Update response body

Learn how to return a customized response body and how replace specific values in the body.

In this guide, you use the following methods to transform a JSON body:

  • Directly access fields in the JSON body and inject them into a custom JSON body.
  • Use the replace_with_random function to replace specific patterns in the JSON body.

Before you begin

  1. Follow the Get started guide to install kgateway.

  2. Follow the Sample app guide to create a gateway proxy with an HTTP listener and deploy the httpbin sample app.

  3. Get the external address of the gateway and save it in an environment variable. 

    export INGRESS_GW_ADDRESS=$(kubectl get svc -n kgateway-system http -o jsonpath="{.status.loadBalancer.ingress[0]['hostname','ip']}")
echo $INGRESS_GW_ADDRESS  
    kubectl port-forward deployment/http -n kgateway-system 8080:8080

Update response body

  1. Send a request to the json endpoint of the httpbin app. The request returns a JSON body that you later transform. 

    curl -vi http://$INGRESS_GW_ADDRESS:8080/json \
 -H "host: www.example.com:8080" 
    curl -vi localhost:8080/json \
-H "host: www.example.com"

    Example output:

    {
  "slideshow": {
    "author": "Yours Truly",
    "date": "date of publication",
   "slides": [
      {
        "title": "Wake up to WonderWidgets!",
        "type": "all"
      },
      {
        "items": [
          "Why <em>WonderWidgets</em> are great",
          "Who <em>buys</em> WonderWidgets"
        ],
        "title": "Overview",
        "type": "all"
      }
    ],
    "title": "Sample Slide Show"
  }
}

  2. Create a TrafficPolicy resource with your transformation rules:

    • A new body is created in the response with the values of the author, title, and slides fields.
    • To extract the values, you use dot notation. Because the transformation sets body.parseAs: AsJson, the response body is parsed into the template context, so you can access the fields directly without defining extractors. Without parseAs: AsJson, the rustformation engine treats the body as a string and the dot-notation access does not resolve.
    kubectl apply -f- <<EOF
apiVersion: gateway.kgateway.dev/v1alpha1
kind: TrafficPolicy
metadata:
  name: transformation
  namespace: httpbin
spec:
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: httpbin
  transformation:
    response:
      body: 
        parseAs: AsJson
        value: '{"author": "{{ slideshow.author }}", "title": "{{ slideshow.title }}", "slides": "{{ slideshow.slides }}}'
EOF

  3. Send a request to the json endpoint of the httpbin app again. Verify that you see the transformed response body. 

    curl -vi http://$INGRESS_GW_ADDRESS:8080/json \
 -H "host: www.example.com:8080" 
    curl -vi localhost:8080/json \
-H "host: www.example.com"

    Example output:

    {"author": "Yours Truly", "title": "Sample Slide Show", "slides":
"[{"title":"Wake up to WonderWidgets!","type":"all"},{"items":
["Why <em>WonderWidgets</em> are great","Who <em>buys</em> WonderWidgets"],
"title":"Overview","type":"all"}]}

  4. Update the TrafficPolicy resource to replace the all pattern with a random string.

    kubectl apply -f- <<EOF
apiVersion: gateway.kgateway.dev/v1alpha1
kind: TrafficPolicy
metadata:
  name: transformation
  namespace: httpbin
spec:
  targetRefs:
  - group: gateway.networking.k8s.io
    kind: HTTPRoute
    name: httpbin
  transformation:
    response:
      body: 
        parseAs: AsJson
        value: '{{ replace_with_random(body(), "all") }}'
EOF

  5. Send another request to the json endpoint of the httpbin app. Verify that every all value is replaced with a random string. 

    curl -vi http://$INGRESS_GW_ADDRESS:8080/json \
 -H "host: www.example.com:8080" 
    curl -vi localhost:8080/json \
-H "host: www.example.com"

    Example output:

    {
  "slideshow": {
    "author": "Yours Truly",
    "date": "date of publication",
    "slides": [
      {
        "title": "Wake up to WonderWidgets!",
        "type": "5pESMzYNtg8W9AG/eVZ13A"
      },
      {
        "items": [
          "Why <em>WonderWidgets</em> are great",
          "Who <em>buys</em> WonderWidgets"
        ],
        "title": "Overview",
        "type": "5pESMzYNtg8W9AG/eVZ13A"
      }
    ],
    "title": "Sample Slide Show"
  }
}

Cleanup

You can remove the resources that you created in this guide. 
kubectl delete TrafficPolicy transformation -n httpbin
Change response status