Revision Control with Arch: Introduction to Arch

Arch quickly is becoming one of the most powerful tools in the free software developer's collection. This is the first in a series of three articles that teaches basic use of Arch for distributed development, to manage shared archives and script automated systems around Arch projects.

This article shows you how to get code from a public Arch archive, contribute changesets upstream and make a local branch of a project for disconnected use. In addition, it provides techniques to improve performance of both local and remote archives.

Revision control is the business of change management within a project. The ability to examine work done on a project, compare development paths and replicate and undo changes is a fundamental part of free software development. With so many potential contributors and such rapid release of changes, the tools developers use to manipulate these changes have had to evolve quickly.

Early revision control was handled with tape backups. Old versions of a project would be dragged out of backup archives and compared line by line with the new copy. The process of restoring a backup from tape is not quick, so this is not an efficient method by any means.

To work around this lag, many developers kept old copies of files around for comparison, and this was soon integrated into early development tools. File-based revision control, such as that used by the Emacs editor, uses numbered backup files so you can compare foo.c~7~ with foo.c~8~ to see what changed. Versioned backup files even were integrated into the filesystem on some early proprietary operating systems.

For nearly two decades, the preferred format for third-party contributions to free software projects has been a patch file, sometimes called a diff. Given two files, the diff program generates a listing that highlights the differences between them. To apply the changes specified in the diff output, a user need only run it through the patch program.

In the 1990s, the Concurrent Versions System (CVS) became the default for managing the changes of a core group of developers. CVS stores a list of patches along with attribution information and a changelog. A primitive system of branching and merging allows users to experiment with various lines of development and then fold successful efforts back into the main project.

CVS has its limitations, and they are becoming a burden for many projects. First, it does not store any metadata changes, such as the permissions of a file or the renaming of a file. In addition, check-ins are not grouped together in a set, making it difficult to examine a change that spanned multiple files and directories. Finally, nearly all operations on a remote CVS repository require that a new connection be opened to the server, making it difficult for disconnected use.

Efforts such as the Subversion Project have come a long way toward fixing the flaws found in CVS. Subversion is effectively a CVS++, and it supports file metadata change logging and atomic check-ins. What it still requires is a centralized server on the network that all clients connect to for revision management operations.

A new generation of revision control systems has sprung up in the past few years, all operating on a distributed model. Distributed revision control systems do away with a single centralized repository in favor of a peer-to-peer architecture. Each developer keeps a repository, and the tools allow easy manipulation of changes between systems over the network.

Projects such as Monotone, DARCS and Arch are finding popularity in a world where free software development happens outside of well-connected universities, and laptops are much more common.

One of the most promising distributed systems today is GNU Arch. Arch handles disconnected use by encouraging users to create archives on their local machines, and it provides powerful tools for manipulating projects between archives. Arch lacks any sort of dedicated server process and uses a portable subset of filesystem operations to manipulate the archive. Archives are simply directories that can be made available over the network using your preferred remote filesystem protocol. In addition, Arch supports archive access over HTTP, FTP and SFTP.

One advantage to not having a dedicated dæmon is that no new code is given privilege on your server machine. Thus, your security concerns are with your SSH dæmon or Web server, which most system administrators already are keeping tabs on.

Another advantage is that for most tasks no root privilege is needed to make use of Arch. Developers can begin using it on their own machines and publish archives without even installing Arch on the Web server machine. This affects the pattern of adoption as well. Using CVS or Subversion is a top-down decision made for an entire project team, although Arch can be adopted by one or two developers at a time until everyone in the group is up to speed.