The clearinghouse: a decentralized agent for locating named objects in a distributed environment

The problem of naming and locating objects in a distributed environment is considered, and the clearinghouse, a decentralized agent for supporting the naming of these "network-visible" objects, is described. The objects "known" to the clearinghouse are of many types and include workstations, file servers, print servers, mail servers, clearinghouse servers, and human user. All objects known to the clearinghouse are named using the same convention, and the clearinghouse provides information about objects in a uniform fashion, regardless of their type. The clearinghouse also supports aliases. The clearinghouse binds a name to a set of properties of various types. For instance, the name of a user may be associated with the location of his local workstation, mailbox, and nonlocation information such as password and comments. The clearinghouse is decentralized and replicated. Tha t is, instead of one global clearinghouse server, there are many local clearinghouse servers, each storing a copy of a portion of the global database. The totality of services supplied by these clearinghouse servers is called "the clearinghouse." Decentralization and replication increase efficiency, security, and reliability. A request to the clearinghouse to bind a name to its set of properties may originate anywhere in the system and be directed to any clearinghouse server. A clearinghouse client need not be concerned with the question of which clearinghouse server actually contains the binding-the clearinghouse stub in the client in conjunction with distributed clearinghouse servers automatically fmds the mapping ff it exists. Updates to the various copies of a mapping may occur asynchronously and be interleaved with requests for bindings of names to properties; updates to the various copies are not treated as indivisible transactions. Any resulting inconsistency between the various copies is only transient: the clearinghouse automatically arbitrates between conflicting updates to restore consistency.

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