Keep not only your project's source code under version control, but also its web pages, documentation, FAQ, design notes, and anything else that people might want to edit. Keep them right next to the source code, in the same repository tree. Any piece of information worth writing down is worth versioning—that is, any piece of information that could change. Things that don't change should be archived, not versioned. For example, an email, once posted, does not change; therefore, versioning it wouldn't make sense (unless it becomes part of some larger, evolving document).

The reason versioning everything together in one place is important is so people only have to learn one mechanism for submitting changes. Often a contributor will start out making edits to the web pages or documentation, and move to small code contributions later, for example. When the project uses the same system for all kinds of submissions, people only have to learn the ropes once. Versioning everything together also means that new features can be committed together with their documentation updates, that branching the code will branch the documentation too, etc.

Don't keep generated files under version control. They are not truly editable data, since they are produced programmatically from other files. For example, some build systems create configure based on the template configure.in. To make a change to the configure, one would edit configure.in and then regenerate; thus, only the template configure.in is an "editable file." Just version the templates—if you version the result files as well, people will inevitably forget to regenerate when they commit a change to a template, and the resulting inconsistencies will cause no end of confusion.^[15]

The rule that all editable data should be kept under version control has one unfortunate exception: the bug tracker. Bug databases hold plenty of editable data, but for technical reasons generally cannot store that data in the main version control system. (Some trackers have primitive versioning features of their own, however, independent of the project's main repository.)

The project's repository should be browsable on the Web. This means not only the ability to see the latest revisions of the project's files, but to go back in time and look at earlier revisions, view the differences between revisions, read log messages for selected changes, etc.

Browsability is important because it is a lightweight portal to project data. If the repository cannot be viewed through a web browser, then someone wanting to inspect a particular file (say, to see if a certain bugfix had made it into the code) would first have to install version control client software locally, which could turn their simple query from a two-minute task into a half-hour or longer task.

Browsability also implies canonical URLs for viewing specific revisions of files, and for viewing the latest revision at any given time. This can be very useful in technical discussions or when pointing people to documentation. For example, instead of saying "For tips on debugging the server, see the www/hacking.html file in your working copy," one can say "For tips on debugging the server, see http://subversion.apache.org/docs/community-guide/," giving a URL that always points to the latest revision of the hacking.html file. The URL is better because it is completely unambiguous, and avoids the question of whether the addressee has an up-to-date working copy.

Some version control systems come with built-in repository-browsing mechanisms, while others rely on third-party tools to do it. Three such tools are ViewCVS (http://viewcvs.sourceforge.net/), CVSWeb (http://www.freebsd.org/projects/cvsweb.html), and WebSVN (http://websvn.tigris.org/). The first works with both CVS and Subversion, the second with CVS only, and the third with Subversion only.

Every commit to the repository should generate an email showing who made the change, when they made it, what files and directories changed, and how they changed. The email should go to a special mailing list devoted to commit emails, separate from the mailing lists to which humans post. Developers and other interested parties should be encouraged to subscribe to the commits list, as it is the most effective way to keep up with what's happening in the project at the code level. Aside from the obvious technical benefits of peer review (see הסעיף שנקרא “Practice Conspicuous Code Review”), commit emails help create a sense of community, because they establish a shared environment in which people can react to events (commits) that they know are visible to others as well.

The specifics of setting up commit emails will vary depending on your version control system, but usually there's a script or other packaged facility for doing it. If you're having trouble finding it, try looking for documentation on hooks, specifically a post-commit hook, also called the loginfo hook in CVS. Post-commit hooks are a general means of launching automated tasks in response to commits. The hook is triggered by an individual commit, is fed all the information about that commit, and is then free to use that information to do anything—for example, to send out an email.

With pre-packaged commit email systems, you may want to modify some of the default behaviors:

Non-expert version control users are sometimes a bit afraid of branching and merging. This is probably a side effect of CVS's popularity: CVS's interface for branching and merging is somewhat counterintuitive, so many people have learned to avoid those operations entirely.

If you are among those people, resolve right now to conquer any fears you may have and take the time to learn how to do branching and merging. They are not difficult operations, once you get used to them, and they become increasingly important as a project acquires more developers.

Branches are valuable because they turn a scarce resource—working room in the project's code—into an abundant one. Normally, all developers work together in the same sandbox, constructing the same castle. When someone wants to add a new drawbridge, but can't convince everyone else that it would be an improvement, branching makes it possible for her to go to an isolated corner and try it out. If the effort succeeds, she can invite the other developers to examine the result. If everyone agrees that the result is good, they can tell the version control system to move ("merge") the drawbridge from the branch castle over to the main castle.

It's easy to see how this ability helps collaborative development. People need the freedom to try new things without feeling like they're interfering with others' work. Equally importantly, there are times when code needs to be isolated from the usual development churn, in order to get a bug fixed or a release stabilized (see הסעיף שנקרא “Stabilizing a Release” and הסעיף שנקרא “Maintaining Multiple Release Lines” in פרק 7, Packaging, Releasing, and Daily Development) without worrying about tracking a moving target.

Use branches liberally, and encourage others to use them. But also make sure that a given branch is only active for exactly as long as needed. Every active branch is a slight drain on the community's attention. Even those who are not working in a branch still maintain a peripheral awareness of what's going on in it. Such awareness is desirable, of course, and commit emails should be sent out for branch commits just as for any other commit. But branches should not become a mechanism for dividing the development community. With rare exceptions, the eventual goal of most branches should be to merge their changes back into the main line and disappear.

