How can I receive an email alert when one or more nodes are down?

You have 2 ways to receive an email alert when the nodes are down.

Cloudtrax dashboard notifications (only for WiFi Location Monitor)

The first way is to enable the Send email alerts switch under the Configure > General section of the Cloudtrax dashboard and add, at the same time, your email address inside the text field. Anyway please be aware that sometime it takes a little bit to get email alerts from Cloudtrax when one or multiple nodes are down, because Cloudtrax check the status of the nodes only once in a while. So if you need an immediate feedback, this solution is not very helpful.

Using the nodes API call (Wi-Fi Location Monitor and Bluetooth Beacon Tracker)

The second (and reliable) way to get immediate alerts regarding the status of the nodes is to create a simple script that calls (i.e. every 120 seconds) the Accuware Wi-Fi Location Monitor nodes API or the Accuware Bluetooth Beacon Tracker nodes API. Once you have the JSON array with all the nodes, you just have to iterate on it and check if at least one node has been seen down for more than X seconds by using a rule like the one shown in the pseudo code below: