Developing Elements¶
Conform to the following conventions:
Use the environment for overridable defaults, prefixing environment variable names with
DIB_
. For example: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 listed below) 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 useset -e
andcp -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 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¶
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
post-root.d
block-device.d
pre-finalise.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=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
Contents placed under
$TMP_HOOKS_PATH
will be available at/tmp/in_target.d
inside the chroot.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 servicesClean the cache left by the package manager to reduce the size of the image.
runs: in chroot
post-root.d
Run code outside the chroot. This is a good place to perform tasks that cannot run inside the chroot and must run after installing things. The root filesystem content is rooted at
$TMP_BUILD_DIR/mnt
.runs: outside 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}
pre-finalise.d
Final tuning of the root filesystem, outside the chroot. Filesystem content has been copied into the final file system which is rooted at
$TMP_BUILD_DIR/mnt
. You might do things like re-mount a cache directory that was used during the build in this phase (with subsequent unmount incleanup.d
).
runs: outside chroot
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=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,
dpkg and opensuse 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, theopensuse
element exportsDISTRO_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.
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_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 source-repositories for more information.
Finding other elements¶
DIB exposes an internal $IMAGE_ELEMENT_YAML
variable which
provides elements access to the full set of included elements and
their paths. This can be used to process local in-element files
across all the elements (pkg-map
for example).
import os
import yaml
elements = yaml.load(os.getenv('IMAGE_ELEMENT_YAML'))
for element, path in elements:
...
For elements written in Bash, there is a function
get_image_element_array
that can be used to instantiate an
associative-array of elements and paths (note arrays can not be
exported in bash).
# note eval to expand the result of the get function
eval declare -A image_elements=($(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 manifests 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:
export ELEMENTS_PATH=tripleo-image-elements/elements disk-image-create rhel 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