Java™ Application Development on Linux - Dator

484Chapter 21Introduction to Enterprise JavaBeansKey to directory services is the concept of a context. A context is a set ofbindings with a common name, expressed in a common way. In our filesystemexample, /etc is a context. A context may contain other contexts that followthe same naming convention. For example, /etc/sysconfig is a context thatis a subcontext of /etc. Likewise, multitool.net is a subcontext of thenet context.A context is distinguished by having a naming convention for itself andits subcontexts, and it must have means of creating bindings, removingbindings, and querying or listing bindings.Since JNDI is designed to operate across multiple naming and directorysystems, it is necessary to talk about naming systems and namespaces. A namingsystem is a connected set of contexts that use the same naming convention.Thus, Internet domain names are a naming system, UNIX filenames are anaming system, and so on. A namespace is a set of names in a naming system.These terms will have significance later when we’ll talk about JNDI.A naming system binds a name to a value. Directory services bind a directoryobject to one or more attributes. A naming service could be thought of asa simple case of a directory where “name” and “value” are the attributes of thedirectory object. A directory can store many more attributes (bindings) for agiven name than can a naming service. Directory services also (in general)support the notion of searches and queries.A directory object represents an object in the computing environment. Thismight be a server, a printer, a user, a router, whatever. Each object would havea set of attributes that describe the object. A directory is a connected set ofdirectory objects.In the directories we know about (see Sections 21.3.2.4 and 21.3.2.5 forthe limits of our knowledge), directory objects are arranged in a hierarchy, sothat they serve as naming contexts as well as directory objects.21.3.2 Common Directory ServicesNow that you have seen the concepts, we can cover a few common implementationsof naming and directory services.21.3.2.1 Domain Name Service (DNS)This is probably the most familiar naming and directory system. It is used allthe time to resolve Internet host names to IP addresses, and it is commonlyused to obtain the names of mail servers for domains. It also has less often used