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, 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 thus 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).
ClearCase also supports snapshot views which are just a copies of a directory tree spanning one or several VOBs. These can be used while disconnected from the network and later resynchronized to the VOB when a connection is reestablished.
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.
In the recent years, a UCM extension has been developed, supporting a more traditional model.