CST(27)-Distributed File and Global Directory Service

Distributed File Service: A new generation of network file servers promises to introduce even more transparency into file systems. The DFS...

Distributed File Service:

A new generation of network file servers promises to introduce even more transparency into file systems. The DFS file naming scheme is location-independent. Each file has a unique identifier that is consistent across the network. File use the DCE global namespace just like the rest of the network resources. DFS provide a Local File System (LFS) with many advanced features including replication that make file system highly available. A snapshot of the file system can reside on the client which can operate on files even if the server is down. LFS also provides transactional log support.

Global Directory Service:

This essential component keeps track of all resources and knows where everything is. Ideally a distributed directory would provide a single system image that can be used by all network applications such as email, system management, and RPC. s

In a client/server system names must be unique within the context in which they are used and resolved. In a federated namespace, each name must include its naming authority like the telephone ISD and STD code. It’s a tree like naming scheme.

The directory service is implemented as a distributed, replicated and object database. It is distributed to allow different administration domains to control their environment. It is replicated to provide high availability and performance when needed. Its an object database in the sense that everything that is tracked is an instance of object class.

A typical directory is implemented as a set of named entries and their associated attributes. Modern NOS directories also have APIs and user interfaces that allow programs to locate entities on the network by querying on the name or attributes. In each name there is a global component and a local component. The global component is the name by which the local directory is known at intergalactic level. The global component manages a federation of loosely coupled local directories. In addition a gateway agent can reside on each local directory and can forward queries for non-local names to a global directory.

Typically a directory maintains a master copy and read-only shadow replicas. Two types of synchronization schemes are used to refresh the replicas.
  1. Immediate replicas causes any update to the master to be immediately shadowed on all replicas.
  2. Skulking causes a periodic propagation to all replicas of all changes made on the master.
X.500 Global Directory Standard:

The industry standard for global directories, X.500 is based on a replicated distributed database. Programs can access the directory services using X/Open Directory Services API. The XDS API allow programs to read, compare, update, add, and remove directory entries. X/Open Management API can be used to define and navigate through the information objects in an X.500 directory.

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CST(27)-Distributed File and Global Directory Service
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