add input bindings how to

howto/trigger-app-with-input-binding/Readme.md

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+# Create an event-driven app using input bindings
+
+Using bindings, your code can be triggered with incoming events from different resources which can be anything: a queue, messaging pipeline, cloud-service, filesystem etc.
+
+This is ideal for event-driven processing, data pipeplines or just generally reacting to events and doing further processing.
+
+Dapr bindings allow you to:
+
+* Receive events without including specific SDKs or libraries
+* Replace bindings without changing your code
+* Focus on business logic and not the event resource implementation
+
+For more info on bindings, read [this](../concepts/bindings/bindings.md) link.<br>
+For a complete sample showing bindings, visit this [link](<PLACEHOLDER>).
+
+## 1. Create a binding
+
+An input binding represents an event resource that Dapr uses to read events from and push to your application.
+
+For the purpose of this HowTo, we'll use a Kafka binding. You can find a list of the different binding specs [here](../concepts/bindings/specs).
+
+Create the following YAML file, named binding.yaml, and save this to the /components sub-folder in your application directory:
+
+*Note: When running in Kubernetes, apply this file to your cluster using `kubectl apply -f binding.yaml`*
+
+```
+apiVersion: actions.io/v1alpha1
+kind: Component
+metadata:
+  name: myEvent
+spec:
+  type: bindings.kafka
+  metadata:
+  - name: topics
+    value: topic1
+  - name: brokers
+    value: localhost:9092
+  - name: consumerGroup
+    value: group1
+```
+
+Here, you create a new binding component with the name of `myEvent`.<br>
+Inside the `metadata` section, configure the Kafka related properties such as the topics to listen on, the brokers and more.
+
+## 2. Listen for incoming events
+
+Now configure your applicaiton to receive incoming events. If using HTTP, you need to listen on a `POST` endpoint with the name of the binding as specifiied in `metadata.name` in the file.  In this example, this is `myEvent`.
+
+*The following example shows how you would listen for the event in Node.js, but this is applicable to any programming language*
+
+```
+const express = require('express')
+const bodyParser = require('body-parser')
+const app = express()
+app.use(bodyParser.json())
+
+const port = 3000
+
+app.post('/myEvent', (req, res) => {
+    console.log(req.body)
+    res.status(200).send()
+})
+
+app.listen(port, () => console.log(`Kafka consumer app listening on port ${port}!`))
+```
+
+#### Acknowleding an event
+
+In order to tell Dapr that you successfully processed an event in your application, return a `200 OK` response from your HTTP handler.
+
+```
+res.status(200).send()
+```
+#### Rejecting an event
+
+In order to tell Dapr that the event wasn't processed correctly in your application and schedule it for redelivery, return any response different from `200 OK`. For example, a `500 Error`.
+
+```
+res.status(500).send()
+```
+
+### Event delivery Guarantees
+Event delivery guarantees are controlled by the binding implementation. Depending on the binding implementation, the event delivery can be exactly once or at least once.