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.
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.