Version Control

Version Control
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Introduction

Motivation:
- We need to manage documents (code, documentation, etc...) that are created by numerous people and change over time
- We need to manage the documents for multiple products/projects and multiple variations of those products/projects
Other Names:
- Revision Control
- Source Control

Some Important Concepts

Repository:
- An "official" collection of documents/files
Working Copy:
- A copy (outside of a repository) of a document/file (the collection of which may be in a working tree on the file system)
Check-Out:
- Making a working copy
Check-In/Commit:
- Adding/returning a file to the repository

Approaches

Locking/Exclusivity/Single Check-Out:
- Only one person can check a file out at a time
- Other people can still view the file
- Nobody else can check the file in (until the original person has done so)
Multiple Check-Outs:
- Multiple people can check-out the same file
- The first check-in always succeeds
- Subsequent check-ins have to be merged (sometimes manually)

Architecture

Centralized/Client-Server:
- There is a a single, authoritative repository
Distributed/Peer-to-Peer:
- Every peer has a repository
- Repositories must be synchronized at some point

Some Terminology

Branches/Forks:
- Copies that will change in different ways in the future (e.g., products being developed for different platforms but have the same origin)
Change/Diff/Delta:
- A particular modification
Change List/Update/Patch/Commit:
- An atomic set of changes
Conflict:
- Changes that can not be reconciled automatically (and, hence, must be resolved)
Head/Tip:
- The most recent commit to either the trunk or a branch
Trunk:
- The original code and revisions (i.e., before the first branch/fork)

Some Examples

The Early Systems:
- Source Code Control System (SCCS) -- 1972, Marc Rochkind, Bell Labs
- Revision Control System (RCS) -- 1982, Walter Tichy, Purdue
Concurrent Versioning System (CVS):
- Released in 1986 (Dick Grune)
- Centralized, Exclusive
- Non-atomic (i.e., doesn't have "all or nothing" transactions)

Some Examples (cont.)

Subversion:
- Centralized, Multiple Check-Out
- Released in 2001
- Conceived as a replacement for CVS
Git:
- Distributed, Multiple Check-Out
- Released in 2005
- Based on experience with BitKeeper (which was used to develop the Linux kernel but stopped being free)
- Initiated by Linus Torvalds
Mercurial:
- Distributed, Multiple Check-Out
- Released in 2005
- Based on experience with BitKeeper (which was used to develop the Linux kernel but stopped being free)
- Initiated by Matt Mackall