var properties = PropertiesService.getScriptProperties();

var service = "MyService"
var url = "http://example.com";
var email = "you@gmail.com";

function parseDate(date_string) {
  return new Date(Date.parse(date_string)).toISOString();
}

function startFailure() {
  var failures = parseInt(properties.getProperty('failures'));
  properties.setProperty('failures', failures + 1);
  var failure_start = new Date();
  if (failures == 1) {
    properties.setProperty('failure_start', failure_start.toISOString());
  }
  
  var message = "Failed to reach endpoint (attempt " + (failures + 1) + ")";
  console.log(message);
  if ( failures < 3 ) {
    if (failures == 2) {
      message = message + " Alert will be silenced until recovery.";
    }
    MailApp.sendEmail(email, service + " is UNHEALTHY", message);
  }
}

function endFailure() {
  var failures = parseInt(properties.getProperty('failures'));
  if (failures > 0) {
    var failure_end = new Date();
    var failure_start = properties.getProperty('failure_start');
    var message = "Downtime lasted from " + parseDate(failure_start) + " to " + failure_end.toISOString();
    MailApp.sendEmail(email, service + " returned to HEALTHY", message);
  }
  properties.setProperty('failures', 0);
  properties.deleteProperty('failure_start');
}

function checkService() {
  try {
    var response = UrlFetchApp.fetch(url, { 
      muteHttpExceptions: true,
      validateHttpsCertificates: false,
      followRedirects: true 
    });
    endFailure();
  } catch (e) {
    startFailure();
  }
}