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: DSEE, which was ported to Unix and further developed by Atria Software after HP 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.
ClearCase novelty was in its versioned file system, which can be mounted as a virtual file system through a view, selecting a consistent set of versions and allowing for the production of derived objects. The 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).
An other 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).
In the recent years, a UCM extension has been developed, supporting a more traditional model.
