- Clearcase operates in two modes (1) "Dynamic View" with a mounted "Multi-Versioned File System" (MVFS) which operates like a shared drive with a file system which supports change management or (2) "Snapshot View" which downloads the repository to the local filesystem in a similar fashion to Subversion.
When using a "Dynamic View", Clearcase recommends that each user create and manage a new
branch every time the developer wishes to work on code. This requires
writing a new Clearcase "Configuration Specification" (config-spec) to use the branch. Although scripts
are available to simplify the task, Clearcase is still a complex system
requiring constant expert support. If the developer is working on two
tasks, he will be switching config-specs when switching tasks.
Subversion does not require this as the developer is always working
with a local copy which can not be disturbed by the check-in of another
developer. Both systems might require a merge if another developer has
altered the same code. Thus for small projects without expert dedicated
Clearcase support, Subversion is the clear winner. It is the Clearcase config-spec rules definition and generation required to view the CM repository which many find burdensome.
Subversion is simple to administer and simple to use.
- Rapid development may lead to developers checking in code
which breaks the build. Subversion always assigns a new version number
to each check-in. Thus previous states can be recreated, checked out or
viewed. Clearcase requires a "Clearcase Label" to be manually created
to identify the current state. Any previous state of the code is lost
completely in Clearcase as the system does not identify which versions
of each file go together to define a build at any point in time, unless
a "Clearcase Label" is applied. Use of Clearcase UCM will track code changes for a task.
- Cost. Subversion is free vs Clearcase license fees. It
also seems to take a larger staff and more expertise to run a Clearcase
installation.
- Subversion is easy to use in an Enterprise (nation
wide or global) environment as it uses HTTP based web technologies. Clearcase "Dynamic Views" with MVFS uses
a feature called "Multi-site" for this purpose. Since Clearcase dynamic
views are a local mounted file system (MVFS) mirroring a shared
Clearcase server file system, it does not lend itself to networks which
can have unexpected latencies and momentary delays such as the
internet. Clearcase multi-site overcomes this and is used to tie
together remote locations. This requires a highly skilled Clearcase
administrator. One could use Clearcase "Snapshot Views" to overcome this complexity.
A recent Clearcase addition is the Remote Client. This is an Eclipse based client to interface with the Clearcase repository and said to simplify the user experience.
No special considerations are necessary for Subversion in this environment.
- Both Subversion and Clearcase have a web interface for
viewing the source code on the server. Subversion's is native as part
of the installation with Apache. It is greatly improved when used with
Insurrection or ViewVC. Clearcase on the other
hand requires each developer to generate a static view on the Clearcase
web server from which one can view the code via the web. The Clearcase
web interface is poor (in 2006) and I can not recommend its use.
- Integration with task / bug tracking:
Subversion integrates with Trac and other task/bug management tools.
Trac and Subversion support two way tracking so that Subversion check-ins can reference a Trac ticket (specify track ticket eg. #165 in the Subversion comment) and Trac tickets can reference the Subversion check-ins (specify the Subversion check-in eg r879) supporting the Trac ticket. Hyperlinks to each other are automatically generated.
Clearcase typically integrates with Rational Clearquest and now has a layer known as UCM to support the reference to all changes made in support of the "activity". UCM claims to remove the requirement that users manage the config-spec for the CM repository view. I have not used UCM but if true it would greatly improve the developer's experience with Clearcase.
Here I find Subversion/Trac to be much simpler while Clearcase/Clearquest can support a much more complex environment with lots of customization possibilities. The level of sophistication can be regarded as too complex and too much of a burden but that will depend on the level of process complexity you are looking to support. - Integration with regression testing: Subversion integrates with Jenkins, a multi-platform automated build and test tool. Jenkins is definitely a time saver as it compiles and tests the build on all platforms. It is perfect for rapid development as it tests the build upon each check-in. A plus for rapid development. There are interfaces available which allow Jenkins to interface with Clearcase. Clearcase typically integrates with Rational Buildforge.
Conclusion: It is the opinion of this author that complexity comes at a cost. Clearcase (and Clearquest/Buildforge) supports many ways of approaching CM with many levels of customization which may be required at some corporations but at the cost of difficulty to administer and use. I find that the open source quartet of Subversion (and ViewVC/Trac/Jenkins) perform at a reasonable level of sophistication without the burden of over complexity on either the programmer or the administrator.
