Build Route53 & DNS Stack

Objective

Implement DNS for blue/green traffic routing. Implementation differs by deployment style — do not mix patterns.

Path Selection

Refer to INFRA_REQUIREMENTS.md § Deployment Style.

EKS Path

Skill: AWS CDK with Python § Route53 Hosted Zone

1. Implement stacks/dns_stack.py

Create hosted zone. Weighted prod/idle records managed by scripts/traffic_switch.py (not CDK) to avoid drift.

2. Create scripts/traffic_switch.py

Actions: switch, rollback, status — swap Route53 weighted record targets.

3. Infra Makefile targets

traffic-switch:  ## Swap prod/idle DNS weights
    python scripts/traffic_switch.py --action switch
traffic-rollback: ## Restore previous weights
    python scripts/traffic_switch.py --action rollback

4. Verify

dig prod.{domain}
dig idle.{domain}
make traffic-switch && dig prod.{domain}  # target changed

Elastic Beanstalk Path

Skill: AWS EB Blue/Green Deployment § Pattern 7: Route53 for EB

1. DNS Mental Model

Public:  https://app.example.com
           ↓ Route53 CNAME (TTL 60s)
         app-prod.eba-xxxxx.elasticbeanstalk.com   ← fixed label name
           ↓ EB swap-environment-cnames
         whichever physical env currently owns that label

Critical: Route53 must point at the EB environment CNAME label ({project}-prod.eba-…), not the ALB ARN directly. ALB-alias breaks CNAME swap — live traffic won't follow promotion.

2. Implement stacks/dns_stack.py

Use an idempotent Lambda custom resource to UPSERT the CNAME (avoids ConflictingResourceExists on re-deploy):

# Route53 CNAME: app.example.com → {project}-prod.eba-xxxxx.elasticbeanstalk.com
# Lambda handler: ListResourceRecordSets + ChangeResourceRecordSets (UPSERT)

HTTPS terminates at the EB-managed ALB (ACM cert on :443 listener). No CloudFront required for basic setup.

3. No traffic_switch.py for EB

Promotion is only aws elasticbeanstalk swap-environment-cnames (see EB skill Pattern 2). Route53 record stays fixed; EB rotates which env owns the {project}-prod label.

4. Verify

dig +short app.example.com
# Must resolve to whichever env currently holds app-prod.eba-… label

make swap   # in app repo
sleep 90    # Route53 TTL + resolver cache
dig +short app.example.com  # same CNAME label, different backend env

Deliverables

• ✅ EKS: hosted zone + traffic_switch.py + make targets
• ✅ EB: CNAME to prod EB label via idempotent Lambda UPSERT
• ✅ EB: documented why ALB-alias Route53 breaks swap
• ✅ CDK tests passing
• ✅ DNS resolving correctly for chosen style