Unix in a Nutshell, 4th Edition

Usage Models

Different systems have different conceptual “models” as to how they’re used.

Older systems such as SCCS and RCS use a “check out with locking” model. These systems were developed before client/server computing, when software development was done on centralized minicomputers and mainframes. In this model, the repository is a central directory on the same machine where the developers work, and each developer checks out a private copy into their own sandbox. In order to avoid two developers making conflicting changes to a file, the file must be locked when it’s checked out. Only one user may lock a particular version of a file at a time. When that user has checked in their changes, they unlock the file so that the next user can check in changes. If necessary, the second user may “break” the first user’s lock, in which case the first user is notified via electronic mail.

This model works well for small projects where developers are co-located and can communicate easily. As long as one developer locks a file when she checks it out, another developer wishing to work with the file will know that he can’t until the first one is done. The drawback is that such locking can slow down development significantly.

Newer systems, such as CVS and Subversion, use a “copy, modify, merge” model. In practice, when two developers wish to work on the same file, they usually end up changing different, unrelated parts of the file. Most of the time each developer can make changes without adversely ...