Make ExecStartPost command to run in background - linux

Question

I have a systemd service for my spring boot application connected to consul server, behind haproxy. consul provides consul-template to automatically update the service location in haproxy configuration file via consul-template command.
consul-template takes a template file and writes to the final haproxy configuration file and then reload the haproxy.
Now, consul-template process needs to run in background always along with my application, so that as the application comes up, it can detect new application startup and update its location in the configuration file.
Here is my systemd service file for this.
[Unit]
Description=myservice
Requires=network-online.target
After=network-online.target
[Service]
Type=forking
PIDFile=/home/dragon/myservice/run/myservice.pid
ExecStart=/home/dragon/myservice/bin/myservice-script start
ExecReload=/home/dragon/myservice/bin/myservice-script reload
ExecStop=/home/dragon/myservice/bin/myservice-script stop
ExecStartPost=consul-template -template '/etc/haproxy/haproxy.cfg.template:/etc/haproxy/haproxy.cfg:sudo systemctl reload haproxy'
User=dragon
[Install]
WantedBy=multi-user.target
Now, when I start systemctl start myservice, my application starts and the call to consul-template also works, but consul-template process doesn't go in background. I have to press Ctl+C and then systemctl comes back and I have both my application and consul-template process running.
Is there way to run the consul-template process in background specified in ExecStartPost?
I was trying to add & at the end of the ExecStartPost command, but then consul-template complains that it is an additional invalid argument and it fails.
I was also trying to make the command as /bin/sh -c "consul-template command here...", but then this also doesn't work. Even nohup in this command wasn't working.
Any help is really appreciated.

Answer 1

A workaround would be to have a bash file as your entrypoint, add all you need in there, then it will all magically work

Answer 2

I was trying to accomplish the same task. I wanted to fire off some HTTP requests to Tomcat once the service had started, so that I could warmup our servers ahead of the first user request.
I went through a lot of trial and error with using trying to use ExecStartPost to fire off an async process, but actually worked. By calling a shell script, I could trigger off background processes, but from my testing Systemd appears to kill the process thread when ExecStartPost finishes, so any child processes end up getting killed too. I tried various combinations of using &, setsid, nohup, etc, even some Perl to try and trigger off the an executable in it's own thread, but as soon as the shell script exite from ExecStartPost any processes running where killed. It's possible there's some solution that would work using ExecStartPost, but I couldn't find it.
However, what did work is creating a new service (like #divinedragon mentions) which piggy backs off the service I wanted to monitor (in this case Tomcat).
Since it took me a little research to get something working the way I wanted, I wanted to share my solution in case it helps someone.
The first step is to create a new service (e.g. /usr/lib/systemd/system/tomcat-service-listener.service):
[Unit]
Description=Tomcat start/stop event listener
# make sure to stop the service when Tomcat stops
BindsTo=tomcat.service
# waits for both Nginx & Tomcat to be started before this service is started
After=nginx.service tomcat.service
[Service]
Type=oneshot
ExecStart=/path/to/your/script.sh start
ExecStop=/path/to/your/script.sh stop
RemainAfterExit=yes
TimeoutStartSec=300
[Install]
# When the service is enabled, forces this service to start when Tomcat is started
WantedBy=tomcat.service
Some notes on what is happening here:
The BindsTo make sure the service gets stopped when Tomcat is stopped. This triggers the ExecStop command.
The After make sure that on server reboot, this service does not start until both Nginx & Tomcat have started.
The WantedBy will create the wants symlink for Tomcat (when the service is enabled), which will force Tomcat to start this service any time it's restarted.
The RemainAfterExit=yes is necessary for the ExecStop to work. If you only care about triggering something when you're service is started and don't care about when the service is stopped, you can set this to no and remove the ExecStop line.
Make the TimeoutStartSec long enough for whatever task you plan on running.
To get this service working, you then need to do the following:
# make the service executable
chmod 664 /usr/lib/systemd/system/tomcat-service-listener.service
# make Systemd aware of the new service
systemctl daemon-reload
# register the service so it's started/stopped with Tomcat
systemctl enable tomcat-service-listener.service
Now all you need script to trigger off the logic you want. In my case, I wanted to warmup some servers once Tomcat started so my /path/to/your/script.sh looks something like:
#!/bin/sh
SCRIPT_MODE="$1"
LOGFILE=/var/logs/myscript.log
log_message() {
local MESSAGE="$1"
echo "$(date '+%Y-%m-%d %H:%M:%S') $MESSAGE" >> "$LOGFILE"
return 0
}
warmup_server() {
local SERVER_ADDRESS="$1"
local SERVER_DESCRIPTION="$2"
log_message "Warming up $SERVER_DESCRIPTION..."
# we want to track the time it took to warm up the server
local START_TIME=$(date +%s)
# server restarts can take a while for all services to start, so we must retry long enough for all relevant services to start
HTTP_STATUS=$(curl --insecure --location --silent --show-error --fail --retry 60 --retry-delay 2 --retry-max-time 240 --output /dev/null --write-out "%{http_code}" '$SERVER_ADDRESS')
# we want to track the time it took to warm up the server
local TOTAL_STARTUP_TIME=$(($(date +%s)-$START_TIME))
log_message "$SERVER_DESCRIPTION started in $TOTAL_STARTUP_TIME seconds... (Status: $HTTP_STATUS)"
return 0
}
# monitor when Tomcat has stopped
if [ "$SCRIPT_MODE" == "stop" ]; then
log_message "Tomcat listener shutting down..."
exit 0
elif [ "$SCRIPT_MODE" == "start" ]; then
log_message "Tomcat listener started..."
fi
# servers to warm up
warmup_server 'https://127.0.0.1' 'Localhost #1'
warmup_server 'https://127.0.0.2' 'Localhost #2'
This seems to be working exactly as I want. The service starts up when the server is reboot and starting/stopping/restarting Tomcat fires off the expected events. Since it's independent of the Tomcat service, I can restart this warmup script if needed. It also doesn't delay the Tomcat startup time, since it is its own service, therefore running asynchronously like I wanted.