Most software has a typical lifecycle: code, test, release, repeat. There are two problems with this process. First, developers need to keep writing new features while quality-assurance teams take time to test supposedly stable versions of the software. New work cannot halt while the software is tested. Second, the team almost always needs to support older, released versions of software; if a bug is discovered in the latest code, it most likely exists in released versions as well, and customers will want to get that bugfix without having to wait for a major new release.

这是版本控制可以做的帮助，典型的过程如下：

整个过程随着软件的成熟不断重复：当2.0完成，一个新的2.0分支被创建，测试、打标签和最终发布，经过许多年，版本库结束了许多版本发布，进入了“维护”模式，许多标签代表了最终的发布版本。

A feature branch is the sort of branch that's been the dominant example in this chapter (the one you've been working on while Sally continues to work on /trunk). It's a temporary branch created to work on a complex change without interfering with the stability of /trunk. Unlike release branches (which may need to be supported forever), feature branches are born, used for a while, merged back to the trunk, then ultimately deleted. They have a finite span of usefulness.

Again, project policies vary widely concerning exactly when it's appropriate to create a feature branch. Some projects never use feature branches at all: commits to /trunk are a free-for-all. The advantage to this system is that it's simple—nobody needs to learn about branching or merging. The disadvantage is that the trunk code is often unstable or unusable. Other projects use branches to an extreme: no change is ever committed to the trunk directly. Even the most trivial changes are created on a short-lived branch, carefully reviewed, and merged to the trunk. Then the branch is deleted. This system guarantees an exceptionally stable and usable trunk at all times, but at the cost of tremendous process overhead.

Most projects take a middle-of-the-road approach. They commonly insist that /trunk compile and pass regression tests at all times. A feature branch is only required when a change requires a large number of destabilizing commits. A good rule of thumb is to ask this question: if the developer worked for days in isolation and then committed the large change all at once (so that /trunk were never destabilized), would it be too large a change to review? If the answer to that question is “yes,” then the change should be developed on a feature branch. As the developer commits incremental changes to the branch, they can be easily reviewed by peers.

最终，有一个问题就是怎样保持一个特性分支“同步”于工作中的主干，在前面提到过，在一个分支上工作数周或几个月是很有风险的，主干的修改也许会持续涌入，因为这一点，两条线的开发会区别巨大，合并分支回到主干会成为一个噩梦。

This situation is best avoided by regularly merging trunk changes to the branch. Make up a policy: once a week, merge the last week's worth of trunk changes to the branch.

At some point, you'll be ready to merge the “synchronized” feature branch back to the trunk. To do this, begin by doing a final merge of the latest trunk changes to the branch. When that's done, the latest versions of branch and trunk will be absolutely identical except for your branch changes. You would then merge back with the --reintegrate option:

$ cd trunk-working-copy

$ svn update
At revision 1910.

$ svn merge --reintegrate http://svn.example.com/repos/calc/branches/mybranch
--- Merging differences between repository URLs into '.':
U    real.c
U    integer.c
A    newdirectory
A    newdirectory/newfile
 U   .
…

可以用另一种考虑这种模式，你每周按时同步分支到主干，类似于在工作拷贝执行svn update的命令，最终的合并操作类似于在工作拷贝运行svn commit，毕竟，工作拷贝不就是一个非常浅的分支吗？只是它一次只可以保存一个修改。