Call a registered external service from Kyma

This guide shows how to call a registered external service from Kyma using a simple Function.

Prerequisites

A registered external service
Your service display name exported as an environment variable
Your Application name exported as an environment variable

Your desired Namespace, cluster domain, and the names for your Function and APIRule exported as environment variables

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export NAMESPACE=default
export CLUSTER_DOMAIN=local.kyma.dev
export FUNCTION_NAME=my-function
export APIRULE_NAME=$FUNCTION_NAME-ar

Istio sidecar injection enabled in your Namespace
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```
kubectl label namespace $NAMESPACE istio-injection=enabled
```

CAUTION: On a local Kyma deployment, skip SSL certificate verification when making a curl call, by adding the -k flag to it. Alternatively, add the Kyma certificates to your local certificate storage on your machine using the kyma import certs command.

Steps

Build a path to access your registered service:
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```
export GATEWAY_URL=http://central-application-gateway.kyma-system:8080/$APP_NAME/$SERVICE_DISPLAY_NAME
```
CAUTION: SERVICE_DISPLAY_NAME in the GATEWAY_URL path must be in its normalized form. This means that, for example, if you used test-basic-auth as the service displayName, you're good to go, but if you used "Test Basic Auth", you must replace it with test-basic-auth in the path.

Create a Function that sends a request to the registered service with the additional path of /uuid. A successful response returns a UUID generated by httpbin.org. To create and register the Function in the desired Namespace, run:

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cat <<EOF | kubectl apply -f -
apiVersion: serverless.kyma-project.io/v1alpha2
kind: Function
metadata:
  name: $FUNCTION_NAME
  namespace: $NAMESPACE
  labels:
    app: $APP_NAME
spec:
  runtime: nodejs16
  source:
    inline:
      source: |-
       const request = require('request');
       module.exports = { main: function (event, context) {
        return new Promise((resolve, reject) => {
            const url = process.env.GATEWAY_URL + "/uuid";
            const options = {
                url: url,
            };
            sendReq(url, resolve, reject)
        })
       } }
       function sendReq(url, resolve, reject) {
        request.get(url, { json: true }, (error, response, body) => {
            if(error){
                resolve(error);
            }
            resolve(response.body);
        })
       }
      dependencies: |-
       {
        "name": "example-1",
        "version": "0.0.1",
        "dependencies": {
          "request": "^2.85.0"
        }
       }
  env:
  - name: GATEWAY_URL
    value: $GATEWAY_URL
EOF

To expose the Function outside the cluster, create an APIRule custom resource.

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cat <<EOF | kubectl apply -f -
apiVersion: gateway.kyma-project.io/v1beta1
kind: APIRule
metadata:
  name: $APIRULE_NAME
  namespace: $NAMESPACE
  labels:
    function: $FUNCTION_NAME
spec:
  gateway: kyma-system/kyma-gateway
  host: $APIRULE_NAME.$CLUSTER_DOMAIN
  rules:
  - path: /.*
    accessStrategies:
    - config: {}
      handler: noop
    methods:
    - GET
  service:
    name: $FUNCTION_NAME
    port: 80
EOF

To verify that everything was set up correctly, you can now call the Function through HTTPS:
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```
curl https://$APIRULE_NAME.$CLUSTER_DOMAIN/
```
NOTE: If you get nothing or no healthy upstream as a response, wait for a few moments for the Function to get ready and call it again.
A successful response returns a UUID generated by httpbin.org:
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```
{
  "uuid": "d44cc373-b26e-4a36-9890-6418d131a285"
}
```