Name And Directory Services

Harshal Jhaveri

OutlineIntroductionName services and Directory servicesResolutionObject attribute and Name structuresName space and Information baseReferences

IntroductionWhat is it? A Name Service is a generic way of describing how a named object can be addressed and subsequently located by using its address.[1]

Why needed? Making a request to a service or accessing an object by means of inter-process communication requires that one must first locate the service or object.

Service? Service are abstractions of objects. They are usually represented by processes with a service access point.

Objects? Object may be users, computers, communication links or other resources such as files.

(Chow, Johnson, 1997)

Name services and Directory services

Services and Objects are normally identified by textual names. Alternatively, if names are unknown, service or object entities can be described by using attributes associated with them.Although services and objects have distinct meanings, their naming issues are similarName and Directory services, in a narrow sense are look-up operations. The terms name service and directory service are often used interchangeably

(Chow, Johnson, 1997)

ResolutionAny object entity in a system must be named and located before it can be used.The operation of locating an object is called a resolution process.Each object entity has a logical address in the OS and physical location in the network.Resolution process has two stages

Name resolution – Maps names to address Address resolution – Maps addresses to routes that show how an object can be physically located.

(Chow, Johnson, 1997)

Object AttributeObject entity is characterized by it attributesName resolution has two special object attributes. Name and AddressName Space – The collection of names which are recognized by a name service, with their corresponding attributes and addresses.

(Chow, Johnson, 1997)

Name Structures

Name Space and Information

Base Using X.500 terminology, the conceptual data model for storing and representing object information is called the Directory Information Base (DIB). The Directory Service (DS) of the CCITT X.500 standard provides structural and syntactic rules for specifying a DIB in a hierarchical Directory Information Tree (DIT).A large name space and its corresponding DIT can be decomposed and distributed into naming domains and naming contexts. Naming contexts are the basic units for distributing the information base to Directory Service Agents (DSAs), which are the servers for the name service.

(Chow, Johnson, 1997)

A name resolution process is initiated by a Directory User Agent (DUA), working on behalf of a user process. The resolution request is sent from one DSA to another until the object is found in the DIT and returned to the DUA. Whether the resolution scheme is structured and name-based or structure-free and attribute-based, the interaction among DSAs can be in one of the four modes shown in the following figure.

(Chow, Johnson, 1997)

1. Randy Chow,Theodore Johnson, “Distributed Operating Systems & Algorithms”, 1998

2. http://en.wikipedia.org/wiki/Flat_memory_model

References