Lambda Provisioned Vs Reserved Concurrency FAQ & Answers

Best practices (2025): (1) Set reserved = 2x provisioned (buffer for bursts, e.g., Provisioned = 50, Reserved = 100), (2) Monitor ConcurrentExecutions metric to track if hitting reserved limit (need increase) or underutilizing provisioned (wasting cost), (3) Use Application Auto Scaling to automatically adjust provisioned concurrency based on schedule/metrics, (4) Test failover behavior - verify on-demand instances kick in correctly when provisioned exhausted (load testing with >provisioned RPS). Common mistake: setting reserved = provisioned exactly (no headroom for bursts) causes throttling when traffic spike exceeds provisioned. Always reserve 2-3x headroom.

Concurrency types (2025): (1) Account-level concurrency - 1,000 concurrent executions default per region (soft limit, increase to 10K-100K via support ticket), shared across all functions in account/region, (2) Reserved concurrency - dedicated allocation for specific function, guarantees availability but caps max concurrency. Example: Set reserved = 100 → function guaranteed 100 concurrent executions, cannot scale beyond 100 (throttles at 101), reduces account pool by 100 (900 left for other functions), (3) Provisioned concurrency - pre-initialized instances (always warm, zero cold starts), must be ≤ reserved concurrency.

Use reserved WITHOUT provisioned: Protect function from consuming all account concurrency (prevent noisy neighbor problem - runaway function throttling others). Example: Background job processing - reserve 200 concurrency to prevent starving critical API functions, but don't need provisioned (cold starts acceptable for async jobs). Use provisioned WITH reserved: Latency-critical APIs requiring <50ms response - provisioned eliminates cold starts, reserved ensures those warm instances can't be starved by account-level throttling.

No - AWS Lambda provisioned concurrency cannot exceed reserved concurrency (hard limit enforced by AWS). Configuration rules: Without reserved concurrency, cannot set provisioned concurrency (AWS blocks with InvalidParameterValueException). With reserved concurrency, provisioned concurrency ≤ reserved concurrency (enforced). Example: Reserved = 50, attempt Provisioned = 60 → AWS rejects with 'Specified provisioned concurrency is greater than reserved concurrency'. Must set reserved concurrency first, then provisioned must be equal or lower.

If traffic exceeds provisioned concurrency but within reserved limit → Lambda scales using on-demand instances (subject to cold starts). Example: Reserved = 100, Provisioned = 50, Traffic = 75 concurrent requests → 50 requests use warm provisioned instances (0ms cold start), 25 requests trigger on-demand instances (200-500ms cold start for Node.js/Python). Cost implications: Reserved concurrency free (just caps max), Provisioned $0.015/GB-hour (50 instances @ 512MB = $162/month) + execution cost, On-demand only charged during execution ($0.0000167/GB-second).