A simplistic way to ensure that a certain tcp host:port is available so that a service can depend on it

wait-for-tcp-port.sh

#!/bin/sh
#/usr/local/bin/wait-for-tcp-port.sh
instance="$1"
host="${instance%%:*}"
port="${instance##*:}"
start=10
loop=$start
while true
do
  nc -zvw3 "$host" "$port" 2>/dev/null >/dev/null
  result=$?
  if [ $result = 0 ]; then
    break
  fi
  if [ $loop = $start ]; then
    printf "Sleeping " >&2
    loop=$(( $loop - 1 ))
  fi
  printf "." >&2
  sleep 1
done
echo >&2

wait-for-tcp-port@.service

#/etc/systemd/system/wait-for-tcp-port@.service
[Unit]
Description=Wait for '%i'

[Service]
TimeoutStartSec=infinity
ExecStartPre=/usr/local/bin/wait-for-tcp-port.sh "%i"
ExecStart=/bin/echo 'Found %i'
RemainAfterExit=yes