lambda_local_runner/docs/lambdas-md/lambda-03-limits.md

Limits — Cheatsheet

Every number worth memorising. The "why it matters" column is the part interviews actually probe.

Per-function compute & storage

Limit	Default	Max	Why it matters
Memory	128 MB	10 240 MB	CPU scales linearly with memory. More memory ≠ just more headroom — at >1769 MB you get a full vCPU; at higher tiers, multiple. Often cheaper to bump memory because duration drops faster than cost rises.
Timeout	3 s	900 s (15 min)	3 s default is too short for almost anything that talks to S3. Set explicitly; don't accept the default. API Gateway caps at 29 s no matter what your function says (see below).
Ephemeral storage (/tmp)	512 MB	10 240 MB	Persists across warm invocations on the same env, vanishes on cold start. Not shared between concurrent envs. Pay per-invocation for >512 MB.
Init phase	10 s hard cap	10 s hard cap	Module-level code (imports, client construction). Heavy ML model loads, custom JIT warmups — measure them or you'll trip this.

Payloads & responses

Limit	Value	Why it matters
Sync invocation request	6 MB	Hard cap on the event body for RequestResponse invocations.
Sync invocation response	6 MB	Truncated silently above this — your handler "succeeds" but the caller gets a 413. lambda_function.py sidesteps this by returning a manifest URL instead of inlining all presigned URLs.
Async invocation event	256 KB	For Event invocations and most event-source-mapped triggers (S3, EventBridge, SNS).
Response streaming	20 MB (soft) / unlimited (with bandwidth cap)	Function URLs and Lambda Streaming response mode break the 6 MB cap by flushing chunks. Not all clients/SDKs support it.
Environment variables	4 KB total	Per function, all keys+values combined. Big config → Parameter Store / Secrets Manager.
Event size (SQS, SNS, EventBridge)	256 KB each	Producer-side limit. Larger payloads → store in S3, send a pointer.

Packaging

Limit	Value	Why it matters
Zip upload (direct)	50 MB	Above this you must upload via S3 first.
Zip unzipped (function + layers)	250 MB	Total of /var/task + all layers extracted. aioboto3+deps is ~50 MB; you have headroom but not infinite.
Container image	10 GB	Per image. Preferred when you'd otherwise blow the 250 MB zip ceiling — e.g. ML deps with native binaries.
Layers	5 per function	Ordering matters: later layers overwrite earlier. Layers count toward the 250 MB unzipped cap.

Concurrency & scaling

Limit	Default	Notes
Account concurrent executions	1 000 / region	Soft quota — request increase via Service Quotas. The single most common throttling cause in production.
Burst concurrency	500–3 000 (region-dependent)	How many fresh environments AWS will spin up immediately at traffic spike. Beyond this, scale-up is +500 envs / min.
Reserved concurrency	0 to account quota	Carves a slice of the account pool for a function. Setting it to 0 effectively disables the function.
Provisioned concurrency	0 by default	Pre-warmed envs. Eliminates cold starts at the cost of paying for idle capacity. Bills as PC-seconds + invocation cost.

Time & rate limits at the edges

Surface	Limit	Why it matters
API Gateway integration timeout	29 s	Caps your effective Lambda timeout when fronted by API GW, regardless of what the Lambda timeout says. Function URLs allow up to 15 min.
Async invocation event age	6 h	If retries don't succeed in this window, the event is dropped (or sent to DLQ / on-failure destination).
Async retry attempts	2 (default)	Total of 3 attempts (initial + 2). Configurable down to 0.
SQS visibility timeout requirement	≥ 6× function timeout	AWS recommendation. Otherwise messages reappear while still being processed.

Memorisation hack. Three numbers cover most interview questions: 15 minutes (timeout), 10 GB (memory and /tmp ceiling), 6 MB (sync payload). Everything else is a footnote until you hit a specific design.