Have you ever tried to run any message broker on Kubernetes? KubeMQ is relatively new solution and is not as popular as competitive tools like RabbitMQ, Kafka or ActiveMQ. However, it has one big advantage over them – it is Kubernetes native message broker, which may be deployed there using a single command without preparing any additional templates or manifests. This convinced me to take a closer look at KubeMQ. Continue reading “Kubernetes Messaging with Java and KubeMQ”
Tag: messaging
Kafka In Microservices With Micronaut
Today we are going to build some microservices communicating with each other asynchronously through Apache Kafka topics. We use Micronaut Framework, which provides dedicated library for integration with Kafka. Let’s take a brief look at the architecture of our sample system. We have 4 microservices: order-service, trip-service, driver-service and passenger-service. The implementation of these applications is very simple. All of them have in-memory storage and connect to the same Kafka instance.
Continue reading “Kafka In Microservices With Micronaut” →
Building and testing message-driven microservices using Spring Cloud Stream
Spring Boot and Spring Cloud give you a great opportunity to build microservices fast using different styles of communication. You can create synchronous REST microservices based on Spring Cloud Netflix libraries as shown in one of my previous articles Quick Guide to Microservices with Spring Boot 2.0, Eureka and Spring Cloud. You can create asynchronous, reactive microservices deployed on Netty with Spring WebFlux project and combine it succesfully with some Spring Cloud libraries as shown in my article Reactive Microservices with Spring WebFlux and Spring Cloud. And finally, you may implement message-driven microservices based on publish/subscribe model using Spring Cloud Stream and message broker like Apache Kafka or RabbitMQ. The last of listed approaches to building microservices is the main subject of this article. I’m going to show you how to effectively build, scale, run and test messaging microservices basing on RabbitMQ broker.
Architecture
For the purpose of demonstrating Spring Cloud Stream features we will design a sample system which uses publish/subscribe model for inter-service communication. We have three microservices: order-service
, product-service
and account-service
. Application order-service
exposes HTTP endpoint that is responsible for processing orders sent to our system. All the incoming orders are processed asynchronously – order-service
prepare and send message to RabbitMQ exchange and then respond to the calling client that the request has been accepted for processing. Applications account-service
and product-service
are listening for the order messages incoming to the exchange. Microservice account-service
is responsible for checking if there are sufficient funds on customer’s account for order realization and then withdrawing cash from this account. Microservice product-service
checks if there is sufficient amount of products in the store, and changes the number of available products after processing order. Both account-service
and product-service
send asynchronous response through RabbitMQ exchange (this time it is one-to-one communication using direct exchange) with a status of operation. Microservice order-service
after receiving response messages sets the appropriate status of the order and exposes it through REST endpoint GET /order/{id}
to the external client.
If you feel that the description of our sample system is a little incomprehensible, here’s the diagram with architecture for clarification.
Enabling Spring Cloud Stream
The recommended way to include Spring Cloud Stream in the project is with a dependency management system. Spring Cloud Stream has an independent release trains management in relation to the whole Spring Cloud framework. However, if we have declared spring-cloud-dependencies
in the Elmhurst.RELEASE
version inside the dependencyManagement
section, we wouldn’t have to declare anything else in pom.xml
. If you prefer to use only the Spring Cloud Stream project, you should define the following section.
<dependencyManagement> <dependencies> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-stream-dependencies</artifactId> <version>Elmhurst.RELEASE</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement>
The next step is to add spring-cloud-stream
artifact to the project dependencies. I also recommend you include at least the spring-cloud-sleuth
library to provide sending messaging with the same traceId
as the source request incoming to order-service
.
<dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-stream</artifactId> </dependency> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-sleuth</artifactId> </dependency>
Spring Cloud Stream programming model
To enable connectivity to a message broker for your application, annotate the main class with @EnableBinding
. The @EnableBinding
annotation takes one or more interfaces as parameters. You may choose between three interfaces provided by Spring Cloud Stream:
- Sink: This is used for marking a service that receives messages from the inbound channel.
- Source: This is used for sending messages to the outbound channel.
- Processor: This can be used in case you need both an inbound channel and an outbound channel, as it extends the Source and Sink interfaces. Because
order-service
sends messages, as well as receives them, its main class has been annotated with@EnableBinding(Processor.class)
.
Here’s the main class of order-service that enables Spring Cloud Stream binding.
