AWS ALB Transform Actions for URL Rewrites

AWS ALB now supports URL rewrites using the transform action - no need to rely on redirects or backend rewrites anymore! This is particularly useful for path-based routing scenarios where you want to modify the request before it reaches your backend.

What can you do?

Transform actions let you modify request headers and paths using pattern matching and variable substitution:

• Rewrite paths (e.g., /old/path → /new/path)
• Add/modify/remove headers
• Append query strings
• Use pattern matching with wildcards and capture groups

Example: AWS Load Balancer Controller

Here’s how to use transforms with the AWS Load Balancer Controller for Kubernetes:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: api-rewrite
  annotations:
    alb.ingress.kubernetes.io/actions.rewrite-api: |
      {
        "Type": "forward",
        "ForwardConfig": {
          "TargetGroups": [{"ServiceName": "api-service", "ServicePort": "80"}]
        },
        "Transform": {
          "Path": {
            "Pattern": "/v1/*",
            "Replacement": "/api/v2/$1"
          }
        }
      }
spec:
  ingressClassName: alb
  rules:
  - http:
      paths:
      - path: /v1
        pathType: Prefix
        backend:
          service:
            name: rewrite-api
            port:
              name: use-annotation

In this example:

• Requests to /v1/users get rewritten to /api/v2/users before reaching the backend
• The $1 captures everything after /v1/ and substitutes it into the replacement pattern

Why is this useful?

• API versioning: Handle multiple API versions at the ALB layer
• Legacy migrations: Rewrite old paths to new endpoints without changing client code
• Microservices routing: Route to services with different path structures
• Cleaner URLs: Present user-friendly URLs while using different internal paths

Learn more

• AWS ALB Transform Actions docs
• AWS Load Balancer Controller annotations