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3.4 Starting a New Project

This and later chapters will show you how to set up and manage a simple project with arch through the specific example of a world hello world program.

3.4.1 Choose a Project Category

As a first step, you must choose a general category to serve as a name for the project. In the examples, we'll use the name:


3.4.2 Choose a Project Branch

arch encourages you to divide up the work on a project into separate branches.

Roughly speaking, branches are a mechanism for splitting the work on a project into two or more, largely independent efforts. Let's suppose, for example, that the hello-world project has two needs:

1) A need to make regular releases of good ol' fashioned hello-world, fixing simple bugs, porting the program, and adding tiny features.

2) A need to begin work on a graphical user interface for hello-world, which is expected to take about a year to complete.

We'd like those two efforts to proceed in parallel, but not get in each other's way. For example, we don't want GUI code to appear in the regular releases until it is working fairly well.

In such a case, we'll use branches: one for regular releases (the mainline branch) and another for GUI features (the gui branch).

There are many other uses for branches, some of which will be described later in the manual. For now, we just need one branch: a branch for the official latest sources of hello-world, which we'll call:

             ^^^^^^^^^^^  ^^^^^^^^
                |            |
                |       branch name
           category name

Notice that the category and branch names are separated by two dashes. In general, category and branch names must: consist only of letters, numbers, and dashes; must begin with a letter; must not themselves contain two dashes; and must not end with a dash.

3.4.3 Choose a Version Number

Finally, you must choose a version number for the version of hello-world that you'll be working on, and create that version in the archive.

Version numbers in arch are not the name of a particular "snapshot" or release of your project – though they are related to that concept. Instead, version numbers are the name of a development line: a sequence of changes that you make while creating a particular release.

In this case, we'll use the name:

                               version number

Notice that version numbers are always positive integers, separated by periods.

3.4.4 Preparing the Archive

Having chosen a name, it's time to prepare the archive for use of that name:

             % tla archive-setup hello-world--mainline--0.1

After that command, we can query the archive to see what we've done:

             % tla categories
             % tla branches hello-world
             % tla versions hello-world--mainline

3.4.5 Why is it Like This

People new to arch are sometimes startled at the rigidity of its archive namespace. Two most common question is:

Why have categories, branches and versions? Why can't I just name my projects with arbitrary string? These questions are best answered by recalling that a revision control system is a librarian. Part of its job is to help people navigate and search through very large collections of projects and source code. In order to make such searching practical, arch defines a cataloging system: categories, branches, and versions. (See What is Revision Control? in Introducing arch.)

This is somewhat analogous to the cataloging systems used in libraries for books, such as the Dewey decimal classification system: it's a hierarchical categorization of everything in the library. It's a uniform way to describe where a given item is stored, and it aids searching by suggesting the relationships between various items. For example, a branch is likely most closely related to other branches in the same category. A version with a higher major version number most likely contains later work than one in the same branch with a lower major version number.

The analogy isn't perfect: book cataloging systems such as Dewey are based on an official list of categories and subcategories, while arch, on the other hand, let's you choose your own category names. Still, like Dewey, arch names are based on the idea of grouping related items together to make them easier to search and navigate. And just as Dewey is intended to capture the most common patterns of how people search through books, arch is intended to capture the most common patterns of how people search through source archives.

3.4.6 How it Works – Creating Categories, Branches, and Versions

What does the command archive-setup actually do? Its conceptually quite simple: it creates new directories in your archive:

             % tla whereis-archive

             % cd `tla whereis-archive`

Categories are top level directories:

             % ls
             =meta-info      hello-world

Branches the next level:

             % ls hello-world

Versions the third:

             % ls hello-world/hello-world--mainline

Versions are themselves directories:

             % ls hello-world/hello-world--mainline/hello-world--mainline--0.1/
             +revision-lock  +version-lock

Note: The lock files (e.g. +revision-lock) are used internally by arch. When adding new data to an archive, arch doesn't simply call mkdir. Instead, it carefully modifies archives to that they are always in a consistent state, regardless of what commands are issued concurrently, or whether or not a command is killed in mid-execution.