.. _developing-elements:
Developing Elements
===================
Conform to the following conventions:
* Use the environment for overridable defaults, prefixing environment variable
names with ``DIB_``. For example:
.. sourcecode:: sh
DIB_MYDEFAULT=${DIB_MYDEFAULT:-default}
If you do not use the ``DIB`` prefix you may find that your overrides are
discarded as the build environment is sanitised.
* Consider that your element co-exists with many others and try to guard
against undefined behaviours. Some examples:
* Two elements use the source-repositories element, but use the same filename
for the source-repositories config file. Files such as these (and indeed the
scripts in the various .d directories :ref:`listed below
<phase-subdirectories>`) should be named such that they are unique. If they
are not unique, when the combined tree is created by disk-image-builder for
injecting into the build environment, one of the files will be overwritten.
* Two elements copy different scripts into ``/usr/local/bin`` with the same
name. If they both use ``set -e`` and ``cp -n`` then the conflict will be
caught and cause the build to fail.
* If your element mounts anything into the image build tree (``$TMP_BUILD_DIR``)
then it will be automatically unmounted when the build tree is unmounted - and
not remounted into the filesystem image - if the mount point is needed again,
your element will need to remount it at that point.
* If caching is required, elements should use a location under
``$DIB_IMAGE_CACHE``.
* Elements should allow for remote data to be cached. When ``$DIB_OFFLINE`` is
set, this cached data should be used if possible.
See the :ref:`dev-global-image-build-variables` section of this document for
more information.
* Elements in the upstream diskimage-builder elements should not create
executables which run before 10- or after 90- in any of the phases if
possible. This is to give downstream elements the ability to easily make
executables which run after our upstream ones.
.. _phase-subdirectories:
Phase Subdirectories
^^^^^^^^^^^^^^^^^^^^
Make as many of the following subdirectories as you need, depending on what
part of the process you need to customise. The subdirectories are executed in
the order given here. Scripts within the subdirectories should be named with a
two-digit numeric prefix, and are executed in numeric order.
Only files which are marked executable (+x) will be run, so other files can be
stored in these directories if needed. As a convention, we try to only store
executable scripts in the phase subdirectories and store data files elsewhere in
the element.
The phases are:
#. ``root.d``
#. ``extra-data.d``
#. ``pre-install.d``
#. ``install.d``
#. ``post-install.d``
#. ``block-device.d``
#. ``finalise.d``
#. ``cleanup.d``
``root.d``
Create or adapt the initial root filesystem content. This is where
alternative distribution support is added, or customisations such as
building on an existing image.
Only one element can use this at a time unless particular care is taken not
to blindly overwrite but instead to adapt the context extracted by other
elements.
* runs: **outside chroot**
* inputs:
* ``$ARCH=i386|amd64|armhf|arm64``
* ``$TARGET_ROOT=/path/to/target/workarea``
``extra-data.d``
Pull in extra data from the host environment that hooks may
need during image creation. This should copy any data (such as SSH keys,
http proxy settings and the like) somewhere under ``$TMP_HOOKS_PATH``.
* runs: **outside chroot**
* inputs: ``$TMP_HOOKS_PATH``
* outputs: None
``pre-install.d``
Run code in the chroot before customisation or packages are installed. A good
place to add apt repositories.
* runs: **in chroot**
``install.d``
Runs after ``pre-install.d`` in the chroot. This is a good place to
install packages, chain into configuration management tools or do other image
specific operations.
* runs: **in chroot**
``post-install.d``
Run code in the chroot. This is a good place to perform tasks you want to
handle after the OS/application install but before the first boot of the
image. Some examples of use would be:
Run ``chkconfig`` to disable unneeded services
Clean the cache left by the package manager to reduce the size of the image.
* runs: **in chroot**
``block-device.d``
Customise the block device that the image will be made on (for example to
make partitions). Runs after the target tree has been fully populated but
before the ``cleanup.d`` phase runs.
* runs: **outside chroot**
* inputs:
* ``$IMAGE_BLOCK_DEVICE={path}``
* ``$TARGET_ROOT={path}``
* outputs: ``$IMAGE_BLOCK_DEVICE={path}``
``finalise.d``
Perform final tuning of the root filesystem. Runs in a chroot after the root
filesystem content has been copied into the mounted filesystem: this is an
appropriate place to reset SELinux metadata, install grub bootloaders and so
on.
Because this happens inside the final image, it is important to limit
operations here to only those necessary to affect the filesystem metadata and
image itself. For most operations, ``post-install.d`` is preferred.
* runs: **in chroot**
``cleanup.d``
Perform cleanup of the root filesystem content. For instance, temporary
settings to use the image build environment HTTP proxy are removed here in
the dpkg element.
