Hello Alpakka

A simple example with Scala, Akka and Alpakka

Table Of Content

• Requirements
• Description
• How the project is built
  • Stack
  • Akka HTTP Server With Producer
  • Consumer
• Running locally
  • Start a kafka cluster using docker-compose
  • Start the consumer
  • Start the akka http server
  • Send POST request to http://localhost:8080/hello
  • Check the logs of the consumer app

Requirements

• docker 20.10.12
• docker-compose 1.25.5
• jdk 11
• scala 2.13.8
• sbt 1.6.2

Description

There are two apps.

• Akka HTTP Server: it starts a http server exposing and endpoint /hello. Sending a GET it will return a hello message. Sending a POST with an empty body, a kafka message will be published in the event.hello topic.
• Consumer: it starts a kafka consumer that subscribes to the event.hello topic

How the project is built

Stack

• scala 2.13.8
• Alpakka 3.0.0

Akka HTTP Server With Producer

Main class for starting the http server is located in com.gaston.hello.akka.http.AkkaHttpServerApp. It doesn't contain any complex logic. It just starts the HTTP Server exposing the /hello endpoint for GET and POST methods. The logic for POST method calls to the producer and this one publish a message in a kafka topic. The Producer has the logic to send the message.

Consumer

Main class for starting the consumer is located in com.gaston.hello.alpakka.kafka.consumer.ConsumerApp. It creates a kafka consumer that subscribes to the event.hello topic, which is where the producer is going to be sending messages. Once the message is received, this one is logged and that's all.

Running locally

Start a kafka cluster using docker-compose

docker-compose up -d

This command will start two containers

• zookeper: port 22181 is exposed
• kafka: port 29092 is exposed

Start the consumer

sbt "runMain com.gaston.hello.alpakka.kafka.consumer.ConsumerApp"

Once the consumer starts it will try to subscribe to event.hello topic

Start the akka http server

sbt "runMain com.gaston.hello.akka.http.AkkaHttpServerApp"

Once the http server is started a http GET or POST request can be sent to http://localhost:8080/hello. Sending a GET will return a hello message. Sending a POST will publish a message in event.hello topic and returns a hello message.

Send POST request to http://localhost:8080/hello

curl -d "" http://localhost:8080/hello

With this command a http POST request is sent to the akka http server. A message is published in event.hello topic and a hello message is returned.

Check the logs of the consumer app

Once the POST request is sent, a log message should be shown like the following one

[INFO ] 2022-03-06 02:22:10,399 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [6f6f930e-dd3a-416b-b506-c3ac0cf8f569], value: [hello at 2022-03-06T05:22:09.872594Z] 
[INFO ] 2022-03-06 02:22:31,257 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [42bce1e8-1da6-4fa3-956d-d5c0669b3a91], value: [hello at 2022-03-06T05:22:31.103983Z] 
[INFO ] 2022-03-06 02:22:31,920 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [7c48bd51-1cee-46df-b5ab-9aeb55b58499], value: [hello at 2022-03-06T05:22:31.831066Z] 
[INFO ] 2022-03-06 02:22:32,317 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-4 - received - key: [489f8b91-2c3b-4880-bd4c-a8dc6eb37413], value: [hello at 2022-03-06T05:22:32.225990Z] 
[INFO ] 2022-03-06 02:22:32,697 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [7e743f01-71cf-4298-8099-e717c042b5e7], value: [hello at 2022-03-06T05:22:32.539574Z] 
[INFO ] 2022-03-06 02:22:32,957 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [15d2ecfa-bbd4-41ee-a7b9-30ad7413a590], value: [hello at 2022-03-06T05:22:32.817235Z] 
[INFO ] 2022-03-06 02:22:33,217 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [54e16bb3-07a7-4935-952c-c61e2030b813], value: [hello at 2022-03-06T05:22:33.059850Z] 
[INFO ] 2022-03-06 02:22:33,357 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [1628b127-fc37-4c22-97bc-29953151deb2], value: [hello at 2022-03-06T05:22:33.273488Z] 
[INFO ] 2022-03-06 02:22:33,617 - com.gaston.hello.alpakka.kafka.consumer.ConsumerApp$ - default-akka.actor.default-dispatcher-5 - received - key: [6342f7d2-9040-4c67-b30e-9940a4e92132], value: [hello at 2022-03-06T05:22:33.520176Z]