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Rational ClearCase is a software tool for revision control (configuration management, SCM etc) of source code and other software development assets. It originally derived from a product of Apollo Computers: DSEE (Domain Software Engineering Environment), which was ported to Unix and further developed by Atria Software after Hewlett-Packard bought Apollo. Atria later merged with Pure Software to form PureAtria. That firm merged with Rational Software, who was purchased by IBM. IBM continues to develop and market ClearCase.
VOBs The ClearCase source code repository is identified as a versioned object base or VOB for short. A VOB contains all the data and metadata associated with a particular software project. A project can make use of several VOBs, a single VOB or multiple projects can share a VOB.
Views ClearCase's novelty was in its versioned file system (called MVFS: Multiversion File System), which can be mounted as a virtual file system through a dynamic view, selecting a consistent set of versions and allowing for the production of derived objects. The dynamic view allows this to map to a Software Configuration. This was a departure from the repository/sandbox model, allowing for the early management of artifacts (before they are being checked in, and not limited to these first order configuration items).
Alternatively, ClearCase supports snapshot views which are just copies of a directory tree spanning one or several VOBs. Snapshot views do not use a virtual file system to provide access to VOB data. Instead, a snapshot view stores a copy of the VOB data locally on the user's computer. Snapshot views can be used while disconnected from the network and later resynchronized to the VOB when a connection is reestablished. This mode of operation is similar to how the widely-used CVS (Concurrent Versions System) software works.
MultiSite Another originality of ClearCase is its MultiSite extension. In this model, there is no real master database, but instead peer replicas, kept in-step in an asynchronous way (with close to no penalty to end-users).
Since replicas are kept in sync in an asynchronous way, ClearCase must keep track of access so that multiple teams do not perform conflicting operations. The paradigm of mastership allows this. Specific data or metadata within a VOB have a property which specifies what particular replica is allowed to modify them. Thus, a VOB object or piece of metadata can only be manipulated by one replica at a time. MultiSite also has the ability to change this property so that different teams can change the mastership of VOB data. This can be done by changing the mastership manually from a "giving" site or by a request for mastership from a "receiving" site.
UCM In the recent years, a UCM (Unified Change Management) extension has been developed, supporting a more traditional model.
Unique features - Build Avoidance: Use of MVFS (Multiversion File System) allows derived objects built in one dynamic view to be automatically "copied over" to another dynamic view requiring "exactly the same" derived object. Two derived objects are deemed to be "exactly same" if they have the same configuration record (ClearCase terminology, also called bill of materials). Shared derived objects will be physically present on the VOB server, and not in the views that reference them. The process of "copying over" is called winking in in ClearCase terminology.
- Unix/Windows Interoperability: VOBs hosted on *nix (Solaris, Linux, AIX, HP-UX, Irix primarily) servers can be accessed from views hosted on Windows clients. VOBs hosted on Windows servers can only be accessed by Unix clients with snapshot views. .
- Integration With Other Rational Products: Other products (originally) from Rational Software, notably ClearQuest and Rational Rose integrate well with ClearCase.
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