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## Starting a New Project

up: arch Meets hello-world next: Starting a New Source Tree prev: Creating a New Archive

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

### 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:

            hello-world

### 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:

            hello-world--mainline
            ^^^^^^^^^^^  ^^^^^^^^
               |            |
               |       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.

### 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:

            hello-world--mainline--0.1
                                   ^^^
                                    |
                              version number

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

### 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
            hello-world

            % tla branches hello-world
            hello-world--mainline

            % tla versions hello-world--mainline
            hello-world--mainline--0.1

### 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?.)

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.

### How it Works -- Creating Categories, Branches, and Versions

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

            % tla whereis-archive lord@emf.net--2003-example
            /home/lord/{archives}/2003-example

            % cd `tla whereis-archive lord@emf.net--2003-example`

Categories are top level directories:

            % ls
            =meta-info      hello-world

Branches the next level:

            % ls hello-world
            hello-world--mainline

Versions the third:

            % ls hello-world/hello-world--mainline
            hello-world--mainline--0.1

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.

arch Meets hello-world: A Tutorial Introduction to The arch Revision Control System
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