Systemd Service Start With Dynamic Port Value From Docker

Question

Using CoreOS, Docker and systemd to manage my services I want to properly perform service discovery. Since CoreOS utilizes etcd (distributed key-value) there is a very convenient way to do this. On systemd's ExecStartPost I can just insert the started service into etcd without problems. My usecase needs something like this:

ExecStartPost=/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": 5555 }' 

which works like a charm.

But this is where my idea popped up. Docker has the power to randomly assign a port if I just run docker run -p 5555 which is awesome since I don't have to set it statically in the *.service file and I could possibly run multiple instances on the same host. What if I could get the randomly assigned port and insert instead of the static 5555?

Turns out I can use the docker port command to get the port and with some formatting we can get just the port with

$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2) 

which works if I set it (using bash) like this:

/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": '$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2)' }' 

but using systemd I just can't get it to work. This is the code I'm using:

ExecStartPost=/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": '$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2)'}' 

Somehow I got something wrong but it's hard to debug since it works when typed in the terminal.

Answer 1

The systemd ExecStart option doesn't do any shell interpolation and thus doesn't understand stuff like $(echo 'Hello'). But, you can start a shell that executes the command to get behavior like this. Try something like:

ExecStartPost=/bin/sh -c "/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": '$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2)'}'" 

Answer 2

I ended up using something like this:

ExecStart=/bin/sh -c "port=$(docker port [CONTAINER-NAME] [CONTAINER-EXPOSED-PORT] | cut -d ':' -f 2); echo Connected to $port, publishing to etcd...; while netstat -lnt | grep :$port >/dev/null; do etcdctl set /services/[CONTAINER-NAME]/[SOME-IDENTIFIER] $port --ttl 60 >/dev/null; sleep 45; done" 

But the answer for my question was indeed that I needed to state which shell to run the command.

Edit: Adding the Go templating logic

[Service] EnvironmentFile=/etc/environment ExecStartPre=/bin/sh -c "until docker port {{.Name}} {{.Port}} >/dev/null 2>&1; do echo Waiting for {{.Name}}; sleep 2; done; port=$(docker port {{.Name}} {{.Port}} | cut -d ':' -f 2); echo Waiting for $port/tcp...; until netstat -lnt | grep :$port >/dev/null; do sleep 1; done" ExecStart=/bin/sh -c "port=$(docker port {{.Name}} {{.Port}} | cut -d ':' -f 2); echo Connected to $COREOS_PRIVATE_IPV4:$port/tcp, publishing to etcd...; while netstat -lnt | grep :$port >/dev/null; do etcdctl set /services/{{.Service}}/{{.Name}} $COREOS_PRIVATE_IPV4:$port --ttl 60 >/dev/null; sleep 45; done" ExecStop=/usr/bin/etcdctl rm --recursive /services/{{.Service}}/{{.Name}}