1. 5.3 Structured Naming
Distributed Systems
Chapter 5
Naming
Prepared by: Ahmed Magdy Ezzeldin
Reference: Distributed Systems Principles and Paradigms 2nd Edition
2. 5.3.1 Name Spaces
●A Name space is a labeled, directed graph with two types of
nodes.
●A leaf node represents a named entity and has the property that it
has no outgoing edges.
● And a directory node that has a number of outgoing edges, each
labeled with a name, and has an associated identifier.
●A directory table holds (edge label, node identifier) for the
directory contents.
●n0 is the root node and name : N label_1 , label_2 , ..... label_n is
called a path. If N is the root then the path is called an absolute
path, otherwise it is called a relative path. We use "/" as a
separator between labels and as a representation for n0.
3. 5.3.1 Name Spaces [continued]
A name space can be strictly hierarchical so that the naming graph
is organized as a tree (one absolute path for each node). In other
cases it can be organized in a directed acyclic graph (not permitted
to have a cycle).
4. 5.3.2 Name Resolution
- Resolution starts by reading the directory table
at the first node of the path and then proceeding
to the next identified node and continue the look-
up until reaching the last node where it returns the
node identifier for the last node in the path.
- Closure Mechanism
Knowing how and where to start name resolution
is generally referred to as a closure mechanism.
Essentially, a closure mechanism deals with
selecting the initial node in a name space from
which name resolution is to start.
6. 5.3.2 Name Resolution [continued]
- Linking and Mounting
There are 2 ways of aliasing naming
1- In a directed graph where there are 2 absolute paths to
a certain node. (Like Hard Links in UNIX file system)
2- In a tree structure, we can represent an entity by a leaf
node that stores an absolute path name of another node. (like
symbolic links in UNIX file system) [Figure 5-11]
A mounted file system corresponds to letting a directory node
store the identifier of a directory node from a different name
space. To mount a foreign name space in a distributed
system requires at least the following information:
1. The name of an access protocol.
2. The name of the server.
3. The name of the mounting point in the foreign name
space. [Figure 5-12]
8. 5.3.3 Implementation of a Name Space
- Name Space Distribution
Name spaces are organized hierarchically and also
partitioned into logical layers:
1- Global layer (Root and its direct children) which is
stable (rarely changed). In this layer availability is more
important than performance as caching can be used.
2- The administrational layer (they represent groups of
entities that belong to the same organization or
administrational unit) stable but not like the global layer.
3- The managerial layer consists of nodes that may
typically change regularly. For example, nodes
representing hosts in the local network belong to this
layer. [Figure 5-13] & [Figure 5-14]
11. 5.3.3 Implementation of a Name Space [continued]
● Implementation of Name Resolution
- Iterative name resolution:
Where the client name resolver requests from the
root name server to respond with the URL parts
identifiers. The root server replies with what it can
resolve then the response is sent back to the
client that sends a new request to the next name
server that has been resolved by the root server
and so on until the whole URL has been resolved.
[Figure 5-15]
13. 5.3.3 Implementation of a Name Space [continued]
- Recursive Name resolution:
Where it is the responsibility of the name server
not the client to resolve the name by requesting
the rest of the name to be resolved from the other
servers recursively which adds a performance
demand on each server as it has to resolve the
whole URL, but caching can be used more
effectively than iterative name resolution.
[Figure 5-16] & [Figure 5-17] & [Figure 5-18]
17. 5.3.4 The DNS Name Space [continued]
● The DNS name space is hierarchically organized as a
rooted tree.
● A label is a case-insensitive string made up of
alphanumeric characters.
● A label has a maximum length of 63 characters.
The length of a complete path name is restricted to 255
characters.
● The string of a path name consists of its labels, starting
with the rightmost one, and separating the labels by a dot.
● The root is represented by a dot. e.g. the path name root:
<com, google, web, s1>, is represented by the string
"s1.web.google.com.", which includes the rightmost dot to
indicate the root node (the dot is removed for readability).
● A subtree is called a domain; a path name to its root node
is called a domain name. [Figure 5-19]
19. 5.3.4 The DNS Name Space [continued]
- DNS Implementation
● The DNS name space is divided into a global
layer and an administrational layer.
●Each zone is implemented by a name server,
which is virtually always replicated for availability.
●A DNS database is implemented as a (small)
collection of files, a file for each zone.
