Lua(JIT), Perl, Erlang (RabbitMQ) all playing nicely in Home Alone

Home Alone is a melting pot of technology, all playing together.  RabbitMQ is the transport and Linux is the foundation.

The base station (hub for all the house sensors) runs a stripped down Ubuntu 12.04 Server (to fit on a 1GB eMMC flash drive) and hosts RabbitMQ (as temporary storage and transport) along with the sensor  reading code, written in LuaJIT (calling libusb directly -- no C).  The LuaJIT talks to  RabbitMQ via STOMP (it could has just as well used AMQP, but I was itching to test a pure Lua STOMP library I've developed). The server is efficient and cranks along seamlessly.  With RabbitMQ, you can say that each base station is running Erlang!

The collected sensor data is sent via a RabbitMQ shovel to a RabbitMQ in the cloud.  There the sensor data is read by a "logger", which simply dumps the data directly into daily rolling log files.

Any house monitoring app on the cloud server is able to "subscribe" to sensor data, but currently the apps periodically read the daily log (building up a picture of the house in memory), determine the state of the house (is the stove on? are monitoring motion? is anyone home?), generate appropriate events, feed them into RabbitMQ event queues  and terminate.

This batch approach is used to keep the system manageable (performance-wise) since each arriving event doesn't trigger monitoring apps directly.  The monitoring apps are essentially cron'd to run periodically every few minutes. (Each base station is required to send data or a heartbeat every 5 minutes -- this is how we determine if the base station is running properly.)  This approach should scale well (but not fantastically well).

The first house monitors were written in Lua, but I rewrote them in Perl.  Why?  The house monitors are mostly pure logic.  They model the house and I needed a richer language than Lua (quick: how do you sort a table in Lua based on a time stamp and then walk that table in sorted order?).

Perl is old hat, but still has too much built in facility for describing data structures and algorithms to be ignored.  I've could have used Python, but I enjoy coding in Perl more. "Modern Perl" looks nice...

But, it really doesn't matter as the architecture allows using any programming language you want.  The monitors are essentially just modules in a pipeline data flow.  I may be using Haskell for some monitors.  I want to use a language that expresses what a monitor needs to do, but as succinctly as possible.

Telling a Story with Sensors

More than just reporting events, your sensors should tell a story.

Collecting temperature and motion data from around your house can tell you a lot about how you live.

My elderly monitoring system strives to tell you (the care taker) an ongoing story about what your loved one is doing during the day.  From just motion and temperature, the system can tell:

• Did she have breakfast this morning? When?
• Is she getting regular meals (spending time in the kitchen)?
• Did she get up at her usual time in the morning?
• Maybe she is isn't feeling well and is still in bed.
• Is she wandering about the home?
• Did she leave the home?
• Is she taking naps in the living room chair?
• Did she fall asleep last night in the living room chair?
• Is is too cold or too hot in the house?
• Is she constantly changing the thermostat?
• Did she leave the stove on unattended?
• Did she leave the water running in the bathroom (causing a flood)?

This is much more useful than just alert events.  At the end of the day, a care taker should be able to track the well being of the elderly, not just be notified when something is wrong.