AWS Lambda Function Concurrency and Throttling

Example Answer

Concurrency is the number of requests being handled at the same time. Reserved concurrency sets aside capacity for a function and also caps how much it can consume; it does not initialize warm environments. Provisioned concurrency prepares execution environments in advance and has an additional cost. I would use reserved concurrency to protect a downstream database and provisioned concurrency when a measured latency target justifies it. Neither removes every failure or guarantees that all traffic avoids initialization: bursts can exceed the provisioned allocation, and environment resets still matter. I would monitor throttles and the traffic actually served by provisioned capacity.

Design example

If a dependency can safely sustain 40 simultaneous requests, setting a function cap near that tested limit may protect it. That cap must account for other clients and the connections each invocation uses. Separately choose a provisioned allocation based on the expected interactive load; do not equate it with the total concurrency ceiling.

Test traffic below and above the allocation, and verify what happens at the reserved limit. A synchronous throttle response needs a caller policy; queued work needs age and backlog monitoring.

Practice follow-up

What would you measure before increasing reserved or provisioned concurrency?

Sources

Lambda reserved concurrency; Lambda provisioned concurrency behavior; Lambda lifecycle and resets.