It is week four of the IoT course and students of Case Western Reserve University have successfully built their very own touch-screen controlled LAMPi’s. This week students will take a major step towards greater functionality and control.
Demoing Previous Week’s Work
Students began the week by demoing their assignment from the previous week – building a Python service to act as an abstraction layer between the LAMPi hardware and the various user interfaces. You can check out the post for the week here: Week 3
Learning IoT the hard way – Why we are not using an IoT platform
This week’s lecture started with a brief introduction to available commercial IoT platforms. For this course, students will forego working with a commercial IoT platform, and delve directly into Amazon Web Services. By having the students build their own platform on top of AWS, they will gain a greater understanding of the various components involved, which will allow them to make informed decisions when dealing with commercial IoT platforms in a professional capacity.
To the cloud – Amazon EC2
The cloud! Yes, that cloud. Those nebulous servers in the sky. Students will be maintaining their cloud connectivity using AWS Elastic Compute Clusters, or EC2. At the beginning of week 4, the services running on LAMPi were locally controlled through a MQTT broker called Mosquitto. The goal for this week is to bridge the MQTT broker running on the Raspberry Pi, to another MQTT broker running on EC2. Luckily for our students, with a little configuration, Mosquitto offers this functionality.
Students began by setting up free tier EC2 instances running Ubuntu. After configuring AWS security groups for their instances, they installed the Mosquitto command line utilities mosquitto_pub and mosquitto_sub.
Bridging between Mosquitto brokers is done through editing configuration files on both the host and the remote. When bridging, the user can configure what topics are transmitted in which direction — for instance, we can specify that only lamp state updates go out to the cloud, and only configuration changes are received from the cloud. Any topics not configured do not get to cross the bridge and will remain on the system they originated. The configuration to the remote follows the structure here:
topic <pattern> <direction> <qos> <local-prefix> <remote-prefix>
cleansession <true or false> (defaults to false)
Following this pattern will open the bridge between the MQTT brokers. From that point, depending on the configuration of “In” and “Out” directions, messages will be passed from the appropriate publisher to its subscribers. It is worthwhile to note that, although the broker is now up and running on EC2, there is not yet any functionality beyond simply passing messages. This will be the topic of the next lesson. Part of the reason for this is that configuration of Mosquitto can be complex and that complexity is compounded by the configuration of the whole system — security, supervisord, pigpiod, etc. By doing this in steps, we can test our system at each step of the way. For now, a few more steps are required to ensure our path from the Raspberry Pi and EC2 instance remain unbroken.
Unexpected Events – Last Will and Testament
The MQTT specification defines a mechanism called a Will. In the event that a client disconnects unexpectedly the MQTT server will publish the Will message. The students will use this mechanism to announce the connection state of the lamp. In Python, the code looks like this:
client = mqtt.Client(client_id=CLIENT_ID, protocol=MQTT_VERSION)
“0”, qos=1, retain=True)
Service management – Supervisor
The students were introduced to a Linux service called Supervisor which is responsible for ensuring that all of the processes that we care about stay running. If the LAMPi is unplugged, or the cloud service restarts for some reason, a proper Supervisor setup will ensure that all systems are restarted and running. These concepts were applied to the LAMPi’s by creating a Supervisor D configuration file that will keep the lamp service and the UI running.
What is up next?