Set asynchronous communication between Functions

This tutorial demonstrates how to connect two Functions asynchronously. It is based on the in-cluster Eventing example.

The example provides a very simple scenario of asynchronous communication between two Functions. The first Function accepts the incoming traffic via HTTP, sanitizes the payload, and publishes the content as an in-cluster event using Kyma Eventing. The second Function is a message receiver. It subscribes to the given event type and stores the payload.

This tutorial shows only one possible use case. There are many more use cases on how to orchestrate your application logic into specialized Functions and benefit from decoupled, re-usable components and event-driven architecture.

Prerequisites

Kyma CLI
Kyma installed locally or on a cluster
Istio sidecar injection enabled in the Namespace in which you want to deploy the Functions

Steps

Export the KUBECONFIG variable:
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```
export KUBECONFIG={KUBECONFIG_PATH}
```
Create the emitter and receiver folders in your project.

Create the emitter Function

Go to the emitter folder and run Kyma CLI init command to initialize the scaffold for your first Function:
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```
kyma init function
```
The init command creates these files in your workspace folder:

config.yaml with the Function's configuration
NOTE: See the detailed description of all fields available in the config.yaml file.
handler.js with the Function's code and the simple "Hello Serverless" logic
package.json with the Function's dependencies

In the config.yaml file, configure an APIRule to expose your Function to the incoming traffic over HTTP. Provide the subdomain name in the host property:

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apiRules:
  - name: incoming-http-trigger
    service:
      host: incoming
    rules:
      - methods:
          - GET
        accessStrategies:
          - handler: allow

Provide your Function logic in the handler.js file:
NOTE: In this example, there's no sanitization logic. The sanitize Function is just a placeholder.
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```
module.exports = {
   main: async function (event, context) {
      let sanitisedData = sanitise(event.data)
      const eventType = "sap.kyma.custom.acme.payload.sanitised.v1";
      const eventSource = "kyma";
      
      return await event.emitCloudEvent(eventType, eventSource, sanitisedData)
            .then(resp => {
               return "Event sent";
            }).catch(err=> {
               console.error(err)
               return err;
            });
   }
}
let sanitise = (data)=>{
   console.log(`sanitising data...`)
   console.log(data)
   return data
}
```
NOTE: The sap.kyma.custom.acme.payload.sanitised.v1 is a sample event type declared by the emitter Function when publishing events. You can choose a different one that better suits your use case. Keep in mind the constraints described on the Event names page. The receiver subscribes to the event type to consume the events.
NOTE: The event object provides convenience functions to build and publish events. To send the event, build the Cloud Event. To learn more, read Function's specification. In addition, your eventOut.source key must point to “kyma” to use Kyma in-cluster Eventing. NOTE: There is a require('axios') line even though the Function code is not using it directly. This is needed for the auto-instrumentation to properly handle the outgoing requests sent using the publishCloudEvent method (which uses axios library under the hood). Without the axios import the Function still works, but the published events are not reflected in Jaeger.
Apply your emitter Function:
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```
kyma apply function
```
Your Function is now built and deployed in Kyma runtime. Kyma exposes it through the APIRule. The incoming payloads are processed by your emitter Function. It then sends the sanitized content to the workload that subscribes to the selected event type. In our case, it's the receiver Function.

Test the first Function. Send the payload and see if your HTTP traffic is accepted:

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export KYMA_DOMAIN={KYMA_DOMAIN_VARIABLE}
curl -X POST https://incoming.${KYMA_DOMAIN} -H 'Content-Type: application/json' -d '{"foo":"bar"}'

Create the receiver Function

Go to your receiver folder and run Kyma CLI init command to initialize the scaffold for your second Function:
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```
kyma init function
```
The init command creates the same files as in the emitter folder.
In the config.yaml file, configure event types your Function will subscribe to:

v1alpha1
v1alpha2

Apply your receiver Function:
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```
kyma apply function
```
The Function is configured, built, and deployed in Kyma runtime. The Subscription becomes active and all events with the selected type are processed by the Function.

Test the whole setup

Send a payload to the first Function. For example, use the POST request mentioned above. As the Functions are joined by the in-cluster Eventing, the payload is processed in sequence by both of your Functions. In the Function's logs, you can see that both sanitization logic (using the first Function) and the storing logic (using the second Function) are executed.