[hapless]

To be fare, $0 up front and pay for usage used to be called "time sharing," in which the mainframe you buy time on is located on someone else's premises.

Only large and deep-pocketed users opted to lease a mainframe for their exclusive, local, use.

[ucaetano]

"Time sharing" is different from "pay for usage".

"Time sharing" is "pay X per month and use up to Y hours".

"Pay per usage" is "pay X per hour of usage, no minimum requirements".

In a time-share it can be very hard to break out of the contract.

[0xCMP]

So in a sense "reserved instances" are more like time sharing then.

[ucaetano]

Yes, far more flexible, but in some sense yes.

The key difference between cloud and the traditional mainframe model is the decoupling between usage and the assets.

Time sharing is buying a share of an asset. Reserved instances is pre-commiting to computing utilization.