What do “branch”, “tag” and “trunk” mean in Subversion repositories?

Question

I\'ve seen these words a lot around Subversion (and I guess general repository) discussions.
I have been using SVN for my projects for the last few year

Answer 1

First of all, as @AndrewFinnell and @KenLiu point out, in SVN the directory names themselves mean nothing -- "trunk, branches and tags" are simply a common convention that is used by most repositories. Not all projects use all of the directories (it's reasonably common not to use "tags" at all), and in fact, nothing is stopping you from calling them anything you'd like, though breaking convention is often confusing.

I'll describe probably the most common usage scenario of branches and tags, and give an example scenario of how they are used.

• Trunk: The main development area. This is where your next major release of the code lives, and generally has all the newest features.

• Branches: Every time you release a major version, it gets a branch created. This allows you to do bug fixes and make a new release without having to release the newest - possibly unfinished or untested - features.

• Tags: Every time you release a version (final release, release candidates (RC), and betas) you make a tag for it. This gives you a point-in-time copy of the code as it was at that state, allowing you to go back and reproduce any bugs if necessary in a past version, or re-release a past version exactly as it was. Branches and tags in SVN are lightweight - on the server, it does not make a full copy of the files, just a marker saying "these files were copied at this revision" that only takes up a few bytes. With this in mind, you should never be concerned about creating a tag for any released code. As I said earlier, tags are often omitted and instead, a changelog or other document clarifies the revision number when a release is made.

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For example, let's say you start a new project. You start working in "trunk", on what will eventually be released as version 1.0.

• trunk/ - development version, soon to be 1.0
• branches/ - empty

Once 1.0.0 is finished, you branch trunk into a new "1.0" branch, and create a "1.0.0" tag. Now work on what will eventually be 1.1 continues in trunk.

• trunk/ - development version, soon to be 1.1
• branches/1.0 - 1.0.0 release version
• tags/1.0.0 - 1.0.0 release version

You come across some bugs in the code, and fix them in trunk, and then merge the fixes over to the 1.0 branch. You can also do the opposite, and fix the bugs in the 1.0 branch and then merge them back to trunk, but commonly projects stick with merging one-way only to lessen the chance of missing something. Sometimes a bug can only be fixed in 1.0 because it is obsolete in 1.1. It doesn't really matter: you only want to make sure that you don't release 1.1 with the same bugs that have been fixed in 1.0.

• trunk/ - development version, soon to be 1.1
• branches/1.0 - upcoming 1.0.1 release
• tags/1.0.0 - 1.0.0 release version

Once you find enough bugs (or maybe one critical bug), you decide to do a 1.0.1 release. So you make a tag "1.0.1" from the 1.0 branch, and release the code. At this point, trunk will contain what will be 1.1, and the "1.0" branch contains 1.0.1 code. The next time you release an update to 1.0, it would be 1.0.2.

• trunk/ - development version, soon to be 1.1
• branches/1.0 - upcoming 1.0.2 release
• tags/1.0.0 - 1.0.0 release version
• tags/1.0.1 - 1.0.1 release version

Eventually you are almost ready to release 1.1, but you want to do a beta first. In this case, you likely do a "1.1" branch, and a "1.1beta1" tag. Now, work on what will be 1.2 (or 2.0 maybe) continues in trunk, but work on 1.1 continues in the "1.1" branch.

• trunk/ - development version, soon to be 1.2
• branches/1.0 - upcoming 1.0.2 release
• branches/1.1 - upcoming 1.1.0 release
• tags/1.0.0 - 1.0.0 release version
• tags/1.0.1 - 1.0.1 release version
• tags/1.1beta1 - 1.1 beta 1 release version

Once you release 1.1 final, you do a "1.1" tag from the "1.1" branch.

You can also continue to maintain 1.0 if you'd like, porting bug fixes between all three branches (1.0, 1.1, and trunk). The important takeaway is that for every main version of the software you are maintaining, you have a branch that contains the latest version of code for that version.

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Another use of branches is for features. This is where you branch trunk (or one of your release branches) and work on a new feature in isolation. Once the feature is completed, you merge it back in and remove the branch.

• trunk/ - development version, soon to be 1.2
• branches/1.1 - upcoming 1.1.0 release
• branches/ui-rewrite - experimental feature branch

The idea of this is when you're working on something disruptive (that would hold up or interfere with other people from doing their work), something experimental (that may not even make it in), or possibly just something that takes a long time (and you're afraid if it holding up a 1.2 release when you're ready to branch 1.2 from trunk), you can do it in isolation in branch. Generally you keep it up to date with trunk by merging changes into it all the time, which makes it easier to re-integrate (merge back to trunk) when you're finished.

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Also note, the versioning scheme I used here is just one of many. Some teams would do bug fix/maintenance releases as 1.1, 1.2, etc., and major changes as 1.x, 2.x, etc. The usage here is the same, but you may name the branch "1" or "1.x" instead of "1.0" or "1.0.x". (Aside, semantic versioning is a good guide on how to do version numbers).

Answer 2

The trunk directory is the directory that you're probably most familiar with, because it is used to hold the most recent changes. Your main codebase should be in trunk.

The branches directory is for holding your branches, whatever they may be.

The tags directory is basically for tagging a certain set of files. You do this for things like releases, where you want "1.0" to be these files at these revisions and "1.1" to be these files at these revisions. You usually don't modify tags once they're made. For more information on tags, see Chapter 4. Branching and Merging (in Version Control with Subversion).

Answer 3

One of the reasons why everyone has a slightly different definition is because Subversion implements zero support for branches and tags. Subversion basically says: We looked at full-featured branches and tags in other systems and did not found them useful, so we did not implement anything. Just make a copy into a new directory with a name convention instead. Then of course everyone is free to have slightly different conventions. To understand the difference between a real tag and a mere copy + naming convention see the Wikipedia entry Subversion tags & branches.

Answer 4

The trunk is the development line that holds the latest source code and features. It should have the latest bug fixes in it as well as the latest features added to the project.

The branches are usually used to do something away from the trunk (or other development line) that would otherwise break the build. New features are often built in a branch and then merged back into the trunk. Branches often contain code that are not necessarily approved for the development line it branched from. For example, a programmer could try an optimization on something in a branch and only merge back in the development line once the optimization is satisfactory.

The tags are snapshots of the repository at a particular time. No development should occur on these. They are most often used to take a copy of what was released to a client so that you can easily have access to what a client is using.

Here's a link to a very good guide to repositories:

• Source Control HOWTO

The articles in Wikipedia are also worth reading.

Answer 5

In addition to what Nick has said you can find out more at Streamed Lines: Branching Patterns for Parallel Software Development

In this figure main is the trunk, rel1-maint is a branch and 1.0 is a tag.

Answer 6

Tag = a defined slice in time, usually used for releases

I think this is what one typically means by "tag". But in Subversion:

They don't really have any formal meaning. A folder is a folder to SVN.

which I find rather confusing: a revision control system that knows nothing about branches or tags. From an implementation point of view, I think the Subversion way of creating "copies" is very clever, but me having to know about it is what I'd call a leaky abstraction.

Or perhaps I've just been using CVS far too long.