Chapter 9: Commitment Discounts, Savings Plans & Reserved Instances
Compute Savings Plans vs EC2 Savings Plans vs RIs, the 75-80% coverage curve, and upfront cash optimization.
#1. Executive Summary & Strategy
Running 100% of cloud compute on pure On-Demand rates is the most expensive way to operate software. AWS, Azure, and Google Cloud offer substantial rate discounts (25% to 72%) in exchange for a 1-year or 3-year hourly spend commitment.
However, over-committing to rigid Reserved Instances (RIs) can create architectural handcuffs, locking engineering teams into obsolete instance families (e.g., sticking with x86 m5 instances when Graviton c7g offers 40% superior price-performance).
#2. Savings Plans vs Reserved Instances Taxonomy
| Commitment Type | Flexibility | Typical Discount | Best Use Case |
|---|---|---|---|
| Compute Savings Plan | Region, instance family, OS, and compute type (EC2, Fargate, Lambda) can change freely. | Up to 66% (3-Yr) | Standard baseline for fast-evolving microservice environments. |
| EC2 Instance Savings Plan | Locked to specific instance family within a single region (e.g. c7g in us-east-1). | Up to 72% (3-Yr) | High-throughput, predictable workloads (Kafka brokers, databases). |
| Convertible Reserved Instances | Can exchange instance families via console manual action. | Up to 54% (3-Yr) | Legacy enterprise contracts. |
| Standard Reserved Instances | Zero flexibility. Locked to exact AZ and instance type. | Up to 72% | Monolithic legacy databases with zero planned migration. |
#3. The 75-80% Commitment Coverage Sweet Spot
Never attempt 100% commitment coverage. Workloads fluctuate with seasonal traffic, marketing campaigns, and architectural changes.
Spend ($)
^
│ Peak Traffic (Covered by Spot & On-Demand)
│ /\
│ / \ /\
│ ───────── ────── ──────── <-- 80% Commitment Threshold (Compute Savings Plans)
│ /////////////////////////////
│ ///// BASELINE USAGE //////// <-- Guaranteed 100% Utilization of Commitment
│ /////////////////////////////
└───────────────────────────────> Time
By covering only the stable bottom 75-80% of baseline compute, you ensure that every dollar committed is fully consumed without unutilized commitment breakage.
#4. No-Upfront vs All-Upfront Financial Analysis
- No-Upfront: Preserves working capital; offers ~85% of the total maximum discount. Recommended for startups and growth-stage companies.
- Partial-Upfront: Balance between capital outlay and discount depth.
- All-Upfront: Maximizes cash discount, but carries highest opportunity cost of capital.