Skip to main content

Internal ALBs

services.alb gives an environment an emulated ALB with a working data plane — created through the real elbv2 API (target groups, listeners, describe-load-balancers), with live traffic routed to your app:

services:
  alb:
    - name: hello-alb
      targetApp: hello     # must be one of apps:

What you get​

  • Control plane: aws elbv2 describe-load-balancers, describe-target-health, listener rules with path-pattern / host-header / http-header / query-string conditions and forward / redirect / fixed-response actions.
  • Data plane: requests to http://aws:4566/_alb/<name>/ (or with Host header <name>.alb.localhost) proxy through the ALB to the target app with ALB-style X-Forwarded-For/Proto/Port and a per-request X-Amzn-Trace-Id, balanced across targets. Target HTTP errors pass through; connection failures surface as 502, like a real ALB.
  • A browser URL: on clusters with Traefik (k3s/k3d), mayfly exposes each ALB at http://<name>.<namespace>.localtest.me/ (via an Ingress plus a Host-rewrite middleware, created automatically) and records it in the secret as ALB_PUBLIC_URL.

The alb-<name> secret carries ALB_URL (in-cluster), ALB_DNS_NAME, ALB_HOST, and ALB_TARGET_APP.

Requirements​

The stock MiniStack image routes ALB traffic to Lambda targets only — mayfly's patched emulator image adds HTTP proxying for instance/ip targets (a one-file overlay, emulator/patches/alb.py, submitted upstream). mayfly up errors clearly if a spec declares alb: while on the stock emulator image.

Testing recipes​

# balancing (fresh connection each request; pod name comes back in JSON)
for i in $(seq 8); do
  curl -s -H 'Connection: close' http://hello-alb.<ns>.localtest.me/json | jq -r .pod
done | sort | uniq -c

# listener rules: add a fixed-response and watch routing change
aws elbv2 create-rule --listener-arn "$L_ARN" --priority 10 \
  --conditions Field=path-pattern,Values='/maintenance*' \
  --actions Type=fixed-response,FixedResponseConfig='{StatusCode=503,MessageBody=down}'

For real AWS ALBs later, apps take ingress: {className: alb, annotations: {...}, host: ...} — mayfly emits a standard networking.k8s.io/v1 Ingress the AWS Load Balancer Controller consumes as-is (host: "*" matches any host, useful against the raw ALB DNS name before Route53 exists).