Distributed Computing Environment
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The Distributed Computing Environment (DCE) is a software system developed in the early 1990s by a consortium that included Apollo Computer (later part of Hewlett-Packard), IBM, Digital Equipment Corporation, and others. The DCE supplies a framework and toolkit for developing client/server applications. The framework includes a remote procedure call (RPC) mechanism known as DCE/RPC, a naming (directory) service, a time service, an authentication service (based on Kerberos), an authorization service and a distributed file system (DFS) known as DCE/DFS. DCE/RPC was derived from an earlier RPC system called the Network Computing System (NCS) created at Apollo Computer. The naming service was derived from work done at DEC. DCE DFS was based on the Andrew file system (AFS), originally developed at Carnegie Mellon University, and later extended by Transarc Corporation (which was later merged into IBM). DCE 1.2.2 was released on 12 January 2005 under a free software license (the LGPL) by The Open Group. DCE 1.1 was available much earlier under the OSF BSD license, and resulted in FreeDCE [1] being available since 2000. FreeDCE contains an implementation of DCOM.
To understand why DCE is useful, one must look at its closest competitor - Kerberos. Like DCE, Kerberos is a distributed computing application. It provides an authentication system for a network of machines - much like Sun's Network Information Service or LDAP. Kerberos is an authentication system only - it can identify the entity requesting resources to the server, but it cannot do authorization. That has to be implemented at each individual server. If for example, in a system that uses Kerberos authentication, a user A authenticates himself and requests resource R on machine M1, then M1 has to be set up to authorize A to access R on M1. If R is a shared resource that's available on machine M2 also, then M2 has to explicitly authorize A to access resource R. Kerberos does not provide a way to allow one to share authorization settings across its domain. DCE can. It does this by supporting Access Control Lists (ACLs).
The largest unit of management in DCE is a cell. The highest privileges within a cell are assigned to a role called cell administrator. Typically this privileges are held by a DCE principal called cell_admin. Note that this need not be a real OS-level user. The cell_admin has all privileges over all DCE resources within the cell. Privileges can be awarded to or removed from the following categories : user_obj, group_obj, other_obj, any_other for any given DCE resource. The first three correspond to the owner, group member, and any other DCE principal respectively. The last group contains any non-DCE principal. Multiple cells can be configured to communicate and share resources with each other. All principals from external cells are treated as "foreign" users and privileges can be awarded / removed accordingly. In addition to this, specific users or groups can be assigned privileges on any DCE resource - something which is not possible with the traditional UNIX filesystem.
There are three major components of DCE within every cell: (1) the security server (which is responsible for authentication) (2) The Cell Directory Server (CDS) (which is the respository of resources and ACLs) and (3) The Distributed Time Server which provides an accurate clock for proper functioning of the entire cell. Modern DCE implementations such as IBM's are fully capable of interoperating with Kerberos as the security server, LDAP for the CDS and the Network Time Protocol implementations for the time server.
While it is possible to implement a distributed file system using plain old DCE by defining files to the CDS and defining the appropriate ACLs on them, this is not user-friendly. DCE/DFS (Distributed Filesystem - not to be confused with the Microsoft product called DFS which is NOT interoperable with DCE) is a DCE based application which provides a distributed filesystem on DCE. DCE/DFS can support replicas of a fileset (the DCE/DFS equivalent of a filesystem) on multiple DFS servers - there is one read-write copy and zero or more read only copies. Replication is supported between the read-write and the read-only copies. In addition, DCE/DFS also supports what are called "backup" filesets, which if defined for a fileset are capable of storing a version of the fileset as it was prior to the last replication.
DCE/DFS is believed to be the world's only distributed filesystem that correctly implements the full POSIX filesystem semantics - including byte range locking. DCE/DFS was sufficiently reliable and stable to be utilised by IBM to run the back-end filesystem for the 1996 Olympics web site, seamlessly and automatically distributed (and edited!) worldwide in different timezones.