Naming

1
Naming

• Chapter 4

2
Naming

• Names play an important role in all computer
  systems.
• E.g., they can be used to share resources, to
  uniquely identify entities, to refer to
  locations, and so on.
• An important issue with naming is that a name can
  be resolved to the entity it refers to.
• In a distributed system, the implementation of a
  naming system is itself often distributed to
  achieve efficiency and scalability of the naming
  system. Three major issues covered in this part
• The organisation and implementation of
  human-friendly names (e.g., file names and WWW
  URLs).
• How to use location-independent names (or
  identifiers) to locate mobile entities.
• How to use naming system to support automatic
  collection of unreferenced objects (garbage
  collection).

3
Names, identifiers, and addresses

• A name in a distributed system is a string of
  bits or characters that is used to refer to an
  entity
• e.g., a host, printer, file, process, user,
  network connection, etc.
• An entity may have multiple access points.
• An access point is yet another special kind of
  entity.
• The name of an access point is called an address.
• An entity may have a unique identifier, which is
  a special kind of name that has the following
  properties
• An identifier refers to at most one entity.
• Each entity is referred to by at most one
  identifier.
• An identifier always refers to the same entity
  (i.e., it is never reused).
• Addresses and identifiers are represented in
  machine-readable form-- bit strings (e.g.,
  Ethernet, IP, and RAM addresses)
• Human-friendly names (e.g. file, DNS names and
  URLs) are represented as a character string.

4
Name Spaces (1)
Names are organised into a name space, which can
be represented as a labeled, directed graph with
leaf and directory nodes.

• A general naming graph with a single root node.

5
Name Spaces (2)

• The general organization of the UNIX file system
  implementation on a logical disk of contiguous
  disk blocks.

6
Name resolution Linking and Mounting (1)

• Name spaces offer a convenient mechanism for
  storing and retrieving information about entities
  by means of names.
• The process of looking up a name is called name
  resolution.
• An alias is another name for the same entity,
  which can be implemented by
• multiple absolute paths (hard links in Unix), or
• multiple leaf nodes storing absolute path names
  (symbolic links in Unix)

• The concept of a symbolic link explained in a
  naming graph.

7
Linking and Mounting (2)

• Name resolution can be used to merge different
  name spaces in a transparent way.
• To mount a foreign name space, we can let a
  directory node store the identifier (e.g. URL) of
  a directory node (normally the root) from the
  foreign name space.

• Mounting remote name spaces through a specific
  process protocol.

8
Linking and Mounting (3)

• Organization of the DEC Global Name Service

9
Name Space Distribution (1)

• An example partitioning of the DNS name space,
  including Internet-accessible files, into three
  layers.

10
Name Space Distribution (2)

• A comparison between name servers for
  implementing nodes from a large-scale name space
  partitioned into a global layer, an
  administrational layer, and a managerial layer.

11
Implementation of Name Resolution (1)

• The principle of iterative name resolution.

12
Implementation of Name Resolution (2)

• The principle of recursive name resolution.

13
Implementation of Name Resolution (3)

• Recursive name resolution of ltnl, vu, cs, ftpgt.
  Name servers cache intermediate results for
  subsequent lookups.

14
Implementation of Name Resolution (4)

• The comparison between recursive and iterative
  name resolution with respect to communication
  costs.

15
The DNS Name Space

• The most important types of resource records
  forming the contents of nodes in the DNS name
  space.

16
DNS Implementation (1)

• An excerpt from the DNS database for the zone
  cs.vu.nl.

17
DNS Implementation (2)

• Part of the description for the vu.nl domain
  which contains the cs.vu.nl domain.

18
The X.500 Name Space (1)

• A simple example of a X.500 directory entry using
  X.500 naming conventions.

19
The X.500 Name Space (2)

• Part of the directory information tree.

20
The X.500 Name Space (3)

• Two directory entries having Host_Name as RDN.

21
Naming versus Locating Entities

• Direct, single level mapping between names and
  addresses.
• T-level mapping using identities.

22
Simple solutions Broadcasting and Multicasting

• A location service accepts an identifier as input
  and returns the current address of the identified
  entity.
• Simple solutions exist to work in a local area
  network.
• Address Resolution Protocol (ARP) to map the IP
  address of a machine to its data-link address,
  which uses broadcasting.
• Multicasting can be used to locate entities in
  point-to-point networks (such as the Internet).
• Each multicasting address can be associated with
  multiple replicated entities.

23
Forwarding Pointers (1)

• The principle of forwarding pointers using
  (proxy, skeleton) pairs.

24
Forwarding Pointers (2)

• Redirecting a forwarding pointer, by storing a
  shortcut in a proxy.

25
Home-Based Approaches

• The principle of Mobile IP.

26
Hierarchical Approaches (1)

• Hierarchical organisation of a location service
  into domains, each having an associated directory
  node.

27
Hierarchical Approaches (2)

• An example of storing information of a
  (replicated) entity having two addresses in
  different leaf domains.

28
Hierarchical Approaches (3)

• Looking up a location in a hierarchically
  organised location service.

29
Hierarchical Approaches (4)

• An insert request is forwarded to the first node
  that knows about entity E.
• A chain of forwarding pointers to the leaf node
  is created.

30
Pointer Caches (1)

• Caching a reference to a directory node of the
  lowest-level domain in which an entity will
  reside most of the time.

31
Pointer Caches (2)

• A cache entry that needs to be invalidated
  because it returns a nonlocal address, while such
  an address is available.

32
Scalability Issues

• The scalability issues related to uniformly
  placing subnodes of a partitioned root node
  across the network covered by a location service.

33
The Problem of Unreferenced Objects

• An example of a graph representing objects
  containing references to each other.

34
Reference Counting (1)

• The problem of maintaining a proper reference
  count in the presence of unreliable communication.

35
Reference Counting (2)

• Copying a reference to another process and
  incrementing the counter too late
• A solution.

36
Advanced Referencing Counting (1)

• The initial assignment of weights in weighted
  reference counting
• Weight assignment when creating a new reference.

37
Advanced Referencing Counting (2)

• Weight assignment when copying a reference.

38
Advanced Referencing Counting (3)

• Creating an indirection when the partial weight
  of a reference has reached 1.

39
Advanced Referencing Counting (4)

• Creating and copying a remote reference in
  generation reference counting.

40
Tracing in Groups (1)

• Initial marking of skeletons.

41
Tracing in Groups (2)

• After local propagation in each process.

42
Tracing in Groups (3)

• Final marking.