.. _db_overview:

Overview
========

.. note:: **Abstract**

   This document provides a conceptual overview of the steps
   of creating an image with {kiwi}. It also explains the terminology
   regarding the concept and process when building system images
   with {kiwi} |version|.

.. toctree::
   :maxdepth: 1

   overview/workflow

Conceptual Overview
-------------------

A system image (usually called an "image") is a *complete installation* of a Linux
system within a file. The image represents an operating system and,
optionally, contains the "final" configuration.

{kiwi} creates images in a two-step process:

1. The first step, the *prepare operation*, generates a so-called
   *unpacked image tree* (directory) using the information provided in
   the image description.

2. The second step, the *create operation*, creates the *packed image* or
   *image* in the specified format based on the unpacked image and the
   information provided in the configuration file.

The image creation process with {kiwi} is automated and does not require any
user interaction. The information required for the image creation process is
provided by the image description.


Terminology
-----------

Appliance
   An appliance is a ready-to-use image of an operating system,
   including a pre-configured application for a specific use case.
   The appliance is provided as an image file and needs to be
   deployed to or activated in the target system or service.

Image
   The result of a {kiwi} build process.

Image Description
   A specification to define an appliance. The image description is a
   collection of human-readable files in a directory. At least one XML
   file, :file:`config.xml` or :file:`.kiwi`, is required. In addition,
   there may be other files, like scripts or configuration data.
   These can be used to customize certain parts of either the {kiwi}
   build process or the initial startup behavior of the image.

Overlay Files
   A directory structure with files and subdirectories stored as part
   of the Image Description. This directory structure is packaged as
   a file, :file:`root.tar.gz`, or stored inside a directory named
   :file:`root`. Additional overlay directories for selected profiles
   are also supported and are taken into account if the directory
   name matches the name of the profile. The content of each of the
   directory structures is copied on top of the existing filesystem
   (overlayed) of the appliance root. This also includes
   permissions and attributes as a supplement.

{kiwi}
   An OS appliance builder.

Virtualization Technology
   Software-simulated computer hardware. A virtual machine acts like
   a real computer but is separated from the physical hardware.
   Within this documentation, the QEMU virtualization system is
   used. Another popular alternative is VirtualBox.