* runs: outside chroot
* inputs:
* ``$ARCH=i386|amd64|armhf|arm64``
* ``$TARGET_ROOT=/path/to/target/workarea``
Other Subdirectories
^^^^^^^^^^^^^^^^^^^^
Elements may have other subdirectories that are processed by specific elements
rather than the diskimage-builder tools themselves.
One example of this is the ``bin`` directory. The `rpm-distro`,
:doc:`../elements/dpkg/README` and :doc:`../elements/opensuse/README` elements
install all files found in the ``bin`` directory into ``/usr/local/bin`` within
the image as executable files.
Environment Variables
^^^^^^^^^^^^^^^^^^^^^
To set environment variables for other hooks, add a file to your
element ``environment.d``. This directory contains bash script
snippets that are sourced before running scripts in each phase. Note
that because environment includes are sourced together, they should
not set global flags like ``set -x`` because they will affect all
preceeding imports.
Dependencies
^^^^^^^^^^^^
Each element can use the following files to define or affect dependencies:
``element-deps``
A plain text, newline separated list of elements which will be added to the
list of elements built into the image at image creation time.
``element-provides``
A plain text, newline separated list of elements which are provided by this
element. These elements will be excluded from elements built into the image
at image creation time.
For example if element A depends on element B and element C includes element B
in its ``element-provides`` file and A and C are included when building an
image, then B is not used.
Operating system elements
^^^^^^^^^^^^^^^^^^^^^^^^^
Some elements define the base structure for an operating system -- for example,
the ``opensuse`` element builds a base openSUSE system. Such elements have
more requirements than the other elements:
* they must have ``operating-system`` in their element-provides, so this
indicates they are an "operating system".
* they must export the ``DISTRO_NAME`` environment variable with the name
of the distribution built, using an environment.d script. For example,
the ``opensuse`` element exports ``DISTRO_NAME=opensuse``.
Ramdisk Elements
^^^^^^^^^^^^^^^^
Ramdisk elements support the following files in their element directories:
``binary-deps.d``
Text files listing executables required to be fed into the ramdisk. These
need to be present in ``$PATH`` in the build chroot (i.e. need to be installed
by your elements as described above).
``init.d``
POSIX shell script fragments that will be appended to the default script
executed as the ramdisk is booted (``/init``).
``ramdisk-install.d``
Called to copy files into the ramdisk. The variable ``$TMP_MOUNT_PATH`` points
to the root of the tree that will be packed into the ramdisk.
``udev.d``
``udev`` rules files that will be copied into the ramdisk.
Element coding standard
^^^^^^^^^^^^^^^^^^^^^^^
- lines should not include trailing whitespace.
- there should be no hard tabs in the file.
- indents are 4 spaces, and all indentation should be some multiple of
them.
- `do` and `then` keywords should be on the same line as the if, while or
for conditions.
.. _dev-global-image-build-variables:
Global image-build variables
----------------------------
``DIB_OFFLINE``
This is always set. When not empty, any operations that perform remote data
access should avoid it if possible. If not possible the operation should still
be attempted as the user may have an external cache able to keep the operation
functional.
``DIB_IMAGE_ROOT_FS_UUID``
This contains the UUID of the root filesystem, when diskimage-builder is
building a disk image. This works only for ext filesystems.
``DIB_IMAGE_CACHE``
Path to where cached inputs to the build process are stored. Defaults to
``~/.cache/image_create``.
Structure of an element
-----------------------
The above-mentioned global content can be further broken down in a way that
encourages composition of elements and reusability of their components. One
possible approach to this would be to label elements as either a "driver",
"service", or "config" element. Below are some examples.
- Driver-specific elements should only contain the necessary bits for that
driver::
elements/
driver-mellanox/
init - modprobe line
install.d/
10-mlx - package installation
- An element that installs and configures Nova might be a bit more complex,
containing several scripts across several phases::
elements/
service-nova/
source-repository-nova - register a source repository
pre-install.d/
50-my-ppa - add a PPA
install.d/
10-user - common Nova user accts
50-my-pack - install packages from my PPA
60-nova - install nova and some dependencies
- In the general case, configuration should probably be handled either by the
meta-data service (eg, o-r-c) or via normal CM tools
(eg, salt). That being said, it may occasionally be desirable to create a
set of elements which express a distinct configuration of the same software
components.
In this way, depending on the hardware and in which availability zone it is
to be deployed, an image would be composed of:
* zero or more driver-elements
* one or more service-elements
* zero or more config-elements
It should be noted that this is merely a naming convention to assist in
managing elements. Diskimage-builder is not, and should not be, functionally
dependent upon specific element names.
diskimage-builder has the ability to retrieve source code for an element and
place it into a directory on the target image during the extra-data phase. The
default location/branch can then be overridden by the process running
diskimage-builder, making it possible to use the same element to track more
then one branch of a git repository or to get source for a local cache. See
:doc:`../elements/source-repositories/README` for more information.
