When more than one person works on a software project
things often get complicated. Often, two people try to
edit the same file simultaneously. One solution, known
as file locking or reserved checkouts, is
to allow only one person to edit each file at a time.
This is the only solution with some version control
systems, including RCS and SCCS. Currently
the usual way to get reserved checkouts with CVS
cvs admin -l command (see section admin options). This is not as nicely integrated into
CVS as the watch features, described below, but it
seems that most people with a need for reserved
checkouts find it adequate.
It also may be possible to use the watches
features described below, together with suitable
procedures (not enforced by software), to avoid having
two people edit at the same time.
The default model with CVS is known as unreserved checkouts. In this model, developers can edit their own working copy of a file simultaneously. The first person that commits his changes has no automatic way of knowing that another has started to edit it. Others will get an error message when they try to commit the file. They must then use CVS commands to bring their working copy up to date with the repository revision. This process is almost automatic.
CVS also supports mechanisms which facilitate various kinds of communication, without actually enforcing rules like reserved checkouts do.
The rest of this chapter describes how these various models work, and some of the issues involved in choosing between them.