AWS Lambda – Concurrency & Throttling

Concurrency

• Default limit: 1,000 concurrent executions per Region (can be increased via support request).

• Reserved concurrency can be set per function to guarantee capacity and cap usage.

Throttling Behavior

• If concurrency limit is reached:
• Synchronous invocation → returns ThrottleError (HTTP 429).
• Asynchronous invocation → Lambda retries automatically; on repeated failure, events can be sent to a Dead Letter Queue (DLQ) or on-failure destination.

Why Reserved Concurrency Matters

• Without it, high traffic in one function can starve others of capacity.

• Reserved concurrency protects critical functions from throttling during spikes.

Asynchronous Invocations – Retry Logic

• If throttled (429) or failing with 5xx errors:
• Event is retried for up to 6 hours with exponential backoff (starting at 1s, up to 5 min).
• Common trigger: S3 event floods, causing multiple parallel Lambda invocations.