mjhjmtu

In SysV I can use a condition to ensure an application does not try to start before the database is up and running. I give the init script some time to wait, then finally give up after a period of time if the database service is still unavailable.

start() {
local exec=/path/to/exec
local tries=1
[ -x $exec ] || exit 5
echo -n $"Starting $prog: "


#check communication to database
if ! [ 2>/dev/null : < /dev/tcp/$dbHost/$dbPort ]
 then
 while ! [ 2>/dev/null : < /dev/tcp/$dbHost/$dbPort ] && [ ! $tries -ge 5 ]
 do
 >&2 echo -e "Could not connect to the database on $dbHostnWaiting 10 seconds to check database status, attempt $tries"
 sleep 10
 ((tries++))
 done
 sleep 10
 if ! (: < /dev/tcp/$dbHost/$dbPort ) 2>/dev/null
 then
 >&2 echo -e "Could not connect to the database on $dbHost aborting startup of $exec"
 exit 1
 fi
fi

I've been searching for a similar scenario in documentation and google, but have not found anything that is not referencing local services.

Have you looked at ExecStartPre in the systemd service documentation?

I would suggest putting your database test in a script, have it use exit 0 on success and exit 1 on failure and then run it with ExecStartPre. You would then start your application using ExecStart.

@GracefulRestart's answer is the best if you only have one service depending on the availability of the database. However, if you have multiple services that have this requirement, make a oneshot service that all of the services can then have a Requires= dependency to:

/etc/systemd/system/portopen@.service

[Unit]
Description=Checks database availability on %I
After=network.target
Requires=network.target

[Service]
Type=oneshot
RemainAfterExit=true
ExecStart=/path/to/portopen.sh %I

[Install]
WantedBy=default.target

/path/to/portopen.sh

#!/bin/bash
dbhost=$1:-localhost
dbport=$2:-5678
maxtries=$3:-5
wait=$4:-10
tries=1

# Check communication to database
while ! [ 2>/dev/null : < /dev/tcp/$dbhost/$dbport ]; do
 echo "Unable to connect to database on $dbhost TCP/$dbport (attempt $tries): retrying in $wait seconds" >&2
 (( tries++ ))
 if [[ $tries -le $maxtries ]]; then
 sleep $wait
 else
 echo "Unable to connect to database on $dbhost TCP/$dbport: aborting"
 exit 1
 fi
done

I made the script a bit more flexible in case you change or add database servers, or the port changes, or you want to change the number of retries on a per-service level. If you don't need that, just call the service portopen.service and remove the %I parts.

Let's say your database server is on foobar and your database application runs on foobarapp.service. Make the following changes to foobarapp.service:

# systemctl edit foobarapp.service

[in editor]

[Unit]
After=portopen@foobar.service
Requires=portopen@foobar.service

Then reload systemd and start and enable the check:

# systemctl daemon-reload
# systemctl enable portopen@foobar.service

You can then restart foobarapp.service whenever you want. It should only start if portopen@foobar.service returns successfully.

If it doesn't already exist, the database application service foobarapp.service would then look like this:

/etc/systemd/system/foobarapp.service

[Unit]
Description=Foobar database application
# Add any other dependencies here
After=portopen@foobar.service
Requires=portopen@foobar.service

[Service]
# If it is a daemon, use "forking" instead
Type=simple
ExecStart=/path/to/exec

[Install]
WantedBy=default.target