restart_zero_downtime.sh

#!/bin/bash

PORT_FILE=/home/deployer/PORT
APP_DIR=/var/www/cryptoprocessing
APP_PIDS_DIR=$APP_DIR/tmp/pids
UPSTREAM_CONFIG_FILE=/home/deployer/UPSTREAM_SERVERS
PORT=$(cat $PORT_FILE)

if [ "$PORT" == "3000" ]
then
  NEW_PORT=3001
else
  NEW_PORT=3000
fi

echo "$(date +%s%N) Starting new process"
cd $APP_DIR
/bin/bash -l -i -c "cd $APP_DIR && rails s -e production -P $APP_PIDS_DIR/$NEW_PORT.pid -p $NEW_PORT -d"

echo "Waiting"
sleep 2

echo "$(date +%s%N) Killing old process"
CURRENT_PID_FILE=$APP_PIDS_DIR/$PORT.pid
CURRENT_PID=$(cat $CURRENT_PID_FILE)
kill -9 $CURRENT_PID

echo "$(date +%s%N) Writing new port to $PORT_FILE"
echo $NEW_PORT > $PORT_FILE

echo "$(date +%s%N) Updating config and reloading Nginx"
echo -e "server 127.0.0.1:$NEW_PORT;\nserver 127.0.0.1:$PORT;" > $UPSTREAM_CONFIG_FILE
sudo nginx -s reload