Finding other elements
----------------------
DIB exposes an internal ``$IMAGE_ELEMENT_YAML`` variable which
provides elements access to the full set of included elements and
their paths. This can be used to process local in-element files
across all the elements (``pkg-map`` for example).
.. code-block:: python
import os
import yaml
elements = yaml.load(os.getenv('IMAGE_ELEMENT_YAML'))
for element, path in elements:
...
For elements written in Bash, there is a function
``get_image_element_array`` that can be used to instantiate an
associative-array of elements and paths (note arrays can not be
exported in bash).
.. code-block:: bash
# note eval to expand the result of the get function
eval declare -A image_elements=($(get_image_element_array))
for i in ${!image_elements[$i]}; do
element=$i
path=${image_elements[$i]}
done
Debugging elements
------------------
Export ``break`` to drop to a shell during the image build. Break points can be
set either before or after any of the hook points by exporting
"break=[before|after]-hook-name". Multiple break points can be specified as a
comma-delimited string. Some examples:
* ``break=before-block-device-size`` will break before the block device size
hooks are called.
* ``break=before-pre-install`` will break before the pre-install hooks.
* ``break=after-error`` will break after an error during an in target hookpoint.
The :doc:`../elements/manifests/README` element will make a range of
manifest information generated by other elements available for
inspection inside and outside the built image. Environment and
command line arguments are captured as described in the documentation
and can be useful for debugging.
Images are built such that the Linux kernel is instructed not to switch into
graphical consoles (i.e. it will not activate KMS). This maximises
compatibility with remote console interception hardware, such as HP's iLO.
However, you will typically only see kernel messages on the console - init
daemons (e.g. upstart) will usually be instructed to output to a serial
console so nova's console-log command can function. There is an element in the
tripleo-image-elements repository called "remove-serial-console" which will
force all boot messages to appear on the main console.
Ramdisk images can be debugged at run-time by passing ``troubleshoot`` as a
kernel command line argument, or by pressing "t" when an error is reached. This
will spawn a shell on the console (this can be extremely useful when network
interfaces or disks are not detected correctly).
Testing Elements
----------------
An element can have functional tests encapsulated inside the element itself. The
tests can be written either as shell or python unit tests.
shell
"""""
In order to create a test case, follow these steps:
* Create a directory called ``test-elements`` inside your element.
* Inside the test-elements directory, create a directory with the name of your
test case. The test case directory should have the same structure as an
element. For example::
elements/apt-sources/test-elements/test-case-1
* Assert state during each of the element build phases you would like to test.
You can exit 1 to indicate a failure.
* To exit early and indicate a success, touch a file
``/tmp/dib-test-should-fail`` in the image chroot, then exit 1.
Tests are run with ``tools/run_functests.sh``. Running
``run_functests.sh -l`` will show available tests (the example above
would be called ``apt-sources/test-case-1``, for example). Specify
your test (or a series of tests as separate arguments) on the command
line to run it. If it should not be run as part of the default CI
run, you can submit a change with it added to ``DEFAULT_SKIP_TESTS``
in that file.
Running the functional tests is time consuming. Multiple parallel
jobs can be started by specifying ``-j <job count>``. Each of the
jobs uses a lot resources (CPU, disk space, RAM) - therefore the job
count must carefully be chosen.
python
""""""
To run functional tests locally, install and start docker, then use
the following tox command::
tox -efunc
Note that running functional tests requires *sudo* rights, thus you may be
asked for your password.
To run functional tests for one element, append its name to the command::
tox -efunc ironic-agent
Additionally, elements can be tested using python unittests. To create a
a python test:
* Create a directory called ``tests`` in the element directory.
* Create an empty file called ``__init__.py`` to make it into a python
package.
* Create your test files as ``test\whatever.py``, using regular python test
code.
To run all the tests use testr - ``testr run``. To run just some tests provide
one or more regex filters - tests matching any of them are run -
``testr run apt-proxy``.
Third party elements
--------------------
Additional elements can be incorporated by setting ``ELEMENTS_PATH``, for
example if one were building tripleo-images, the variable would be set like:
.. sourcecode:: sh
export ELEMENTS_PATH=tripleo-image-elements/elements
disk-image-create rhel7 cinder-api
Linting
-------
You should always run ``bin/dib-lint`` over your elements. It will
warn you of common issues.
sudo
""""
Using ``sudo`` outside the chroot environment can cause breakout
issues where you accidentally modify parts of the host
system. ``dib-lint`` will warn if it sees ``sudo`` calls that do not
use the path arguments given to elements running outside the chroot.
To disable the error for a call you know is safe, add
::
# dib-lint: safe_sudo
to the end of the ``sudo`` command line. To disable the check for an
entire file, add
::
# dib-lint: disable=safe_sudo