版本控制系统的核心任务是实现协作编辑和数据共享，但是不同的系统使用不同的策略实现这个目的。我们有许多理由要去理解这些策略的区别，首先，如果你遇到了其他类似Subversion的系统，可以帮助你比较现有的版本控制系统。此外，可以帮助你更有效的使用Subversion，因为Subversion本身支持不同的工作方式。
所有的版本控制系统都需要解决这样一个基础问题：怎样让系统允许用户共享信息，而不会让他们因意外而互相干扰？版本库里意外覆盖别人的更改非常的容易。
Consider the scenario shown in 图 1.2 “需要避免的问题”. Suppose we have two coworkers, Harry and Sally. They each decide to edit the same repository file at the same time. If Harry saves his changes to the repository first, then it's possible that (a few moments later) Sally could accidentally overwrite them with her own new version of the file. While Harry's version of the file won't be lost forever (because the system remembers every change), any changes Harry made won't be present in Sally's newer version of the file, because she never saw Harry's changes to begin with. Harry's work is still effectively lost—or at least missing from the latest version of the file—and probably by accident. This is definitely a situation we want to avoid!
Many version control systems use a lock-modify-unlock model to address the problem of many authors clobbering each other's work. In this model, the repository allows only one person to change a file at a time. This exclusivity policy is managed using locks. Harry must “lock” a file before he can begin making changes to it. If Harry has locked a file, then Sally cannot also lock it, and therefore cannot make any changes to that file. All she can do is read the file and wait for Harry to finish his changes and release his lock. After Harry unlocks the file, Sally can take her turn by locking and editing the file. 图 1.3 “锁定-修改-解锁 方案” demonstrates this simple solution.
The problem with the lock-modify-unlock model is that it's a bit restrictive and often becomes a roadblock for users:
锁定可能导致管理问题。有时候Harry会锁住文件然后忘了此事，这就是说Sally一直等待解锁来编辑这些文件，她在这里僵住了。然后Harry去旅行了，现在Sally只好去找管理员放开锁，这种情况会导致不必要的耽搁和时间浪费。
锁定可能导致不必要的线性化开发。如果Harry编辑一个文件的开始，Sally想编辑同一个文件的结尾，这种修改不会冲突，设想修改可以正确的合并到一起，他们可以轻松的并行工作而没有太多的坏处，没有必要让他们轮流工作。
Locking may create a false sense of security. Suppose Harry locks and edits file A, while Sally simultaneously locks and edits file B. But what if A and B depend on one another, and the changes made to each are semantically incompatible? Suddenly A and B don't work together anymore. The locking system was powerless to prevent the problem—yet it somehow provided a false sense of security. It's easy for Harry and Sally to imagine that by locking files, each is beginning a safe, insulated task, and thus they need not bother discussing their incompatible changes early on. Locking often becomes a substitute for real communication.
Subversion, CVS, and many other version control systems use a copy-modify-merge model as an alternative to locking. In this model, each user's client contacts the project repository and creates a personal working copy—a local reflection of the repository's files and directories. Users then work simultaneously and independently, modifying their private copies. Finally, the private copies are merged together into a new, final version. The version control system often assists with the merging, but ultimately, a human being is responsible for making it happen correctly.
Here's an example. Say that Harry and Sally each create working copies of the same project, copied from the repository. They work concurrently and make changes to the same file A within their copies. Sally saves her changes to the repository first. When Harry attempts to save his changes later, the repository informs him that his file A is out-of-date. In other words, that file A in the repository has somehow changed since he last copied it. So Harry asks his client to merge any new changes from the repository into his working copy of file A. Chances are that Sally's changes don't overlap with his own; once he has both sets of changes integrated, he saves his working copy back to the repository. 图 1.4 “拷贝-修改-合并 方案” and 图 1.5 “拷贝-修改-合并 方案(续)” show this process.
But what if Sally's changes do overlap with Harry's changes? What then? This situation is called a conflict, and it's usually not much of a problem. When Harry asks his client to merge the latest repository changes into his working copy, his copy of file A is somehow flagged as being in a state of conflict: he'll be able to see both sets of conflicting changes and manually choose between them. Note that software can't automatically resolve conflicts; only humans are capable of understanding and making the necessary intelligent choices. Once Harry has manually resolved the overlapping changes—perhaps after a discussion with Sally—he can safely save the merged file back to the repository.
拷贝-修改-合并模型感觉有一点混乱，但在实践中，通常运行的很平稳，用户可以并行的工作，不必等待别人，当工作在同一个文件上时，也很少会有交迭发生，冲突并不频繁，处理冲突的时间远比等待解锁花费的时间少。
最后，一切都要归结到一条重要的因素：用户交流。当用户交流贫乏，语法和语义的冲突就会增加，没有系统可以强制用户完美的交流，没有系统可以检测语义上的冲突，所以没有任何证据能够承诺锁定系统可以防止冲突，实践中，锁定除了约束了生产力，并没有做什么事。