Behavior & Limits

Guarantees

Stateless Execution. Each function invocation runs in an isolated environment. Do not rely on in-memory state persisting between invocations — use KV or Storage for persistent state.

At-Least-Once Invocation (Triggers). Queue and storage triggers deliver events at least once. Your handler must be idempotent — the same event may be delivered more than once.

Automatic Scaling. Functions scale up with incoming requests and scale to zero when idle. You do not configure concurrency or instance counts.

Binding Access. Functions can only access resources that are explicitly link()ed in the stack definition. This is enforced at the infrastructure level via IAM/RBAC.

Limits

Limit	Value	Notes
Max request body	Platform-dependent	Lambda: 6 MB sync, 256 KB async. Cloud Run: 32 MB. Container Apps: varies.
Max response body	Platform-dependent	Lambda: 6 MB. Cloud Run: 32 MB.
Max execution time	Platform-dependent	Lambda: 15 min. Cloud Run: 60 min. Container Apps: varies.
Queue triggers per function	1	A function can be triggered by at most one queue.
Concurrent invocations	Platform-dependent	Lambda: 1,000 default (requestable). Cloud Run: per-instance concurrency configurable.

Platform Details

AWS (Lambda)

Runtime: Container image (OCI) on ARM64 (Graviton) for better price-performance.
Cold starts: typically 1-3 seconds for the first invocation after idle.
Payload limits: 6 MB synchronous, 256 KB asynchronous invocation.
Max execution time: 15 minutes.
Function-to-function invocation uses the Lambda Invoke API directly (not HTTP).
Queue triggers: SQS event source mapping. One message per invocation. Visibility timeout is auto-calculated: max(30s, min(12h, function_timeout × 6)).
If the function's ECR image is in a different region than the function, Alien automatically handles cross-region image rewriting.

GCP (Cloud Run)

Runtime: Container image (OCI).
Cold starts: typically 1-2 seconds.
Payload limits: 32 MB request/response.
Max execution time: 60 minutes.
Function-to-function invocation uses direct HTTP calls to the private service URL.
Queue triggers: Pub/Sub push subscription.
Supports per-instance concurrency (multiple requests per container).

Azure (Container Apps)

Runtime: Container image (OCI).
Function-to-function invocation uses direct HTTP calls to the private Container App URL.
Queue triggers: Service Bus integration via KEDA.

Local

Runs as a native process (spawned from extracted OCI image).
Dynamic port assignment via --port flag.
Automatic restart on crash via background monitor.
Triggers supported via LocalTriggerService.
Full environment variable injection and OTLP telemetry configuration.

Trigger Support Matrix

Trigger Type	AWS	GCP	Azure
Queue → Function	SQS event source	Pub/Sub push	Service Bus + KEDA
Storage → Function	S3 notifications	GCS notifications	Dapr blob storage binding
Cron → Function	EventBridge	Cloud Scheduler	Dapr cron binding

HTTP semantics for invocation. Function-to-function calls use HTTP request/response semantics (method, path, headers, body) even when the underlying transport isn't HTTP (e.g., Lambda Invoke API). This keeps the API portable and familiar.

One queue source per function. A function can consume from at most one queue, but can have multiple triggers of different types (e.g. a queue trigger + a cron trigger). If you need to consume from multiple queues, create multiple functions.

Concurrency is optional. By default, Alien lets the cloud provider manage scaling. You can set .concurrencyLimit() to cap concurrent executions — this maps to reserved concurrency on Lambda, max instances on Cloud Run, and max replicas on Container Apps.