Directory services allow entities to be described through attribute-value pairs, known as attribute-based naming. Attributes can be used to search for entities like email messages that have attributes for sender, recipient, subject, etc. Discovery services register and lookup services in distributed systems through attributes. Jini discovery service uses multicast to locate lookup services and services register attributes with lookup services. The Global Name Service (GNS) provided a distributed directory system with a tree structure of directories and references to support resource location and email addressing across changing organizational structures.

1. Presented by
Ramchandra Regmi
Mizoram University

2. Directory Services
There are many ways in which descriptions can be
provided, but a popular one in distributed systems is to
describe an entity in terms of (attribute, value) pairs,
generally referred to as attribute-based naming.
Attribute-based naming systems are also known as
directory services, whereas systems that support
structured naming are generally called naming systems.
With directory services, entities have a set of associated
attributes that can be used for searching.
For example, in an e-mail system, messages can be tagged
with attributes for the sender, recipient, subject, and so
on.

3. What it all boils down to is that designing an appropriate
set of attributes is not trivial. In most cases, attribute
design has to be done manually.
In the distributed systems, one particularly relevant
development is the resource description framework
(RDF). Fundamental to the RDF model is that resources are
described as triplets consisting of a subject, a predicate,
and an object.
For example, (Person, name, Alice) describes a resource
Person whose name is Alice.
Cont..

4. Discovery services
Directory service that registers services provided in a
spontaneous networking environment.
Provide an interface for automatically registering and
de-registering services, as well as an interface for clients to
look up the services they require.
Ex : a printer (or the service that manages it) may register its
attributes with the discovery service as follows
:‘resourceClass = printer, type=laser, color=yes, resolution=600dpi,
location=room101,url=http://www.hotelNW.com/services/printer98’

5. Jini Discovery Service
Designed to be used for spontaneous networking
Entirely java-based.
Computers communicate by means of RMI, and can
download code if necessary.
Discovery-related components in a Jini system are
look up services.
A Jini service (such as printing service) may be
registered with many look up services.

6. Cont..
How to locate the look up service ?
Using a well-known multicast IP address.
Multicast the request using a “TTL” value.
Look up services announce their existence to the
same multicast address.
Service instance is configured with one or more group
names such as ‘finance’, ‘sales’, ‘admin’, which act as
scoping labels.
Limited period of time.

7. Global Name Service (GNS)
Designed and implemented by Lampson and colleagues at
the DEC Systems Research Center (1986).
Provide facilities for resource location, email addressing
and authentication.
When the naming database grows from small to large
scale, the structure of name space may change.
the service should accommodate it.
The structure of the name space may change during that
time to reflect changes in organizational structures.
Difficult to maintain complete consistency between all
copies of a database entry.

8. GNS Structure
Tree of directories holding names and values.
Multi-part pathnames refer to the root or relative
working directory (like Unix file system).
Unique Directory Identifier (DI).
A directory contains list of names and references .
Leaves of tree contain value trees (structured values).

9. Fig4:GNS directory tree and value tree
UK FR
AC
QMWDI: 322
Peter.Smith
passwordmailboxes
DI: 599 (EC)
DI: 574DI: 543
DI: 437
Alpha GammaBeta

10. References:
[1] Distributed Systems, Principle and Paradigms by
Andrew S. Tanenbaum/Second Edition.
 [2] https://books.google.co.in/books?id=O-
FyIBTjVOYC&printsec=frontcover&dq=distributed+syste
ms+principles+and+paradigms+4th+edition&hl=en&sa=
X&redir_esc=y#v=onepage&q=Directory%20Service&f=fa
lse [Accessed on 24/10/2015].
[3] Distributed Systems, Concept and design by George
Coulouris/ Fourth Edition.