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.. _boot_stages:
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 five
stages to boot:
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:
* The file ``/etc/cloud/cloud-init.disabled`` exists
* The kernel command line as found 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.
Again, these mechanisms for disabling cloud-init at runtime currently only
exist in systemd.
Local
=====
+------------------+----------------------------------------------------------+
| systemd service | ``cloud-init-local.service`` |
+---------+--------+----------------------------------------------------------+
| runs | as soon as possible with ``/`` mounted read-write |
+---------+--------+----------------------------------------------------------+
| blocks | as much of boot as possible, *must* block network |
+---------+--------+----------------------------------------------------------+
| modules | none |
+---------+--------+----------------------------------------------------------+
The purpose of the local stage is to:
* 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 determines the network configuration to be used. That
network configuration can come from:
* **datasource**: cloud provided network configuration via metadata
* **fallback**: 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
* **none**: network configuration can be disabled by writing the file
``/etc/cloud/cloud.cfg`` with the content:
``network: {config: disabled}``
If this is an instance's first boot, then the selected network configuration
is rendered. This includes clearing of all previous (stale) configuration
including persistent device naming with old mac addresses.
This stage must block network bring-up or any stale configuration might
already have been applied. That could have negative effects such as DHCP
hooks or broadcast of an old hostname. It would also put the system in
an odd state to recover from as it may then have to restart network
devices.
Cloud-init then exits and expects for the continued boot of the operating
system to bring network configuration up as configured.
**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 can 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 | *cloud_init_modules* in ``/etc/cloud/cloud.cfg`` |
+---------+--------+----------------------------------------------------------+
This stage requires all configured networking to be online, as it will fully
process any user-data that is found. Here, processing means:
* retrieve any ``#include`` or ``#include-once`` (recursively) including http
* decompress any compressed content
* run any part-handler found.
This stage runs the ``disk_setup`` and ``mounts`` modules which may partition
and format disks and configure mount points (such as in ``/etc/fstab``).
Those modules cannot run earlier as they may receive configuration input
from sources only available via network. For example, a user may have
provided user-data in a network resource that describes how local mounts
should be done.
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 those necessary for
cloud-init to run should not be done until after this stage.
A part-handler will run at this stage, as will boot-hooks 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 |
+---------+--------+----------------------------------------------------------+
| blocks | nothing |
+---------+--------+----------------------------------------------------------+
| modules | *cloud_config_modules* in ``/etc/cloud/cloud.cfg`` |
+---------+--------+----------------------------------------------------------+
This stage runs config modules only. Modules that do not really have an
effect on other stages of boot are run here.
Final
=====
+------------------+----------------------------------------------------------+
| systemd service | ``cloud-final.service`` |
+---------+--------+----------------------------------------------------------+
| runs | as final part of boot (traditional "rc.local") |
+---------+--------+----------------------------------------------------------+
| blocks | nothing |
+---------+--------+----------------------------------------------------------+
| modules | *cloud_final_modules* in ``/etc/cloud/cloud.cfg`` |
+---------+--------+----------------------------------------------------------+
This stage runs as late in boot as possible. 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``).
For scripts external to cloud-init looking to wait until cloud-init is
finished, the ``cloud-init status`` subcommand can help block external
scripts until cloud-init is done without having to write your own systemd
units dependency chains. See :ref:`cli_status` for more info.
.. vi: textwidth=79
|