Merging has an important corollary: never commit the same change twice. That is, a given change should enter the version control system exactly once. The revision (or set of revisions) in which the change entered is its unique identifier from then on. If it needs to be applied to branches other than the one on which it entered, then it should be merged from its original entry point to those other destinations—as opposed to committing a textually identical change, which would have the same effect in the code, but would make accurate bookkeeping and release management impossible.

The practical effects of this advice differ from one version control system to another. In some systems, merges are special events, fundamentally distinct from commits, and carry their own metadata with them. In others, the results of merges are committed the same way other changes are committed, so the primary means of distinguishing a "merge commit" from a "new change commit" is in the log message. In a merge's log message, don't repeat the log message of the original change. Instead, just indicate that this is a merge, and give the identifying revision of the original change, with at most a one-sentence summary of its effect. If someone wants to see the full log message, she should consult the original revision.

The reason it's important to avoid repeating the log message is that log messages are sometimes edited after they've been committed. If a change's log message were repeated at each merge destination, then even if someone edited the original message, she'd still leave all the repeats uncorrected—which would only cause confusion down the road.

The same principle applies to reverting a change. If a change is withdrawn from the code, then the log message for the reversion should merely state that some specific revision(s) is being reverted, not describe the actual code change that results from the reversion, since the semantics of the change can be derived by reading the original log message and change. Of course, the reversion's log message should also state the reason why the change is being reverted, but it should not duplicate anything from the original change's log message. If possible, go back and edit the original change's log message to point out that it was reverted.

All of the above implies that you should use a consistent syntax for referring to revisions. This is helpful not only in log messages, but in emails, the bug tracker, and elsewhere. If you're using CVS, I suggest "path/to/file/in/project/tree:REV", where REV is a CVS revision number such as "1.76". If you're using Subversion, the standard syntax for revision 1729 is "r1729" (file paths are not needed because Subversion uses global revision numbers). In other systems, there is usually a standard syntax for expressing the changeset name. Whatever the appropriate syntax is for your system, encourage people to use it when referring to changes. Consistent expression of change names makes project bookkeeping much easier (as we will see in פרק 6, Communications and פרק 7, Packaging, Releasing, and Daily Development), and since a lot of the bookkeeping will be done by volunteers, it needs to be as easy as possible.

See also הסעיף שנקרא “Releases and Daily Development” in פרק 7, Packaging, Releasing, and Daily Development.

Most version control systems offer a feature whereby certain people can be allowed or disallowed from committing in specific sub-areas of the repository. Following the principle that when handed a hammer, people start looking around for nails, many projects use this feature with abandon, carefully granting people access to just those areas where they have been approved to commit, and making sure they can't commit anywhere else. (See הסעיף שנקרא “Committers” in פרק 8, Managing Volunteers for how projects decide who can commit where.)

There is probably little harm done by exercising such tight control, but a more relaxed policy is fine too. Some projects simply use an honor system: when a person is granted commit access, even for a sub-area of the repository, what they actually receive is a password that allows them to commit anywhere in the project. They're just asked to keep their commits in their area. Remember that there is no real risk here: in an active project, all commits are reviewed anyway. If someone commits where they're not supposed to, others will notice it and say something. If a change needs to be undone, that's simple enough—everything's under version control anyway, so just revert.

There are several advantages to the relaxed approach. First, as developers expand into other areas (which they usually will if they stay with the project), there is no administrative overhead to granting them wider privileges. Once the decision is made, the person can just start committing in the new area right away.

Second, expansion can be done in a more fine-grained manner. Generally, a committer in area X who wants to expand to area Y will start posting patches against Y and asking for review. If someone who already has commit access to area Y sees such a patch and approves of it, they can just tell the submitter to commit the change directly (mentioning the reviewer/approver's name in the log message, of course). That way, the commit will come from the person who actually wrote the change, which is preferable from both an information management standpoint and from a crediting standpoint.

Last, and perhaps most important, using the honor system encourages an atmosphere of trust and mutual respect. Giving someone commit access to a subdomain is a statement about their technical preparedness—it says: "We see you have expertise to make commits in a certain domain, so go for it." But imposing strict authorization controls says: "Not only are we asserting a limit on your expertise, we're also a bit suspicious about your intentions." That's not the sort of statement you want to make if you can avoid it. Bringing someone into the project as a committer is an opportunity to initiate them into a circle of mutual trust. A good way to do that is to give them more power than they're supposed to use, then inform them that it's up to them to stay within the stated limits.

The Subversion project has operated on the honor system way for more than four years, with 33 full and 43 partial committers as of this writing. The only distinction the system actually enforces is between committers and non-committers; further subdivisions are maintained solely by humans. Yet we've never had a problem with someone deliberately committing outside their domain. Once or twice there's been an innocent misunderstanding about the extent of someone's commit privileges, but it's always been resolved quickly and amiably.

Obviously, in situations where self-policing is impractical, you must rely on hard authorization controls. But such situations are rare. Even when there are millions of lines of code and hundreds or thousands of developers, a commit to any given code module should still be reviewed by those who work on that module, and they can recognize if someone committed there who wasn't supposed to. If regular commit review isn't happening, then the project has bigger problems to deal with than the authorization system anyway.

In summary, don't spend too much time fiddling with the version control authorization system, unless you have a specific reason to. It usually won't bring much tangible benefit, and there are advantages to relying on human controls instead.

None of this should be taken to mean that the restrictions themselves are unimportant, of course. It would be bad for a project to encourage people to commit in areas where they're not qualified. Furthermore, in many projects, full (unrestricted) commit access has a special status: it implies voting rights on project-wide questions. This political aspect of commit access is discussed more in הסעיף שנקרא “Who Votes?” in פרק 4, Social and Political Infrastructure.