Atomic system updates are at least half of how “Atomic Hosts” earn their Fallout-flavored appellation. Where a standard Fedora, RHEL or CentOS host gets its updates from a sack of RPMs downloaded from various repositories and exploded out where appropriate, the Atomic editions of these distros consume this same software in pre-exploded-and-composed-into-an-image form.

One tricky element of consuming your RPMs in a single blob is choosing a package or two to add beyond what’s been composed into the image. I wanted to do this straightaway after learning about the atomic host concept, and I (semi)helpfully documented my progress with composing custom trees in a few different spots, most recently at: Compose Your Own Atomic Updates.

This works pretty well, but composing and rebasing to a tree of your own is sort of a heavy approach. Shouldn’t you be able to compose just part of a tree, and, like, overlay those packages on your atomic host?

OSTree mastermind Colin Walters has whipped up just such a utility, and today, I took it for a spin with CentOS Atomic Host.

I started with a CentOS Atomic Host vagrant box, which, as you’ll see, doesn’t include the fortune-mod package:

[laptop-host]$ vagrant init centos/atomic-host

[laptop-host]$ vagrant up

[laptop-host]$ vagrant ssh

[atomic-vm]$ fortune
bash: fortune: command not found

I need to grab Colin’s tool from git, which is also not included in the CentOS Atomic Host, but which is available in the friendly centos/tools container. For a bit of info about the Fedora flavor of this container, see here.

[atomic-vm]$ sudo atomic run centos/tools

[tools-container]$ cd /root

[tools-container]$ git clone https://github.com/cgwalters/atomic-pkglayer/

[tools-container]$ cd atomic-pkglayer

[tools-container]$ git checkout v2016.1

atomic-pkglayer requires ostree to function, and this package is missing from the centos/tools container, so I need to grab it from the repo below. Also, fortune-mod lives in EPEL, so I’ll install that repo as well.

[tools-container]$ curl -O https://raw.githubusercontent.com/CentOS/sig-atomic-buildscripts/downstream/rhel-atomic-rebuild.repo

[tools-container]$ mv rhel-atomic-rebuild.repo /etc/yum.repos.d/

[tools-container]$ yum install ostree epel-release -y

Now I need to grab all the rpms required for fortune-mod, and install them to a pkglayer, before exiting my tools container, rebooting my atomic VM, and logging back in to the rebooted atomic VM:

[tools-container]$ mkdir pkgs

[tools-container]$ yumdownloader --resolve --destdir=pkgs fortune-mod

[tools-container]$ /root/atomic-pkglayer/atomic-pkglayer pkgs/*rpm

[tools-container]$ exit

[atomic-vm]$ sudo reboot

[laptop-host]$ vagrant ssh

Now, for some fortune:

[atomic-vm]$ fortune
Rune's Rule:
    If you don't care where you are, you ain't lost.

You can see my local overlay:

[atomic-vm]$ sudo atomic host status
TIMESTAMP (UTC)         VERSION        ID             OSNAME                 REFSPEC                                                     
* 2016-01-29 00:30:06     local          0aa16a3e42     centos-atomic-host     <unknown origin type>                                       
  2015-10-01 09:32:09     7.20151001     1e9838ce88     centos-atomic-host     centos-atomic-host:centos-atomic-host/7/x86_64/standard     

The system is left in an un-upgradable state — I’ll need to rollback before I can grab updates again, so this overlay is temporary:

[atomic-vm]$ sudo atomic host upgrade
error: No origin/refspec in current deployment origin; cannot upgrade via ostree

[atomic-vm]$ sudo atomic host rollback
Moving '1e9838ce8879112c47c72503bbade0830e6f06dc20f5cabbf6da40a373550f69.0' to be first deployment
Transaction complete; bootconfig swap: no deployment count change: 0
Removed:
  fortune-mod-1.99.1-17.el7.x86_64
  recode-3.6-38.el7.x86_64
Successfully reset deployment order; run "systemctl reboot" to start a reboot

[atomic-vm]$ sudo systemctl reboot

[laptop-host]$ vagrant ssh

Post-rollback, the fortune command is missing once again, and my system is ready for upgrades again:

[atomic-vm]$ fortune
bash: fortune: command not found

[atomic-vm]$ sudo atomic host upgrade
Updating from: centos-atomic-host:centos-atomic-host/7/x86_64/standard