# Tom Lord's Hackery

# An Overview of GNU Arch

A Brief Description of the `GNU Arch`
Revision Control System

Below is a brief list of the features that most distinguish Gnu Arch from competing systems. This list presumes that you are somewhat familiar with revision control systems in general. (See also, What is Revision Control?)

## Whole Tree Changesets: Atomic Commits and Add/Delete/Rename Handling

Transactions in Gnu Arch are on whole source trees. For example, let's suppose that you modify three files in order to fix a bug. When you commit this change, you commit all three files at once. Arch records a new revision that contains exactly the modifications to those three files (relative to some earlier revision).

Atomic, whole-tree commits are very convenient. To continue the example: you could record the name of the new revision in your bug database as the revision that contains the bug-fix. Or you could send the name of the new revision to a fellow-programmer, who can then merge exactly those changes and no others into his own branch. A project maintainer can ask arch to display exactly those changes in order to review your work.

Whole tree changesets in Arch correctly handle file, directory, and symbolic link additions, removals, and renames. "Correctly handle" means not only that such changes are recorded as part of the changeset, but also that you can merge changes back and forth between versions that do and do not have those tree-structure changes, and the merging process will take that into account.

## Symbolic Revision Names

Arch assigns revisions meaningful, symbolic names that are globally unique among a community of arch users. Thus, it is always possible to refer unambiguously to any given revision, in any Arch archive. Strange annoyances such as CVS's "even/odd length" branch and revision numbering scheme or Subversions global repository revision number are avoided in Arch.

## Easy Branching and Smart Merging

Creating a branch in Arch can be accomplished with a single, fast command.

Merging between branches in Arch is history sensitive which means that Arch helps to avoid spurious merge conflicts by avoiding redundent merges whenever it can.

In fact, there are many different kinds of merging that are useful in the world (e.g., CVS-like updates, cherry-picking, vendor-branch-style tracking, mutual 3-way merges between branches...). Rather than provide just one merge command, Arch provides a toolbox of several merge commands, allowing you to pick the right tool for each job.

## Versioned Tagging

Tags in Arch are as easy to create as branches (in fact, the same mechanism is used).

Tags can be "moved" in Arch and, when the are, data about their old location is preserved. In other words, tags are revision controlled just like source.

## Lightweight, Standard Servers; Simple On-disk Formats

Arch uses a collection of stone-simple on-disk formats for archives and ancillary databases. It does not require or use a relational database, hash-table database, object database, or CVS-like ",v" files.

Consequently, creating a new local archive is utterly trivial: a single arch command does it, basically by creating some new directories.

And, consequently, remote archives do not require an Arch specific server! Your existing `SFTP`, `FTP`, or `WebDav` server can be used as an Arch server. Your existing `HTTP` server can be used as a read-only Arch server.

## Distributed Operation

Branches, in Arch, are not required to be in the same archive as the branched-from version. In fact, creating a branch requires nothing more than read-only access to the branched-from version.

Operation across archive boundaries in Arch is essentially seemless: commands operate equally well on local and remote archives and on versions scattered across multiple archives.

For public and inter-organizational development projects distributed operation is an especially valuable feature: contributors can form and publish their work on their own branches of your mainline sources -- without requiring that they have write access to your archive. You can merge in their changes as easily as if they had been committed to your own archives.

## Integrity Checking and Signed Revisions

Archive revisions are protected against storage-media corruption by checksum files which are verified during ever retrieval. Additionally, committed revisions can be cryptographically signed by their creators, and those signatures verified on retrieval.

## Easy Mirroring

It is trivial to set-up and incrementally update read-only mirrors of any arch archive. A common pattern of use in public projects is to improve performance by creating private local copies of remote archives.

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## GNU Arch

what is GNU Arch?

GNU Arch Bug Database

popular GNU Arch links

GNU Arch development roadmap

popular GNU Arch writings