[jdoconnor]

consider not storing running_balance (or at least expect it to be written to a LOT). Based on the above description that's going to be a hot table with a write/update of 1 per second per row (of active customers).

If you are thinking sql, that running balance could be calculated from a VERY tall and skinny table of time_spent per session (which would be calculated in the application) or an append only table of (billing_account_id, timestamp) where you write a new row every second.

[soco]

Does the running balance have to be sql? That's why we invented event streaming...

[punkpeye]

I should have mentioned: thankfully, there are no long-running sessions in this design. The longest session I could imagine would be 1 hour, but... they will average 1 minute.

Designing such system for long-running sessions would be a lot more complex, esp if we consider that the price might not be constant during the entire duration of the session.