[playing_colours]

I remember using Akka in the domain of IoT. A persistent actor represented a sensor: state and history of readings. There was a great feature in Akka Persistence: if an actor went idle for a while - no new sensor data, it would be offloaded from memory to the storage (Cassandra). As soon as the sensor started to send new signals again or the sensor state was queried by a user the actor got loaded back to memory.

[vinay_ys]

How much sensor data is kept in memory in the actor object? What's the cost tradeoff between memory vs an SSD? I wonder if SSD based solution would still be cheaper and more scalable than live memory objects based solution.

[playing_colours]

What sensor data and how much depend on the access needs: you can keep N latest records or some rolling aggregated data, if they are frequently accessed by some rule engine. It's certainly not feasible to hold big chunks of sensor data in memory.