What is Revision Control?

As used in software development, a revision control system is a tool for recording, indexing and manipulating the changes (revisions) made to the source code of a software system.

For example, let's suppose that Alice and Bob will be working on the program sort -- perhaps they intend to fix some bugs and add a new feature for handling very long input lines. They agree that Alice will fix the bugs and Bob will add the feature.

Cooperating Programmers Need to Combine their Changes

One straw-man idea -- certainly a poor idea -- is that they make a single copy of sort, both fire up their editors, and they just make changes to that single copy.

This idea won't work well for many reasons but some of the most obvious are: what happens if they want to edit the same file, at the same time? What happens if Alice wants to compile and test a bug-fix, but Bob is in the middle of working on the new feature and his changes won't even compile?

Instead, each programmer should work on a separate copy but then they need some way to combine their efforts.

diff and patch Do the Job, but Crudely

A second idea -- workable, but with problems of its own -- is that Alice and Bob will use the programs diff and patch to combine their work. For example, having fixed a bug, Alice will use diff to extract a description of the changes she made. She can give the output from diff to Bob who can then use patch to add those changes to his copy of the tree.

Cooperating on a program by exchanging diff output and using patch can get the job done. Many (especially small) projects have worked this way. Even today, many projects that internally use revision control still use diff and patch when cooperating with "external contributors".

Still, this approach has its limitations and drawbacks. An example of a limitation is the problem of renaming files. Suppose that Alice, while fixing a bug, had decided to reorganize the source a little bit -- to make it easier to maintain. Perhaps she has created some new subdirectories and moved files around among them. diff and patch will not cope with this situation gracefully. Alice can not easily (or necessarily at all) generate diff output that Bob will find useful. An unfortunate consequence is that Alice is therefore likely to not reorganize the source in the first place -- whatever is gained in future ease of maintenance is hard to justify given the immediate difficulties it will create working with Bob.

An example of a drawback of the diff/patch approach is the cherry-picking problem. Let's suppose that Alice has prepared 10 bug fixes. Of these, 5 are known to be very good, but 5 haven't yet been tested and there is a suspician that they create new problems. Meanwhile, Bob would like to test his new feature code in the context of the known-good bug-fixes from Alice. He wants five of her changes, but not the other five.

For Bob to solve this problem, he'll need to ask Alice to produce diff output for just the 5 known-good fixes and nothing else. To be able to do that easily, Alice will have to have done a lot of bookkeeping in her work -- to have kept around enough information to go back and extract just the known-good changes. And Bob will have to do a lot of bookkeeping too -- to keep track of which of Alice's changes he's taken and which not. With just two programmers, all of this bookkeeping is bad enough. With more programmers, and with more instances of this problem than just Bob's immediate need, the amount of bookkeeping quickly becomes impractical.

As a result of drawbacks like that, projects that use only the diff/patch approach tend to be either very small, or very out of control. When out of control, they have difficulty quickly and reliably producing versions for release, isolating regressions, and so forth.

So, Alice and Bob need something similar to the diff/patch approach -- but that doesn't require quite so much bookkeeping and that doesn't discourage making improvements such as reorganizing source files.

The Solution: Modern Revision Control

Modern revision control systems could be fairly described as improving and automating the diff/patch approach to programmer cooperation. Roughly speaking, modern revision control systems:

1. Record and Catalog diff-like Changes: Whenever Alice or Bob does a "unit of work" -- such as make incremental progress fixing a bug -- they can ask the revision control system to commit that work. The commit process involves creating a diff-like description of the new changes, attaching them to a programmer-supplied description of the nature and purpose of the changes, assigning a standard-form name to the changes (similar to the call number on a library book), and archiving the changes where they can be retrieved later. Query tools allow programmers or managers to view the changes that have been recorded and the descriptions provided of them.

2. Play-back and Combine Changes in Flexible Ways: Given an organized record of changes, problems such as Bob's cherry-picking need become easier to solve. Modern revision control tools give users flexible mechanisms that can be used for tasks such as "Create a tree that combines Bob's latest feature work with Alice's bug-fixes 1, 3, 5, 6, and 8." Recording Alice's and Bob's changes separately in the first place is called branching and combining them is called merging. It's the same problem that can be solved using just diff and patch -- but modern revision control systems contain features that automate this rather complex and error-prone process.

Postscript: "Hey, that's not CVS!"

To people accustomed with older revision control systems, especially something like CVS, this definition of modern revision control may seem unfamiliar and a bit odd. Many people think of revision control as a tool that "gives access to historic version" and as a fancy kind of database where programmers checkin changes that are combined with the changes of others. Indeed, most older systems (and some contemporary ones) make it very difficult, at best, catalog individual changes (such as one of Alice's bug-fixes) or to combine changes in flexible ways (such as to solve Bob's cherry-picking problem).

Modern systems have the basic capabilities of older systems like CVS, but much more beyond those. Whereas CVS makes branching and merging difficult, modern systems make it a natural way to work. Whereas older systems do not help much with cataloging logical changes that may effect multiple files, modern systems use that capability as a foundation. In essense, the familliar capabilities of older systems ("checkin" and "access to historic versions") are just special cases of the basic functionality of modern systems ("recording and cataloging changes" and "playing-back and combining changes in flexible ways").