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Any number of clients connect to the repository, and then read or write to these files.

(book compiled from Revision 2558)

By writing data, a client makes the information available to others; by reading data, the client receives information from others. Why is this interesting? So far, this sounds like the definition of a typical file server. And indeed, the repository is a kind of file server, but it's not your usual breed.

What makes the repository special is that as the files in the repository are changed, the repository remembers each version of those files. When a client reads data from the repository, it normally sees only the latest version of the filesystem tree. But what makes a version control client interesting is that it also has the ability to request previous states of the filesystem from the repository. Most software programs understand how to operate only on a single version of a specific type of file. So how does a version control user interact with an abstract—and, often, remote—repository full of multiple versions of various files in a concrete fashion?

How does his or her word processing software, presentation software, source code editor, web design software, or some other program—all of which trade in the currency of simple data files—get access to such files? The answer is found in the version control construct known as a working copy.

A working copy is, quite literally, a local copy of a particular version of a user's VCS-managed data upon which that user is free to work. The task of managing the working copy and communicating changes made to its contents to and from the repository falls squarely to the version control system's client software. If the primary mission of a version control system is to track the various versions of digital information over time, a very close secondary mission in any modern version control system is to enable collaborative editing and sharing of that data.

But different systems use different strategies to achieve this. It's important to understand these different strategies, for a couple of reasons. First, it will help you compare and contrast existing version control systems, in case you encounter other systems similar to Subversion. Beyond that, it will also help you make more effective use of Subversion, since Subversion itself supports a couple of different ways of working. All version control systems have to solve the same fundamental problem: how will the system allow users to share information, but prevent them from accidentally stepping on each other's feet?

It's all too easy for users to accidentally overwrite each other's changes in the repository. 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, 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!

Information commons

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. If Harry has locked a file, 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. The problem with the lock-modify-unlock model is that it's a bit restrictive and often becomes a roadblock for users:. Locking may cause administrative problems.

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Sometimes Harry will lock a file and then forget about it. Meanwhile, because Sally is still waiting to edit the file, her hands are tied. And then Harry goes on vacation. Now Sally has to get an administrator to release Harry's lock. The situation ends up causing a lot of unnecessary delay and wasted time. Locking may cause unnecessary serialization. What if Harry is editing the beginning of a text file, and Sally simply wants to edit the end of the same file? These changes don't overlap at all. They could easily edit the file simultaneously, and no great harm would come, assuming the changes were properly merged together.

There's no need for them to take turns in this situation. 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. 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, 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. 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. The copy-modify-merge model may sound a bit chaotic, but in practice, it runs extremely smoothly.

Users can work in parallel, never waiting for one another.

When they work on the same files, it turns out that most of their concurrent changes don't overlap at all; conflicts are infrequent. And the amount of time it takes to resolve conflicts is usually far less than the time lost by a locking system. In the end, it all comes down to one critical factor: user communication. When users communicate poorly, both syntactic and semantic conflicts increase. No system can force users to communicate perfectly, and no system can detect semantic conflicts. So there's no point in being lulled into a false sense of security that a locking system will somehow prevent conflicts; in practice, locking seems to inhibit productivity more than anything else.

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While the lock-modify-unlock model is considered generally harmful to collaboration, sometimes locking is appropriate. The copy-modify-merge model is based on the assumption that files are contextually mergeable—that is, that the majority of the files in the repository are line-based text files such as program source code. But for files with binary formats, such as artwork or sound, it's often impossible to merge conflicting changes. In these situations, it really is necessary for users to take strict turns when changing the file.

Without serialized access, somebody ends up wasting time on changes that are ultimately discarded. While Subversion is primarily a copy-modify-merge system, it still recognizes the need to lock an occasional file, and thus provides mechanisms for this. We've mentioned already that Subversion is a modern, network-aware version control system. In this section, we'll begin to introduce the specific ways in which Subversion implements version control. Subversion implements the concept of a version control repository much as any other modern version control system would.

Unlike a working copy, a Subversion repository is an abstract entity, able to be operated upon almost exclusively by Subversion's own libraries and tools. As most of a user's Subversion interactions involve the use of the Subversion client and occur in the context of a working copy, we spend the majority of this book discussing the Subversion working copy and how to manipulate it.

Version Control with Subversion

A Subversion client commits that is, communicates the changes made to any number of files and directories as a single atomic transaction. By atomic transaction, we mean simply this: either all of the changes are accepted into the repository, or none of them is. Subversion tries to retain this atomicity in the face of program crashes, system crashes, network problems, and other users' actions. Each time the repository accepts a commit, this creates a new state of the filesystem tree, called a revision.

Each revision is assigned a unique natural number, one greater than the number assigned to the previous revision. The initial revision of a freshly created repository is numbered 0 and consists of nothing but an empty root directory. Imagine an array of revision numbers, starting at 0, stretching from left to right. Unlike most version control systems, Subversion's revision numbers apply to the entire repository tree , not individual files.