cloud-init-dev team mailing list archive

[Scott Moser <smoser@xxxxxxxxxx>]

updated a bit.
tried to incorporate your suggestions.


Diff comments:

> diff --git a/doc/rtd/topics/boot.rst b/doc/rtd/topics/boot.rst
> new file mode 100644
> index 0000000..693a091
> --- /dev/null
> +++ b/doc/rtd/topics/boot.rst
> @@ -0,0 +1,109 @@
> +============
> +Boot Stages
> +============
> +In order to be able to provide the functionality that it does, cloud-init
> +must be integrated into the boot in fairly controlled way.
> +
> +There are 5 stages.
> +
> +1. **Generator**
> +2. **local**
> +3. **network**
> +4. **config**
> +5. **final**
> +
> +Generator
> +#########
> +When booting under systemd, a 
> +`generator <https://www.freedesktop.org/software/systemd/man/systemd.generator.html>`_
> +will run that determines if cloud-init.target should be included in the
> +boot goals.  By default, this generator will enable cloud-init.  It will
> +not enable cloud-init if either:
> +
> + * A file exists: ``/etc/cloud/cloud-init.disabled``
> + * The kernel command line as fond in /proc/cmdline contains ``cloud-init=disabled``.
> +   When running in a container, the kernel command line is not honored, but
> +   cloud-init will read an environment variable named ``KERNEL_CMDLINE`` in
> +   its place.
> +
> +local
> +#####
> + * **systemd service**: ``cloud-init-local.service``
> + * **runs**: As soon as / is mounted read-write.
> + * **blocks**: as much of boot as possible, *must* block network bringup.
> + * **modules**: none
> +
> +The purpose of the local stage is:
> + * locate "local" data sources.
> + * apply networking configuration to the system (including "Fallback")
> +
> +In most cases, this stage does not do much more than that.  It finds the
> +datasource and renders the described network configuration to disk.

well, but either way, cloud-init renders a network config. even if that is fallback.

> +It then exits and expects for the continued boot of the operating system
> +to bring network configuration up.
> +
> +If this is a new instance, then any previously existing network config

sure

> +should be wiped.  Because network bringup is blocked, we can avoid any
> +previous-instance networking from coming up, and having negative side
> +affects such as dhcp hooks or broadcast of an old hostname.
> +
> +In the event that no local datasource is found, 'Fallback' networking
> +is rendered.  Cloud-init's fallback networking consists of rendering the
> +equivalent to "dhcp on eth0", which was historically the most popular
> +mechanism for network configuration of a guest.
> +
> +**Note**: In the past, local data sources have been only those that were
> +available without network (such as 'ConfigDrive').  However, as seen in
> +the recent additions to the DigitalOcean datasource, even data sources
> +that require a network will operate at this stage.
> +
> +network
> +#######
> + * **systemd service**: ``cloud-init.service``
> + * **runs**: After local stage and configured networking is up.
> + * **blocks**: As much of remaining boot as possible.
> + * **modules**: ``init_modules``
> +
> +This stage requires networking configuration to be up, as it will fully
> +process any user-data that is found.  Such processing means that any
> +``#include`` operations will be done including http.
> +
> +This stage runs the ``disk_setup`` and ``mounts`` modules which may partition
> +and format disks and configure mount points (such as in /etc/fstab).
> +On some clouds such as Azure, this stage will create filesystems to be
> +mounted, including ones that have stale (previous instance) references in
> +/etc/fstab. As such, entries /etc/fstab other than / should not be processed
> +until after this stage is done.
> +
> +A part-handler will run at this stage, as will boothooks including
> +cloud-config ``bootcmd``.  The user of this functionality has to be aware
> +that the system is in the process of booting when their code runs.
> +
> +
> +config
> +######
> + * **systemd service**: ``cloud-config.service``
> + * **runs**: After network stage.
> + * **blocks**: None.
> + * **modules**: ``config_modules``
> +
> +This stage runs config modules only.  Modules that do not really have an
> +affect on other stages of boot are run here.
> +
> +
> +final
> +#####
> + * **systemd service**: ``cloud-final.service``
> + * **runs**: As final part of boot (traditional "rc.local")
> + * **blocks**: None.
> + * **modules**: ``final_modules``
> +
> +This stage runs as late in boot as possible.  The goal is to provide run
> +as if the system had fully booted.  Any scripts that a user is accustomed
> +to running after logging into a system should run correctly here.
> +Things that run here include
> +
> + * package installations
> + * configuration management plugins (puppet, chef, salt-minion)
> + * user-scripts (including ``runcmd``).
> +


-- 
https://code.launchpad.net/~smoser/cloud-init/+git/cloud-init/+merge/310386
Your team cloud init development team is requested to review the proposed merge of ~smoser/cloud-init:doc-boot-stages into cloud-init:master.