diskimage-builder/doc/source/developer/developing_elements.rst

Developing Elements

Conform to the following conventions:

• Use the environment for overridable defaults, prefixing environment variable names with "DIB_". For example: DIB_MYDEFAULT=${DIB_MYDEFAULT:-default} If you do not use the DIB_ prefix you may find that your overrides are discarded as the build environment is sanitised.
• Consider that your element co-exists with many others and try to guard against undefined behaviours. Some examples:
  • Two elements use the source-repositories element, but use the same filename for the source-repositories config file. Files such as these (and indeed the scripts in the various .d directories listed below) should be named such that they are unique. If they are not unique, when the combined tree is created by disk-image-builder for injecting into the build environment, one of the files will be overwritten.
  • Two elements copy different scripts into /usr/local/bin with the same name. If they both use set -e and cp -n then the conflict will be caught and cause the build to fail.
• If your element mounts anything into the image build tree ($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.

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.

• 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=i386armhf $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 and 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 (e.g. to make partitions). Runs after the target tree has been fully populated but before the cleanup hook 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=i386armhf $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 environment.d. This directory contains bash script snippets that are sourced before running scripts in each phase.

DIB exposes an internal IMAGE_ELEMENT variable which provides elements access to the full set of elements that are included in the image build. This can be used to process local in-element files across all the elements (pkg-map for example).

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.

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_ROOT_FS_UUID : this contains the UUID of the root fs, when diskimage-builder is building a disk image. This works only for ext filesystems.

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 elements/source-repositories/README.md for more information.

Debugging elements

The build-time environment and command line arguments are captured by the 'base' element and written to /etc/dib_environment and /etc/dib_arguments inside the image.

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 a in target hookpoint.

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 typicallly 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

Elements can be tested using python. To create a 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

Pending implementation. The idea is to have a search path for elements.