how do I get etcd values into my systemd service on coreOS?

Question

I have two services A and B.

A sets a value in etcd as it's being started, say the public IP address which it gets from an environment file:

ExecStartPost=/usr/bin/etcdctl set /A_ADDR $COREOS_PUBLIC_IPV4

B needs that value as it starts up, as well as its own IP address. So something like this would be nice:

ExecStart=/usr/bin/docker run -e MY_ADDR=$COREOS_PUBLIC_IPV4 -e A_ADDR=$ETCD_A_ADDR mikedewar/B

but that's obviously not possible as etcd variables don't present as systemd environment variables like that. Instead I can do some sort of /usr/bin/bash -c 'run stuff' in my ExecStart but it's awkward especially as I need systemd to expand $COREOS_PUBLIC_IPV4 and my new bash shell to expand $(etcdctl get /A_ADDR). It also reeks of code smell and makes me think I'm missing something important.

Can someone tell me the "right" way of getting values from etcd into my ExecStart declaration?

-- update

So I'm up and running with

ExecStart=/usr/bin/bash -c 'source /etc/environment && /usr/bin/docker run -e A_ADDR=$(/usr/bin/etcdctl get /A_ADDR) -e MY_ADDR=$COREOS_PUBLIC_IPV4 mikedewar/B'

but it's pretty ugly. Still can't believe I'm not missing something..

Answer 1

I've was struggling with the same thing until recently. After reading much of the documentation of CoreOS and systemd, here is a slightly 'cleaner' version of what you're doing:

[Service]
EnvironmentFile=/etc/environment
ExecStart=/bin/sh -c '/usr/bin/docker run -e A_ADDR=$(/usr/bin/etcdctl get /A_ADDR) -e MY_ADDR=$COREOS_PUBLIC_IPV4 mikedewar/B'

Additionally, I have adopted a pattern where my services depend on a systemd 'oneshot' service that will compute some value and write it in to /etc/environment. This allows you to keep more complex shell scripting out of the main service unit and place it into it's own oneshot service unit.

Here are the docs for EnvironmentFile: http://www.freedesktop.org/software/systemd/man/systemd.exec.html#EnvironmentFile=

Finally, a quick gotchya: you must use a shell invocation if you use any variable in your ExecStart/Stop commands. systemd does no shell invocation when executing the command you provide, so variables will not be expanded.

Answer 2

I am currently using such a workaround:

I've created scripts which extracts data from particular etcd directory

#! /bin/sh
for entry in `etcdctl ls /my_dir --recursive`  ; do
  echo ' -e '`grep -o '[^/]*$'  <<< ${entry}`=`etcdctl get ${entry}`
done

its output looks following:

 -e DATABASE_URL=postgres://m:[email protected]:5432/m
 -e WEB_CONCURRENCY=4

So then eventually I can in my init file place that in such way

/bin/sh -c '/usr/bin/docker run -p 9000:9000 $(/home/core/envs.sh) me/myapp -D FOREGROUND'

It's not the most elegant way, and I'd love to know how to improve it, but placing that for loop as a one-liner requires lots of escaping.