Difficulty: Commitment
Description
DynamoDB tables in Provisioned Capacity mode with stable, predictable baseline throughput may benefit from Reserved Capacity purchases — offering up to 54% savings (1-year term) or 77% (3-year term) compared to on-demand provisioned rates. Reserved Capacity is a purely financial commitment that reduces the hourly RCU/WCU rate with no infrastructure changes.
Selection Criteria
The table is in Provisioned Capacity mode (not On-Demand).
The table uses the STANDARD table class — Standard-IA tables are not eligible for Reserved Capacity.
The account has at least 30 days of stable, consistent throughput usage to establish a reliable baseline.
⚠️ Warning — Conflict with Standard-IA recommendation
If a table is in Provisioned Capacity mode AND also qualifies for the Change Storage Type to Standard-IA recommendation, these two optimizations are mutually exclusive: Standard-IA tables cannot benefit from Reserved Capacity. Both recommendations must be surfaced simultaneously with their respective projected savings.
Expected Saving
Up to 54% savings on provisioned throughput (1-year term) or up to 77% (3-year term).
Recommended strategy: reserve 70–80% of the minimum daily usage observed over 30 days to avoid over-committing on capacity that may not always be used.
How it's calculated — example (RCU, us-east-1):
Sample usage data:
region | usage_type | avg_daily_units | max_daily_units | on_demand_cost/yr |
us-east-1 | RCU-Hrs | 4,651 | 8,314 | $52,230 |
us-east-1 | WCU-Hrs | 5,580 | 11,041 | $22,776 |
Pricing comparison (us-east-1):
term | RCU hourly | RCU upfront/unit | WCU hourly | WCU upfront/unit |
On-Demand | $0.00013 | — | $0.00065 | — |
1-year Reserved | $0.000025 | $0.30 | $0.000128 | $1.50 |
3-year Reserved | $0.000016 | $0.36 | $0.000081 | $1.80 |
// Recommended reserved capacity = 70-80% of minimum → 4,600 RCU
Monthly on-demand cost = 4,651 × 24 × 30 × $0.00013 = $4,352.54
Monthly reserved hourly = 4,600 × 24 × 30 × $0.000025 = $828.00
Amortized upfront = (4,600 × $0.30) / 12 = $1,150.00
Excess on-demand = 51 × 24 × 30 × $0.00013 = $0.16
Total monthly reserved cost = $828.00 + $1,150.00 + $0.16 = $1,978.16
Monthly savings = $4,352.54 − $1,978.16 = $2,374.38
Annual savings = $2,374.38 × 12 = $28,492.56 (54.6%)
Operational Impact
Downtime: None — Reserved Capacity is a billing-layer change only. No infrastructure, performance, or availability impact.
Breaking change: None — no application code or configuration changes required.
Financial commitment ⚠️: Reserved Capacity is purchased upfront and charged whether used or not. Under-utilization directly results in wasted spend. Conservative commitment (70–80% of minimum usage) is strongly recommended.
Applies only to: Provisioned Capacity mode + STANDARD table class. On-Demand tables and Standard-IA tables are ineligible.
Recovery: Reserved Capacity cannot be cancelled after purchase. It can however be modified (resized) after the initial purchase within AWS console limits.