Separate five questions
Instance type answers “what machine.” A purchasing option answers “how to pay and whether it can be interrupted.” Capacity Reservation answers “will matching capacity exist in this AZ.” Tenancy answers “is the hardware isolated.”
| Option | Main trade-off | Interruptible | Capacity guarantee | Best fit |
|---|---|---|---|---|
| On-Demand | No long commitment | No | None | Short, unpredictable, non-interruptible |
| Standard / Convertible RI | 1- or 3-year commitment for discount | No | Regional: no; Zonal: yes | Stable EC2 usage |
| Savings Plans | Hourly spend commitment for flexible discount | No | None | Long-term compute usage |
| Spot | Low price for reclamation risk | Yes | None | Retryable, stateless, batch |
| Dedicated Instance | Physical isolation from other customers | No | No host-level control | Compliance isolation |
| Dedicated Host | An entire host with socket/core visibility | No | Host capacity | BYOL and host control |
| Capacity Reservation | Reserve matching capacity in an AZ | No | Yes | Must be able to launch |
Exam boundaries
- Savings Plans and Regional RIs mainly address price, not AZ capacity.
- A capacity reservation adds no long-term discount by itself, and unused capacity is normally billed.
- A matching Zonal RI combines billing benefit with an AZ capacity benefit.
- Spot interruption notices are best effort; continuously checkpoint external state.
- A Spot capacity pool is defined by attributes such as instance type, AZ, and platform; diversification reduces single-pool risk.
- New Spot workloads usually prefer price-capacity-optimized over lowest-price-only allocation.
Decision order
Ask whether interruption is acceptable, then evaluate duration and stability, and finally decide whether hardware isolation or an AZ capacity guarantee is required.
Remember
Discount, capacity, and isolation are independent axes. Savings Plans, RIs, and Capacity Reservations are not the same mechanism.