Core concepts
=============

A simple user model
-------------------

To use Breezy you need to understand four core concepts:

* **Revision** - a snapshot of the files you're working with.

* **Working tree** - the directory containing your version-controlled
  files and sub-directories.

* **Branch** - an ordered set of revisions that describe the history of a
  set of files.

* **Repository** - a store of revisions.

Let's look at each in more detail.

Revision
--------

A revision is a *snapshot* of the state of a tree of files and directories,
including their content and shape. A revision also has some metadata
associated with it, including:

* Who committed it
* When it was committed
* A commit message
* Parent revisions from which it was derived

Revisions are immutable and can be globally, uniquely identified
by a *revision-id*. An example revision-id is::

 pqm@pqm.ubuntu.com-20071129184101-u9506rihe4zbzyyz

Revision-ids are generated at commit time or, for imports from other
systems, at the time of import. While revision-ids are necessary
for internal use and external tool integration, branch-specific
*revision numbers* are the preferred interface for humans.

Revision numbers are dotted decimal identifiers like 1, 42 and 2977.1.59
that trace a path through the revision number graph for a branch.
Revision numbers are generally shorter than revision-ids and,
within a single branch, can be compared with each other to get a sense
of their relationship. For example, revision 10 is the mainline (see below)
revision immediately after revision 9. Revision numbers
are generated on the fly when commands are executing, because they
depend on which revision is the tip (i.e. most recent revision)
in the branch.

See `Specifying revisions <specifying_revisions.html>`_ in the appendices
for a closer look at the numerous ways that revisions and ranges of
revisions can be specified in Breezy, and `Understanding Revision Numbers
<zen.html#understanding-revision-numbers>`_ for a more detailed
description of revision numbering.

.. *TODO: add diagram*

Working Tree
------------

A working tree is a *version-controlled directory* holding files the user
can edit. A working tree is associated with a *branch*.

Many commands use the working tree as their context, e.g. ``commit`` makes
a new revision using the current content of files in the working tree.

.. *TODO: add diagram*

Branch
------

In the simplest case, a branch is an *ordered series of revisions*.
The last revision is known as the *tip*.

Branches may split apart and be *merged* back together, forming a
*graph* of revisions. Technically, the graph shows directed relationships
(between parent and child revisions) and there are no loops, so
you may hear some people refer to it as a *directed acyclic graph* or DAG.

If this name sounds scary, don't worry. The important things
to remember are:

* The primary line of development within the DAG is called
  the *mainline*, *trunk*, or simply the *left hand side* (LHS).

* A branch might have other lines of development and if it does,
  these other lines of development begin at some point and end at
  another point.

.. *TODO: add diagram*

Repository
----------

A repository is simply a *store of revisions*. In the simplest case,
each branch has its own repository. In other cases, it makes sense for
branches to share a repository in order to optimize disk usage.

.. *TODO: add diagram*

Putting the concepts together
-----------------------------

Once you have grasped the concepts above, the various ways of using Breezy
should become easier to understand. The simplest way of using Breezy is
to use a *standalone tree*, which has a working tree, branch, and repository
all in a single location. Other common scenarios include:

* `Shared repositories <branching_a_project.html#a-reminder-about-shared-repositories>`_
  - working tree and branch are colocated, but the repository is in a higher level
  directory.

* `Stacked branches <stacked.html>`_ - branch stores just its
  unique revisions, using its parent's repository for common revisions.

* `Lightweight checkouts <using_checkouts.html#getting-a-lightweight-checkout>`_
  - branch is stored in a different location to the working tree.

The best way to use Breezy, however, depends on your needs. Let's take a
look at some common workflows next.
