4 li 5 #<li> snowhite name server #<ca ,uoguelph, cis, snowhite, li> DNS: name resolution (recursive) Distributed Systems

  18. DNS: resource record types Distributed Systems

  19. Naming versus Locating Entities • Direct, single level mapping between names and addresses. • Two-level mapping using identities. Distributed Systems

  20. Locating a Mobile Entity • Broadcasting: a message containing the ID of the entity is broadcast to each machine and each machine is requested to check whether it has that entity. Suitable for local network. How about moving? • Forward pointer: when an entity moves from A to B, it leaves behind a reference to its new location at B. Chain problem, broken link. • Home Based: a home location keeps track of the current location of an entity. Triangle communication. How about moving? Distributed Systems

  21. Forwarding Pointers (1) The principle of forwarding pointers using (proxy, skeleton) pairs. Distributed Systems

  22. Forwarding Pointers (2) Redirecting a forwarding pointer, by storing a shortcut in a proxy. Distributed Systems

  23. Home-Based Approach Distributed Systems

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