Distributed Systems

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Distributed Systems Principles and Paradigms
Chapter 04 Naming
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Naming Entities

• Names, identifiers, and addresses
• Name resolution
• Name space implementation

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Naming
Essence Names are used to denote entities in a
distributed system. To operate on an entity, we
need to access it at an access point. Access
points are entities that are named by means of an
address. Note A location-independent name for
an entity E, is independent from the addresses of
the access points offered by E.
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Identifiers
Pure name A name that has no meaning at all it
is just a random string. Pure names can be used
for comparison only. Identifier A name having
the following properties - P1 Each identifier
refers to at most one entity - P2 Each entity is
referred to by at most one identifier - P3 An
identifier always refers to the same entity
(prohibits reusing an identifier) Observation
An identifier need not necessarily be a pure
name, i.e., it may have content. Question Can
the content of an identifier ever change?
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Name Space (1/2)
Essence a graph in which a leaf node represents
a (named) entity. A directory node is an entity
that refers to other nodes.
Note A directory node contains a (directory)
table of (edge label, node identifier) pairs.
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Name Space (2/2)
Observation We can easily store all kinds of
attributes in a node, describing aspects of the
entity the node represents

• Type of the entity
• An identifier for that entity
• Address of the entitys location
• Nicknames
• ...

Observation Directory nodes can also have
attributes, besides just storing a directory
table with (edge label, node identifier) pairs.
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Name Resolution
Name Resolution - the process of looking up a
name Problem To resolve a name we need a
directory (initial) node. How do we actually find
that initial node? Closure mechanism The
mechanism to select the implicit context from
which to start name resolution
Question Why are closure mechanisms always
implicit? Observation A closure mechanism may
also determine how name resolution should proceed
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Name Linking (1/2)

• Hard link What we have described so far is a
  path name a name that is resolved by following a
  specific path in a naming graph from one node to
  another.
• Soft link Allows a node O to contain a name of
  another node
• First resolve Os name (leading to O)
• Read the content of O, yielding name
• Name resolution continues with name
• Observations
• The name resolution process determines that we
  read the content of a node, in particular, the
  name in the other node that we need to go to.
• One way or the other, we know where and how to
  start name resolution given name

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Name Linking (2/2)
Observation the path name /home/steen/keys,
which refers to a node containing the absolute
path name /keys, is a symbolic link to node n5.
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Merging Name Spaces (1/3)
Problem We have different name spaces that we
wish to access from any given name
space. Solution 1 Introduce a naming scheme by
which pathnames of different name spaces are
simply concatenated (URLs).
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Merging Name Spaces (2/3)
Solution 2 Introduce nodes that contain the name
of a node in a foreign name space, along with
the information how to select the initial context
in that foreign name space.
Mount point (Directory) node in naming graph
that refers to other naming graph Mounting point
(Directory) node in other naming graph that is
referred to.
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Merging Name Spaces (3/3)
Solution 3 Use only full pathnames, in which the
starting context is explicitly identified, and
merge by adding a new root node (DCEs Global
Name Space).
Note In principle, you always have to start from
the new root
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Name Space Implementation (1/2)
Basic issue Distribute the name resolution
process as well as name space management across
multiple machines, by distributing nodes of the
naming graph. Consider a hierarchical naming
graph and distinguish three levels Global layer
Consists of the high-level directory nodes. Main
aspect is that these directory nodes have to be
jointly managed by different administrations Admin
istrational layer Contains mid-level directory
nodes that can be grouped in such a way that each
group can be assigned to a separate
administration. Managerial layer Consists of
low-level directory nodes within a single
administration. Main issue is effectively mapping
directory nodes to local name servers.
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Name Space Implementation (2/2)
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Iterative Name Resolution

• resolve(dir, name1,, nameK) is sent to Server0
  responsible for dir
• Server0 resolves resolve(dir, name1) ? dir1,
  returning the identification (address) of
  Server1, which stores dir1.
• Client sends resolve(dir1,name2,, nameK) to
  Server1
• etc.

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Recursive Name Resolution

• resolve(dir,name1,,nameK) is sent to Server0
  responsible for dir
• Server0 resolves resolve(dir, name1) ? dir1, and
  sends resolve(dir,name2,,nameK) to Server1,
  which stores dir1.
• Server0 waits for the result from Server1, and
  returns it to the client

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Caching in Recursive Name Resolution

• Also see Figure 4-11 for the comparison between
  recursive and iterative name resolution with
  respect to communication costs.

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Example 1 Internet Domain Name System (DNS)

• used for looking up IP addresses of hosts and
  mail servers in Internet
• comparable to a telephone book (white pages) for
  looking up phone numbers
• DNS name space is hierarchically organized as a
  rooted tree
• The contents of a node is formed by a collection
  of resource records
• Multiple (primary, secondary, etc.) DNS servers
  are usually deployed for an organization to
  increase availability
• nslookup is a utility for querying DNS service

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DNS Resource Records

• Figure 4-12. The most important types of resource
  records forming the contents of nodes in the DNS
  name space.

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Sample DNS Records

• Figure 4-13.
• An excerpt from the DNS database for the zone
  cs.vu.nl.

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Example 2 X.500 Directory Service (1)

• ITU standard for directory services
• provides directory service based on a
  description of properties instead of a full name
  (e.g., yellow pages in telephone book)
• an X.500 directory entry is comparable to a
  resource record in DNS
• Each record is made up of a collection of
  (attribute, value) pairs
• The collection of all entries is called
  Directory Information Base (DIB)
• Each entry in a DIB can be looked up using a
  sequence of naming attributes, which forms a
  globally unique name called Distinguished Name
  (DN). Each naming attribute is called Relative
  Distinguished Name (RDN)
• - e.g., /CKR/OPOSTECH/OUDept. of CSE
• is analogous to the DNS name cse.postech.ac.kr
• X.500 also forms a hierarchy of the collection
  of entries called Directory Information Tree (DIT)

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X.500 Directory Entry Example

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

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A Part of Directory Information Tree
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• Two directory entries having Host_Name as RDN

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Example 2 X.500 Directory Service (2)

• DIT is usually partitioned and distributed
  across multiple servers known as Directory
  Service Agents (DSA)
• Clients are known as Directory User Agents (DUA)
• Directory Access Protocol (DAP) is used between
  DUA and DSA to insert/lookup/modify/delete
  entries in DSA
• traditionally implemented using OSI protocols
• Lightweight Directory Access Protocol (LDAP)
• implemented on top of TCP
• parameters of operations are passed as strings
• has become a de facto standard for
  Internet-based directory services for various
  applications

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