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Version Control with Subversion
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Version Control with Subversion - Chapter 5. Repository Administration

Chapter 5. Repository Administration

The Subversion repository is the central storehouse of versioned data for any number of projects. As such, it becomes an obvious candidate for all the love and attention an administrator can offer. While the repository is generally a low-maintenance item, it is important to understand how to properly configure and care for it so that potential problems are avoided, and actual problems are safely resolved.

In this chapter, we'll discuss how to create and configure a Subversion repository. We'll also talk about repository maintenance, including the use of the svnlook and svnadmin tools (which are provided with Subversion). We'll address some common questions and mistakes, and give some suggestions on how to arrange the data in the repository.

If you plan to access a Subversion repository only in the role of a user whose data is under version control (that is, via a Subversion client), you can skip this chapter altogether. However, if you are, or wish to become, a Subversion repository administrator, [13] you should definitely pay attention to this chapter.

Repository Basics

Before jumping into the broader topic of repository administration, let's further define what a repository is. How does it look? How does it feel? Does it take its tea hot or iced, sweetened, and with lemon? As an administrator, you'll be expected to understand the composition of a repository both from a logical perspective—dealing with how data is represented inside the repository—and from a physical nuts-and-bolts perspective—how a repository looks and acts with respect to non-Subversion tools. The following section covers some of these basic concepts at a very high level.

Understanding Transactions and Revisions

Conceptually speaking, a Subversion repository is a sequence of directory trees. Each tree is a snapshot of how the files and directories versioned in your repository looked at some point in time. These snapshots are created as a result of client operations, and are called revisions.

Every revision begins life as a transaction tree. When doing a commit, a client builds a Subversion transaction that mirrors their local changes (plus any additional changes that might have been made to the repository since the beginning of the client's commit process), and then instructs the repository to store that tree as the next snapshot in the sequence. If the commit succeeds, the transaction is effectively promoted into a new revision tree, and is assigned a new revision number. If the commit fails for some reason, the transaction is destroyed and the client is informed of the failure.

Updates work in a similar way. The client builds a temporary transaction tree that mirrors the state of the working copy. The repository then compares that transaction tree with the revision tree at the requested revision (usually the most recent, or “youngest” tree), and sends back information that informs the client about what changes are needed to transform their working copy into a replica of that revision tree. After the update completes, the temporary transaction is deleted.

The use of transaction trees is the only way to make permanent changes to a repository's versioned filesystem. However, it's important to understand that the lifetime of a transaction is completely flexible. In the case of updates, transactions are temporary trees that are immediately destroyed. In the case of commits, transactions are transformed into permanent revisions (or removed if the commit fails). In the case of an error or bug, it's possible that a transaction can be accidentally left lying around in the repository (not really affecting anything, but still taking up space).

In theory, someday whole workflow applications might revolve around more fine-grained control of transaction lifetime. It is feasible to imagine a system whereby each transaction slated to become a revision is left in stasis well after the client finishes describing its changes to repository. This would enable each new commit to be reviewed by someone else, perhaps a manager or engineering QA team, who can choose to promote the transaction into a revision, or abort it.

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Version Control with Subversion
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