The Tiny Cloud bootstrapper performs critical initialization tasks for cloud instances during their first boot. Unlike the more popular and feature-rich cloud-init, Tiny Cloud seeks to do just what is necessary with a small footprint and minimal dependencies.

A direct descendant of tiny-ec2-bootstrap, Tiny Cloud works with multiple cloud providers. Currently, the following are supported:

AWS (Amazon Web Services)
Azure (Microsoft Azure)
GCP (Google Cloud Platform)
OCI (Oracle Cloud Infrastructure)

Features

The following actions will occur only once, during the initial boot of an instance:

expand the root filesystem to use all available root device space, during the sysinit runlevel
set the instance's hostname from instance metadata
install SSH keys from instance metadata to the cloud user account's authorized_keys file (the user must already exist)
save the instance user-data to a file and if it's a script, execute it at the end of the default runlevel

Optional features, which may not be universally necessary:

manage symlinks from NVMe block devices to /dev/xvd and /dev/sd devices (i.e. AWS Nitro instances)
manage secondary IPv4 and IPv6 addresses on network interfaces

Requirements

As Tiny Cloud is meant to be tiny, it has very few dependencies:

Busybox (ash, wget, etc.)
partx
resize2fs
sfdisk

Tiny Cloud has been developed specifically for use with the Alpine Cloud Images project, and as such, it is currently tailored for use with Alpine Linux, the OpenRC init system, and the ext4 root filesystem. If you would like to see Tiny Cloud supported on additional distributions, init systems, and/or filesystems, please open an issue with your request -- or better yet, submit a merge request!

Installation

Typically, Tiny Cloud is installed and configured when building a cloud image, and is available on Alpine Linux as the tiny-cloud APK...

apk install tiny-cloud

This will install the necessary init scripts, libraries, etc. plus any missing dependencies.

Alternately, you can download a release tarball, and use make to install it.

Next, enable the three primary init scripts...

rc-update add tiny-cloud-early sysinit
rc-update add tiny-cloud default
rc-update add tiny-cloud-final default

Configuration

Tiny Cloud looks expects configuration to be found at /etc/conf.d/tiny-cloud, which documents all tuneable parameters (and their defaults).

However, because Tiny Cloud does not do auto-detection, you must set a value for CLOUD indicating which cloud provider which will be used when instantiating the image. Current valid values are aws, azure, gcp, and oci.

Operation

The first time an instance boots -- either freshly instantiated from an image, or after installation on an existing instance -- Tiny Cloud sets up the instance in three phases...

Early Phase

The tiny-cloud-early init script does not depend on the cloud provider's Instance MetaData Service (IMDS), and does therefore does not have a dependency on networking. During this "early" phase, the root filesystem is expanded, and any necessary mdev rules for device hotplug are set up.

Main Phase

The main tiny-cloud init script does depend on the cloud provider's IMDS data, and sets up instance's hostname and the cloud user's SSH keys before sshd starts.

Final Phase

tiny-cloud-final should be the very last init script to run in the default runlevel, and saves the instance's user data to /var/lib/cloud/user-data.

If the user data is a script that starts with #! (aka "shebang"), it will be executed; its output (combined STDOUT and STDERR) is saved to /var/log/cloud/user-data.log and the exit code can be found in /var/log/cloud/user-data.exit.

If all went well, the very last thing tiny-cloud-final does is touch /var/lib/cloud/.bootstrap-complete into existence.

Further Reboots

After the initial bootstrap of an instance, the init scripts are largely a no-op.

To force the init scripts to re-run on the next boot...

rm -f /var/lib/cloud/.bootstrap-complete

If you're instantiating an instance in order to create a new cloud image (using Packer, or some other means), you will need to remove this file before creating the image to ensure that instances using the new image will also run Tiny Cloud init scripts during their first boot.