Check if one can get a response on a nats channel. I use it for health checking node services. Uses mailgun to email.

gnatsd_service_check.sh

#!/bin/bash
set -ueo pipefail
natsUrl="nats://user:pass@192.168.1.84:4222"
checkTimeout=6

mailgunDomain=CHANGEME
mailgunAPIKey=CHANGEME
deliveryEmailAddress=CHANGEME

SEND_ERROR_MAIL=1

echo "Polling url: ${natsUrl}"

function mailError {
    local service=$1
    mailMessage "!ERROR! - SERVICE NOT RESPONDING: ${service} service not responding"  "ERROR: a service is not responding: ${service}"
}

function mailLazerus {
    local service=$1
    mailMessage "OK - SERVICE IS ALIVE: ${service}"  "Service is responding: ${service}"
}

function mailMessage {
    local subject=$1
    local message=$2
    echo "sending mail via mailgun: ${subject} | ${message}"
    curl -s --user "api:${mailgunAPIKey}" \
        https://api.mailgun.net/v3/${mailgunDomain}/messages \
        -F from="Services Alert <mailgun@${mailgunDomain}>" \
        -F to=${deliveryEmailAddress} \
        -F subject="${subject}" \
        -F text="${message}"
}

for service in blog user
do
    set  +e
    timeout ${checkTimeout} /usr/local/bin/nats-request "${service}.system.up" '{}' -n1 -t -s "${natsUrl}"
    result=$?
    set -e
    echo "Result ${result}"
    if [[ ${result} -ne 1 ]]
    then
        echo "error code ${result} checking ${service}"

        sendMail=0
        if [[ -f ./LAST_ALERT_EMAIL_${service} ]]
        then
            if [[ $(( (`date +%s` - `stat -L --format %Y ./LAST_ALERT_EMAIL_${service}`) > (30*60) )) -eq 1 ]]
            then
                rm ./LAST_ALERT_EMAIL_${service}
                sendMail=${SEND_ERROR_MAIL}
            else
                echo not sending mail as less than 30 minutes since last
            fi
        else
            touch ./LAST_ALERT_EMAIL_${service}
            sendMail=${SEND_ERROR_MAIL}
        fi
        if [[ ${sendMail} -eq ${SEND_ERROR_MAIL} ]]
        then
            mailError ${service}
        fi
    else
        echo "healthy response from ${service}"
        if [[ -f ./LAST_ALERT_EMAIL_${service} ]]
        then
            echo "Sending service ok email"
            mailLazerus ${service}
            rm ./LAST_ALERT_EMAIL_${service}
        fi
    fi
done