Naming Services

Generally, components in distributed systems must find one another to work together. Naming is one of the common services of any distributed system. The names of objects and components are added and stored in the naming service (also called the namespace), and can then be located later by the clients of the system.

It's easier for humans to remember the name www.samspublishing.com than the IP address 165.193.123.117. Naming also adds a level of indirection, where the associated IP can be changed without changing the name. The process of associating an object with a name is called binding. An example of a pervasive naming service is DNS (Domain Naming Service), which is used to refer to a host by its name instead of its numeric IP address. COS (Common Object Services) is another naming service used for CORBA (Common Object Request Broker Architecture) applications to register CORBA objects. NDS (Novell Directory Services) is another product used as a naming service to store user and group information for authentication purposes.

In each system, names are organized in a tree-like fashion, delimited with a special character, and follow a special syntax or naming convention (see Figure 4.1). As an example, a DNS name is read from right to left, and is delimited by a dot (.). Therefore, the fully qualified name www.samspublishing.com starts at the root com, followed by samspublishing, and then www.

In the Windows and DOS file systems file names are read from left to right, starting from the root directory and delimits its segments with a backslash “”. For example, the file myfile.doc, located in the directory c:projects can be written as c:projectsmyfile.doc.

LDAP (Lightweight Directory Access Protocol) has a different naming convention, in which a name is read from right to left, and uses the comma “,” as a delimiter. For example, the LDAP name cn=Lillian Ghaly, o=Diamond, c=US, starts with c=US, followed by o=Diamond, and ends with cn=Lillian Ghaly.

A name is used with a naming system to locate objects. A naming system is simply a collection of objects with unique names. To look up an object in a naming system, you provide a name to the naming system, and the naming system returns the stored object with that name.

A composite name is a sequence of names that spans multiple namespaces. An example of a composite name is www.samspublishing.com:ooksstyejb.doc, which spans the DNS and the Windows file system namespaces. The name components of the composite name are host (www.samspublishing.com), directory (books), and file (styejb.doc).