When you are not willing to pay for monitoring services like AppDynamics (80k /y ) or UptimeMonitor (monthly fee) you still have a cheap shell script based solution.

Shells scripts and Slack are your friends.

Scenario

You have a Tomcat webserver running and you want to take action if it dies and notify of the action to a group of people.

Tools

Crontab, Shell script and Slack

Step 1: Setup crontab

In your linux server add a new crontab crontab -e and edit the following check every minute.

*/1 * * * * /opt/scripts/check.sh

Now, restart your cron service
service crond restart

Step 2: Create the check

Create a new shell script in /opt/scripts/check.sh with the following code:

#! /bin/sh
SERVICE=/etc/init.d/tomcat8

if $SERVICE status | grep -q "not running"; then
    $SERVICE start
    /opt/scripts/slackpost.sh "https://hooks.slack.com/services/your-hook" "#monitor" "Tomcat Automatic Restart"
fi
if $SERVICE status | grep -q "stopped"; then
    $SERVICE start
    /opt/scripts/slackpost.sh "https://hooks.slack.com/services/your-hook" "#monitor" "Tomcat Automatic Restart"
fi

This will find if the service crashed or was not started and it will start it. After staring the service it will send a notification to your slack channel (you can strip this piece if no slack desired)

Step 3: Notify to Slack

Create another script in /opt/scripts/slackpost.sh with the following content:

#!/bin/bash

# Usage: slackpost "" "" ""

webhook_url=$1
if [[ $webhook_url == "" ]]
then
        echo "No webhook_url specified"
        exit 1
fi

shift
channel=$1
if [[ $channel == "" ]]
then
        echo "No channel specified"
        exit 1
fi

shift

text=$*

if [[ $text == "" ]]
then
        echo "No text specified"
        exit 1
fi

escapedText=$(echo $text | sed 's/"/\"/g' | sed "s/'/\'/g" )
json="{\"channel\": \"$channel\", \"text\": \"$escapedText\"}"

curl -s -d "payload=$json" "$webhook_url"

And finally chmod +x /opt/scripts/*sh to make both scripts runnables.

Happy coding!