Naming A name in a distributed system is a string of bits or characters used to refer to an entity. To resolve name a naming system is needed.

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NamingA name in a distributed system is a
string of bits or characters used to refer to an
entity.To resolve name a naming system is
needed.
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• Naming Entities
• An entity can be anything
• An entity can be operated on
• To operate on an entity we need an access point
• An access point in a DS is called address
• An address is a special type of name
• A name for an entity independent from its address
  is called location independent
• A true identifier is a special type of name that
  uniquely identifies an entity having the
  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
• Human friendly name is another type of name

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Name Spaces Names in a DS are organized in a
name space
Root node

• A naming space can be represented as a graph with
  leaf and directory nodes.
• Absolute and relative path names are related to a
  directory node
• A global name denotes the same entity in the
  system
• A local name depends on where the name is being
  used

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Name SpaceUNIX directory node file
directoryleaf node file

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

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Linking and Mounting (1)

• The concept of a symbolic link explained in a
  naming graph within a single name space.

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Linking and Mounting (2)in distributed file
system (NFS)
Name space B
Name space A

• Mounting remote name spaces through a specific
  process protocol.

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Name Space Distribution (1)

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

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Name Space Distribution (2)
Item Global Administrational Managerial
Geographical scale of network Worldwide Organization Department
Total number of nodes Few Many Vast numbers
Responsiveness to lookups Seconds Milliseconds Immediate
Update propagation Lazy Immediate Immediate
Number of replicas Many None or few None
Is client-side caching applied? Yes Yes Sometimes

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

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Implementation of Name Resolution

• The principle of iterative name resolution.

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Implementation of Name Resolution

• The principle of recursive name resolution.

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Implementation of Name Resolution

• The comparison between recursive and iterative
  name resolution

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Naming versus Locating Entities if nodes in
managerial layers vary very often

• Direct, single level mapping between names and
  addresses.
• Two-level mapping using identities.
• Simple example in LAN ARP via
  broadcasting

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Forwarding Pointers when an entity moves it
leaves a reference to its new location
Exit item
Entry items

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

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Home-Based Approachesfor mobile entities in
large scale networks
1
0,3
2bis
2
4

• The principle of Mobile IP (2 tiered case)

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Hierarchical Approaches

• Hierarchical organization of a location service
  into domains, each having an associated directory
  node. Each entity in a domain is rapresented by a
  location record in the directory node. The root
  node has a location record for each entity.

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Hierarchical Approaches

• An entity may have multiple addresses.If an
  entity has an address in different leaf domains
  D1 and D2, then the directory node of the
  smallest domain containing D1 and D2 will have 2
  pointers.

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Hierarchical Approaches

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

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Pointer Caches caching in a hierarchical
location service supporting mobile entities is
not effectivepointer caching let the lookup
start at the directory of the smallest domainin
which a mobile entity moves regularly

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

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Scalability Issueshierarchical location services
have bottlenecks at root directory
partitioning high level nodes

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

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The Problem of Unreferenced Objects

• A graph representing objects containing
  references to each other
• Entities that are not directly or indirectly
  referenced by root nodes have to be removed.

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Removing unreferenced entities

• Reference counting
• Reference listing
• Tracing-based garbage collection
• In a distributed system garbage collection
  requires network communication