# Introduction
LXD uses an image based workflow. It comes with a built-in image store
where the user or external tools can import images.

Containers are then started from those images.

It's possible to spawn remote containers using local images or local
containers using remote images. In such cases, the image may be cached
on the target LXD.

# Caching
When spawning a container from a remote image, the remote image is
downloaded into the local image store with the cached bit set. The image
will be kept locally as a private image until either it's been unused
(no new container spawned) for the number of days set in
images.remote\_cache\_expiry or until the image's expiry is reached
whichever comes first.

LXD keeps track of image usage by updating the last\_used\_at image
property every time a new container is spawned from the image.

# Auto-update
LXD can keep images up to date. By default, any image which comes from a
remote server and was requested through an alias will be automatically
updated by LXD. This can be changed with images.auto\_update\_cached.

On startup and then every 6 hours (unless images.auto\_update\_interval
is set), the LXD daemon will go look for more recent version of all the
images in the store which are marked as auto-update and have a recorded
source server.

When a new image is found, it is downloaded into the image store, the
aliases pointing to the old image are moved to the new one and the old
image is removed from the store.

The user can also request a particular image be kept up to date when
manually copying an image from a remote server.

# Image format
LXD currently supports two LXD-specific image formats.

The first is a unified tarball, where a single tarball
contains both the container rootfs and the needed metadata.

The second is a split model, using two tarballs instead, one containing
the rootfs, the other containing the metadata.

The former is what's produced by LXD itself and what people should be
using for LXD-specific images.

The latter is designed to allow for easy image building from existing
non-LXD rootfs tarballs already available today.

## Unified tarball
Tarball, can be compressed and contains:
 - rootfs/
 - metadata.yaml
 - templates/ (optional)

In this mode, the image identifier is the SHA-256 of the tarball.

## Split tarballs
Two (possibly compressed) tarballs. One for metadata, one for the rootfs.

metadata.tar contains:
 - metadata.yaml
 - templates/ (optional)

rootfs.tar contains a Linux root filesystem at its root.

In this mode the image identifier is the SHA-256 of the concatenation of
the metadata and rootfs tarball (in that order).

## Content
The rootfs directory (or tarball) contains a full file system tree of what will become the container's /.

The templates directory contains pongo2-formatted templates of files inside the container.

metadata.yaml contains information relevant to running the image under
LXD, at the moment, this contains:

    architecture: x86_64
    creation_date: 1424284563
    properties:
      description: Ubuntu 14.04 LTS Intel 64bit
      os: Ubuntu
      release:
        - trusty
        - 14.04
    templates:
      /etc/hosts:
        when:
          - create
          - rename
        template: hosts.tpl
        properties:
          foo: bar
      /etc/hostname:
        when:
          - start
        template: hostname.tpl
      /etc/network/interfaces:
        when:
          - create
        template: interfaces.tpl
        create_only: true

The architecture and creation\_date fields are mandatory, the properties
are just a set of default properties for the image. The os, release,
name and description fields while not mandatory in any way, should be
pretty common.

For templates, the "when" key can be one or more of:
 - create (run at the time a new container is created from the image)
 - copy (run when a container is created from an existing one)
 - start (run every time the container is started)

The templates will always receive the following context:
 - trigger: name of the event which triggered the template (string)
 - path: path of the file being templated (string)
 - container: key/value map of container properties (name, architecture, privileged and ephemeral) (map[string]string)
 - config: key/value map of the container's configuration (map[string]string)
 - devices: key/value map of the devices assigned to this container (map[string]map[string]string)
 - properties: key/value map of the template properties specified in metadata.yaml (map[string]string)

The "create\_only" key can be set to have LXD only only create missing files but not overwrite an existing file.

As a general rule, you should never template a file which is owned by a
package or is otherwise expected to be overwritten by normal operation
of the container.
