Lambda Provisioned Concurrency Latest FAQ & Answers

Why $LATEST unsupported: Provisioned concurrency pre-initializes execution environments with specific function code snapshot. $LATEST changes continuously (every code deployment updates $LATEST), making pre-warming impossible (warm instances would reference stale code after deployment). Published versions are immutable - version 5 code never changes, Lambda can safely cache initialized instances. This architectural constraint makes $LATEST incompatible with provisioned concurrency's pre-warming mechanism.

Configuration workflow (2025): (1) Publish version: aws lambda publish-version --function-name myFunc creates version 6 from current $LATEST, (2) Create/update alias: aws lambda update-alias --function-name myFunc --name prod --function-version 6, (3) Set provisioned concurrency on alias: aws lambda put-provisioned-concurrency-config --function-name myFunc:prod --provisioned-concurrent-executions 10. Provisioned instances now use version 6 code. (4) Deploy new code to $LATEST: update function code, test against $LATEST (cold starts acceptable for testing), (5) Publish and update: when ready - publish version 7, update prod alias → version 7, provisioned concurrency auto-migrates (blue-green deployment, ~1-2 min switchover).

Weighted alias routing for gradual rollout: prod alias routes 90% traffic to version 6 (provisioned), 10% to version 7 (canary, may cold start). Monitor error rates, shift 100% to version 7 when stable. Command: aws lambda update-alias --function-name myFunc --name prod --routing-config AdditionalVersionWeights={'7'=0.1}. Provisioned concurrency only applies to main version (90%), canary traffic (10%) uses on-demand. Cost during blue-green migration: briefly runs both (10 instances v6 + 10 instances v7) during 1-2 min switchover, charged for 20 instances temporarily (minimal cost).

Best practices (2025): (1) Version all production deployments - never invoke production traffic on $LATEST, always use versioned aliases (provides rollback, audit trail, provisioned concurrency eligibility), (2) Alias naming conventions - prod (production), staging (pre-prod testing), canary (experimental rollouts), (3) Automate versioning in CI/CD - publish version + update alias in deployment pipeline, (4) Monitor alias invocations - CloudWatch metric Invocations filtered by alias tracks production traffic separately from $LATEST testing, (5) Delete old versions - Lambda limits 75GB total code storage per region, prune unused versions. SAM AutoPublishAlias: prod with ProvisionedConcurrencyConfig auto-creates version + alias on deploy, eliminates manual commands.