Revision Control with Arch: Introduction to Arch

Before you can begin working in a read-write archive, you must identify yourself to tla:

$ tla my-id "J. Random Hacker <jrh@zork.net>"

Once you have entered your e-mail address, it is time to create an archive for your projects. Arch lets you make many archives, but you can keep as many projects and branches as you like in the same archive.

Archive names have two parts, separated by two hyphens: the first is your e-mail address, and the second is some identifier. Many people like to use the four-digit year as the identifier and roll over to a new archive each year:

$ tla make-archive -l jrh@zork.net--2004 ~/ARCHIVE
$ tla my-default-archive jrh@zork.net--2004

The my-default-archive command makes certain operations on the local archive easier to type.

Arch encourages developers to fork and merge projects using branches. Branches are the primary mechanism for moving code from one archive to another, even over a network. You can use a branch for a complete code fork to pursue an entirely new line of development, or you can use a branch to cache a copy of a project on your laptop so that you can work for a while in an environment that lacks network access.

Published branches are also the primary development communications mechanism for developers who use Arch. Instead of mailing large changeset tarballs or patch files around, a contributor most likely would set up a branch to make local changes and then invite the upstream developers to merge those changes back into the main project. This is where the decentralized and democratic nature of Arch's design shines. Any developer can join the development effort without needing special privilege in the core team's archive.

Before you can branch the lnx-bbc project, you have to set up a space for the project in your archive. The format for a project identifier is similar to that of the archive name: the category (or project name), two dashes, the branch name, two dashes and the version number. It is most likely Tom Lord's experience as a LISP hacker that informed his decision to use these dashes:

$ tla archive-setup lnx-bbc--robot-branch--0.0

This creates a category called lnx-bbc, a branch called robot-branch and a version called 0.0. You did not need to specify jrh@zork.net--2004/ in front of the project name because that is your default archive.

Finally, it is time to tag off the branch from the remote archive. This means the robot-branch begins as a tag pointing to a particular revision of a project in the lnx-bbc-devel@zork.net--gar archive, and all local changes start from that point:

$ tla tag \
  lnx-bbc-devel@zork.net--gar/lnx-bbc--stable--2.1 \
  lnx-bbc--robot-branch--0.0

At this point, running tla abrowse should show your default archive as follows:

jrh@zork.net--2004
  lnx-bbc
    lnx-bbc--robot-branch
      lnx-bbc--robot-branch--0.0
        base-0

You are now ready to check out a copy of your new branch:

$ tla get lnx-bbc--robot-branch robot-branch

At this point, you can go into the robot-branch directory and make some changes:

$ chmod 444 index.txt
$ tla mv faq.txt robofaq.txt
$ echo "ROBOT TIME" > robot-time
$ tla add robot-time
$ tla rm ports.txt

The tla mv command renames a file in such a way that Arch keeps track of the change. It is important to use this command in place of the standard mv. The tla add command prepares a new file to be inserted into the archive, and tla rm schedules removal of a file.

All of these changes can be checked in to your local branch now:

$ tla commit

Your preferred text editor (as specified in the $EDITOR environment variable) will be started up with a template for your check-in log. Once you have filled out the log entry, saving and exiting finalizes the commit.

Now running tla abrowse shows that you have two revisions of the robot branch in the archive, base-0 and patch-1:

jrh@zork.net--2004
  lnx-bbc
    lnx-bbc--robot-branch
      lnx-bbc--robot-branch--0.0
        base-0 .. patch-1