Rational ClearCase

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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, which was purchased by IBM. IBM continues to develop and market ClearCase. ClearCase forms the base of version control for many large and medium sized businesses and can handle projects with hundreds or thousands of developers, but the price is quite steep for smaller companies.

Rational supports two types of SCM configurations, UCM (see below), and base ClearCase. UCM provides an out-of-the-box SCM configuration but is fairly rigid. Base ClearCase supplies the basic tools, and can be configured in many different ways.

ClearCase can run in many different Operating systems ranging from Linux, Solaris, and Windows. It can handle large binary files, extremely large numbers of files, extremely large repository sizes, and is extremely reliable. It handles branching, labeling, and versioning of directories very easily and intuitively, and doesn't clutter each directory with any SCM-only files like .scc or attic files.

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Views

Objects under version control in ClearCase are stored with their histories in repositories called VOBs (versioned object base). ClearCase's novelty was in its versioned file system (called MVFS: Multiversion File System), which can be used to mount VOBs 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.

From the perspective of software on the client computer, a view appears as just another file system. If a file is created in a view by normal OS means (copying or saving from an editor for instance), then ClearCase will create this file as a "view-private" file. It will not be visible in any other view. This allows build systems to operate in the same directory structure as the source code, and ensures that each developer can build completely independently of each other (though if the view is a dynamic one then the normal overhead associated with accessing files over an NFS link applies). A view-private file (or directory for that matter) can be converted into a versioned element at any time, making it visible to other developers.

Each developer typically has one or several views at his disposal. It is sometimes practical to share views between developers, but sharing branches is usually used instead. Having a branch hierarchy is often useful, so an entire development project shares a common development branch, while a smaller team shares a sub-branch, and each developer has his or her private branch. Whenever some change is deemed stable enough for a larger group, it can be merged to the parent branch.

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Configuration specifications

Each view is controlled by its associated configuration specification. This is a text file that specifies what elements (files or directories) should be visible in a view, and which versions of these elements. When deciding which version, if any, of an element should be visible, ClearCase traverses the configuration specification line-by-line from top to bottom, stopping when a match is found. A sample configuration specification could look like this: 

# Show all elements that are checked out to this view, regardless any other rules.
element * CHECKEDOUT

# If an element has a version on the 'module2_dev_branch', then this shall be the visible version in this view.
element * module2_dev_branch

# For all files named 'somefile', regardless of location, always show the latest version on the main branch.
element .../somefile /main/LATEST

# Use a specific version of a specific file. Note: This rule must appear before the next rule to have any effect!
element /vobs/project1/module1/a_header.h /main/proj_dev_branch/my_dev_branch1/14

# Show all elements labeled with the tag 'PROJ1_MOD2_LABEL_1' under the 'project1/module1' path.
# Furthermore, don't allow any checkouts in this path.
element /vobs/project1/module1/... PROJ1_MOD2_LABEL_1 --nocheckout

# Show the 'ANOTHER_LABEL' version of all elements under the 'project1/module2' path.
# If an element is checked out, then branch that element from the currently visible version,
# and add it to the 'module2_dev_branch' branch.
element /vobs/project1/module2/... ANOTHER_LABEL --mkbranch module2_dev_branch

A configuration specification can also reference other configuration specifications using the 'include' statement.

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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.

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UCM

In recent years, a UCM (Unified Change Management) extension has been developed, supporting a more standard SCM model. UCM provides integration between Rational ClearCase and ClearQuest. At the same time high level objects are defined, and there are several facilities for parallel development.

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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. ClearCase also integrates with Microsoft VisualStudio.
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External links

fr:ClearCase

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