Lambda Concurrency¶

Concurrency limit: up to 1000 concurrent executions
Can set a reserved concurrency at the function level
Each invocation over the concurrency limit will trigger a throttle
Throttle behavior
- If syncronous invocation - return ThrottleError - 429
- If asynchronous invocation - retry automaticall and then go to DLQ

Concurrency and Asynchronous Invocations¶

If the function doesn't have enough concurrency available to process all events, additional requests are throttled.
For throttling errors (429) and system errors (500-series), Lambda returns the event to the queue and attempts to run the function again for up to 6 hours.
The retry interval increases exponentially from 1 second after the first attempt to a maximum of 5 minutes.

Cold Start
- New instance -> code is loaded and code outside the handler run (init)
- If the init is large (code, dependencies, SDK) this process can take some time
- First request is served by new instances has higher latency than the rest
Provisioned Concurrency
- Concurrency is allocated before the function is invoked (in advance)
- So the cold start never happens and all invocations have low latency
- Application Auto Scaling can manage concurrency (schedule or target utilization)
Note:
- cold starts in VPC have been dramatically reduced in Oct & Nov 2019
- https://aws.amazon.com/blogs/compute/announcing-improved-vpc-networking-for-aws-lambda-functions/

https://docs.aws.amazon.com/lambda/latest/dg/configuration-concurrency.html