Savings Plans & Reserved

The Commitment Menu

Four levers cut compute cost, each with a different flexibility/ discount trade:

• EC2 Instance Savings Plans — commit to a specific instance family in a region for 1 or 3 years; deepest discount (AWS advertises up to 72%) but tied to the family.
• Compute Savings Plans — commit to a $/hour amount usable by any EC2 instance family/size/OS/region, plus Fargate and Lambda; more flexible, somewhat smaller discount.
• Reserved Instances — the older sibling of Instance SPs: zonal RIs include a capacity reservation, regional RIs give flexibility across sizes in the family.
• Spot — bid on spare capacity at up to ~90% off with interruption anytime (two-minute notice when available); for fault-tolerant, flexible workloads only.

Payment options — all upfront, partial, none — trade cash flow against discount depth.

Matching Workload to Commitment

Walk the decision:

1. Steady baseline, known family (databases, always-on fleets) → EC2 Instance SP or zonal RI — you get discount and, for zonal RIs, capacity certainty.
2. Mixed/unknown compute, multi-account → Compute SP: one commitment covers EC2 across families, Fargate tasks, and Lambda.
3. Interruptible, elastic work (CI, batch, ML training, workers) → Spot, diversified across instance types inside an Auto Scaling group or Spot Fleet so one pool drying up does not halt you.
4. Spiky but latency-sensitive → keep the baseline on SP, burst on-demand.

Verify with Cost Explorer reports: utilization (did you use what you bought?) and coverage (how much of total spend is discounted?).

Pitfalls and Interview Focus

The expensive mistakes: committing to an experimental workload (two-year SP on a service you sunset in six months — sell or exchange before it rots); buying zonal capacity you never consume; mixing Spot into synchronous user paths without checkpointing and graceful fallback to on-demand. Spot interruptions are normal operations — handle the rebalancing signal and make workers idempotent.

Remember discounts apply only to compute, not data transfer, support, or third-party licenses; and Savings Plans apply automatically across the account family, so "which instance does my SP cover" is answered by coverage reports, not spreadsheets.

Interview: state flexibility vs discount as the axis, pick per workload, and mention how you monitor utilization after the purchase.