Книга: Distributed operating systems

10.5.1. Names

10.5.1. Names

Every resource in DCE has a unique name. The set of all names forms the DCE namespace. Each name can have up to five parts, some of which are optional. The five parts are shown in Fig. 10-19.

Fig. 10-19. DCE names can have up to five parts.

The first part is the prefix, which tells whether the name is global to the entire DCE namespace or local to the current cell. The prefix /… indicates a global name, whereas the prefix /.: denotes a local name. A global name must contain the name of the cell needed; a local name must not. When a request comes in to CDS, it can tell from the prefix whether it can handle the request itself or whether it must pass it to the GDA for remote lookup by GDS.

Cell names can be specified either in X.500 notation or in DNS notation. Both systems are highly elaborate, but for our purposes the following brief introduction will suffice. X.500 is an international standard for naming. It was developed within the world of telephone companies to provide future telephone customers with an electronic phonebook. It can be used for naming people, computers, services, cells, or anything else needing a unique name.

Every named entity has a collection of attributes that describe it. These can include its country (e.g., US, GB, DE), its organization (e.g., IBM, Harvard, DOD), its department (e.g., CS, SALES, TAX), as well as more detailed items such as employee number, supervisor, office number, telephone number, and name. Each attribute has a value. An X.500 name is a list of attribute=value items separated by slashes. For example,

/C=US/O=YALE/OU=CSATITLE=PROF/TELEPHONE=3141/OFFICE=210/SURNAME=LIN/

might describe Professor Lin in the Yale Computer Science Department. The attributes C, O, and OU are present in most names and refer to country, organization, and organization unit (department), respectively.

The idea behind X.500 is that a query must supply enough attributes that the target is uniquely specified. In the example above, C, O, OU, and SURNAME might do the job, but C, O, OU, and OFFICE might work, too, if the requester had forgotten the name but remembered the office number. Providing all the attributes except the country and expecting the server to search the entire world for a match is not sporting.

DNS is the Internet's scheme for naming hosts and other resources. It divides the world up into top-level domains consisting of countries and in the United States, EDU (educational institutions), COM (companies), GOV (government), MIL (military sites), plus a few others. These, in turn, have sub-domains such as harvard.edu, princeton.edu, and stanford.edu, and subsub-domains such cs.cmu.edu. Both X.500 and DNS can be used to specify cell names. In Fig. 10-19 the two example cells might be the tax department at IBM and the Laboratory for Computer Science at M.I.T.

The next level of name is usually the name of a standard resource or a junction, which is analogous to a mount point in UNIX, causing the search to switch over to a different naming system, such as the file system or the security system. Finally, comes the resource name itself.

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