Choosing and bumping a Lambda runtime¶

Read this before setting or changing a runtime in any example.

The rule¶

The runtime is never hardcoded. It is one value, declared in examples/<name>/example.yaml under runtime.identifier, and just lint fails if the Terraform variable default or the CloudFormation parameter default disagrees with it. A bump is a one-line manifest edit plus the same value in both IaC paths, and the check proves you did all three.

The default is always GA. Preview runtimes are opt-in per deployment, gated behind allow_preview_runtime (Terraform) and AllowPreviewRuntime (CloudFormation).

Why unplanned runtime work happens, and how to not have it¶

Lambda deprecates a managed language runtime when the language version reaches end of community support. That is the clock that produces the surprise migration. Three things follow:

1. Pick the longest-lived GA runtime, not the newest. The newest is often in preview, and among GA options the newest is not always the longest-lived.
2. Prefer a compiled language on provided.al2023 where the work suits it. An OS-only runtime deprecates on the Amazon Linux clock, and a compiled binary has no language runtime to deprecate at all: moving to the next provided.alXXXX is a rebuild with no source change.
3. Do not copy a deprecation date into code or docs. AWS revises them. The table below exists so a reader understands the shape of the problem; the live table is the authority.

The live table is the authority¶

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

Check it before setting a runtime. What follows is the state that shaped this repo's defaults, not a fact to trust indefinitely.

The same list, with GA/preview classification, lives in common/cloudformation/conformance.yaml, which is what the lint enforces. Edit that one file when a runtime is added or deprecated; do not restate it elsewhere.

Public preview runtimes¶

AWS ships some runtimes in public preview before GA. They are available in every commercial, GovCloud and China region, billed at standard rates, and patched on the same cadence as GA runtimes. They are not covered by the Lambda SLA or technical support, and AWS advises against production use. A preview runtime also has no projected deprecation date, because it has no GA date yet.

So: never a default, always opt-in, and always with the deployer saying so in their own configuration where a reviewer will see it.

# Terraform
runtime               = "python3.15"
allow_preview_runtime = true

# CloudFormation
--parameter-overrides LambdaRuntime=python3.15 AllowPreviewRuntime=true

Without the gate, both IaC paths fail before creating anything, with the reason. In Terraform that is a lifecycle.precondition on the function; in CloudFormation it is a WaitCondition that cannot succeed, because CloudFormation has no assertion primitive.

Go and Rust: there is no managed runtime, and that is fine¶

go1.x was deprecated in January 2024. It is easy to read the current runtimes table, see no Go entry, and conclude Lambda dropped Go. It did not.

Go and Rust run on provided.al2023, the OS-only runtime, via the runtime interface (aws-lambda-go, or lambda_runtime for Rust). The handler binary is named bootstrap. See common/go/README.md.

When a deprecation notice arrives¶

1. Check the live table for the longest-lived GA runtime in the same language.
2. Change runtime.identifier in example.yaml, and the matching default in the example's terraform/variables.tf and cloudformation/template.yaml.
3. Update common/cloudformation/conformance.yaml: move the deprecated identifier out of runtimes.ga, and add the new one.
4. just check. The conformance check fails on any of the three places you missed, and on any template still offering the removed runtime.
5. just package <name> and release. A runtime change needs a redeploy, not just a configuration change, because the deployment package is built against a specific Python or Node version.