engineering-handbook/aws-infrastructure.md

AWS Infrastructure

IaC Strategy

• SAM is the default for new serverless stacks (Lambda + API Gateway + DynamoDB)
• CDK only for complex infrastructure (ECS, VPCs, multi-service compositions)
• Every deployed resource should be managed by CloudFormation
• No manually-created Lambdas, roles, or other resources outside of IaC

Lambda Defaults

These apply to every Lambda in every project. Verify, don't assume.

Setting	Value
Runtime	Python 3.12 or Node 24.x
Architecture	arm64
Log retention	60 days (explicit in IaC template)
Naming	kebab-case, matching the stack name prefix

Never rely on the CloudWatch default for log retention. Always set RetentionInDays explicitly in the template.

CDK Version Policy

Pin aws-cdk-lib to an exact version (no ^, ~, or >=) and let Dependabot keep it current. There is no static "blessed version" — the org standard is the latest release that passes the gates below. Do not add blanket dependabot.yml ignore entries for aws-cdk-lib; that is how pins rot into carrying known vulnerabilities.

Why exact + automated: the exact pin plus the lockfile gives reproducible builds; weekly Dependabot version updates keep the pin moving; CI (npm ci + cdk synth) and the dependency-review check reject a bad release at the PR. A release with broken bundled-dep metadata (e.g. 2.254.0) fails npm ci on its own bump PR; a release bundling a vulnerable transitive dep fails dependency review. Either way, a bad release never merges — the gates do the vetting, not a frozen number in this document.

aws-cdk-lib bundles transitive dependencies (inBundle: true) that npm overrides cannot patch. When a bundled dep has a vulnerability, the only fix is advancing to a release that bundles the patched version — treat the alert as a prompt to merge the next Dependabot bump, never as something to dismiss indefinitely.

If a specific release is known-bad, ignore that version only (ignore: - dependency-name: aws-cdk-lib, versions: ["2.254.0"]) with a comment explaining why, and remove the entry once a fixed release ships.

When upgrading, verify on a branch first:

1. Update package.json to the new version
2. Run rm -rf node_modules package-lock.json && npm install
3. Run npm ci — if it fails, the version is not safe
4. Run npx cdk synth — if it fails, the version is not safe

CloudFormation Outputs

Every stack should export:

• Function ARNs
• Any externally-consumable URLs (API Gateway endpoints, etc.)

S3

• Every non-CloudFormation bucket must have Purpose and ManagedBy tags
• Define lifecycle policies in the IaC template
• Use Glacier Deep Archive for archival data

README

Every repo must have a README that accurately describes:

• Project architecture
• Lambdas and services
• Data flow
• Configuration requirements

Update the README in the same commit where functionality changes. If a README is missing or outdated when you start working on a project, fix it as part of the current work.