28.1.1.4 Merge-based vs Lock-based Version Control

A version control system typically has some mechanism to coordinate between users who want to change the same file. There are two ways to do this: merging and locking.

In a version control system that uses merging, each user may modify a work file at any time. The system lets you merge your work file, which may contain changes that have not been committed, with the latest changes that others have committed.

Older version control systems use a locking scheme instead. Here, work files are normally read-only. To edit a file, you ask the version control system to make it writable for you by locking it; only one user can lock a given file at any given time. This procedure is analogous to, but different from, the locking that Emacs uses to detect simultaneous editing of ordinary files (see Protection against Simultaneous Editing). When you commit your changes, that unlocks the file, and the work file becomes read-only again. Other users may then lock the file to make their own changes.

Both locking and merging systems can have problems when multiple users try to modify the same file at the same time. Locking systems have lock conflicts; a user may try to check a file out and be unable to because it is locked. In merging systems, merge conflicts happen when you commit a change to a file that conflicts with a change committed by someone else after your checkout. Both kinds of conflict have to be resolved by human judgment and communication. Experience has shown that merging is superior to locking, both in convenience to developers and in minimizing the number and severity of conflicts that actually occur.

SCCS always uses locking. RCS is lock-based by default but can be told to operate in a merging style. CVS and Subversion are merge-based by default but can be told to operate in a locking mode. Decentralized version control systems, such as Git and Mercurial, are exclusively merging-based.

VC mode supports both locking and merging version control. The terms “commit” and “update” are used in newer version control systems; older lock-based systems use the terms “check in” and “check out”. VC hides the differences between them as much as possible.