@SpringBootApplication @EnableBinding(Processor.class) public class OrderApplication { public static void main(String[] args) { new SpringApplicationBuilder(OrderApplication.class).web(true).run(args); } }
Adding message broker
In Spring Cloud Stream nomenclature the implementation responsible for integration with specific message broker is called binder. By default, Spring Cloud Stream provides binder implementations for Kafka and RabbitMQ. It is able to automatically detect and use a binder found on the classpath. Any middleware-specific settings can be overridden through external configuration properties in the form supported by Spring Boot, such as application arguments, environment variables, or just the application.yml
file. To include support for RabbitMQ, which used it this article as a message broker, you should add the following dependency to the project.
<dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-stream-rabbit</artifactId> </dependency>
Now, our applications need to connected with one, shared instance of RabbitMQ broker. That’s why I run Docker image with RabbitMQ exposed outside on default 5672 port. It also launches web dashboard available under address http://192.168.99.100:15672.
$ docker run -d --name rabbit -p 15672:15672 -p 5672:5672 rabbitmq:management
We need to override default address of RabbitMQ for every Spring Boot application by settings property spring.rabbitmq.host
to Docker machine IP 192.168.99.100.
spring: rabbitmq: host: 192.168.99.100 port: 5672
Implementing message-driven microservices
Spring Cloud Stream is built on top of Spring Integration project. Spring Integration extends the Spring programming model to support the well-known Enterprise Integration Patterns (EIP). EIP defines a number of components that are typically used for orchestration in distributed systems. You have probably heard about patterns such as message channels, routers, aggregators, or endpoints. Let’s proceed to the implementation.
We begin from order-service
, that is responsible for accepting orders, publishing them on shared topic and then collecting asynchronous responses from downstream services. Here’s the @Service, which builds message and publishes it to the remote topic using Source
bean.
@Service public class OrderSender { @Autowired private Source source; public boolean send(Order order) { return this.source.output().send(MessageBuilder.withPayload(order).build()); } }
That @Service
is called by the controller, which exposes the HTTP endpoints for submitting new orders and getting order with status by id
.
@RestController public class OrderController { private static final Logger LOGGER = LoggerFactory.getLogger(OrderController.class); private ObjectMapper mapper = new ObjectMapper(); @Autowired OrderRepository repository; @Autowired OrderSender sender; @PostMapping public Order process(@RequestBody Order order) throws JsonProcessingException { Order o = repository.add(order); LOGGER.info("Order saved: {}", mapper.writeValueAsString(order)); boolean isSent = sender.send(o); LOGGER.info("Order sent: {}", mapper.writeValueAsString(Collections.singletonMap("isSent", isSent))); return o; } @GetMapping("/{id}") public Order findById(@PathVariable("id") Long id) { return repository.findById(id); } }
Now, let’s take a closer look on consumer side. The message sent by OrderSender
bean from order-service
is received by account-service
and product-service
. To receive the message from topic exchange, we just have to annotate the method that takes the Order object as a parameter with @StreamListener
. We also have to define target channel for listener – in that case it is Processor.INPUT
.
@SpringBootApplication @EnableBinding(Processor.class) public class OrderApplication { private static final Logger LOGGER = LoggerFactory.getLogger(OrderApplication.class); @Autowired OrderService service; public static void main(String[] args) { new SpringApplicationBuilder(OrderApplication.class).web(true).run(args); } @StreamListener(Processor.INPUT) public void receiveOrder(Order order) throws JsonProcessingException { LOGGER.info("Order received: {}", mapper.writeValueAsString(order)); service.process(order); } }
Received order is then processed by AccountService
bean. Order may be accepted or rejected by account-service
dependending on sufficient funds on customer’s account for order’s realization. The response with acceptance status is sent back to order-service
via output channel invoked by the OrderSender
bean.
@Service public class AccountService { private static final Logger LOGGER = LoggerFactory.getLogger(AccountService.class); private ObjectMapper mapper = new ObjectMapper(); @Autowired AccountRepository accountRepository; @Autowired OrderSender orderSender; public void process(final Order order) throws JsonProcessingException { LOGGER.info("Order processed: {}", mapper.writeValueAsString(order)); List accounts = accountRepository.findByCustomer(order.getCustomerId()); Account account = accounts.get(0); LOGGER.info("Account found: {}", mapper.writeValueAsString(account)); if (order.getPrice() <= account.getBalance()) { order.setStatus(OrderStatus.ACCEPTED); account.setBalance(account.getBalance() - order.getPrice()); } else { order.setStatus(OrderStatus.REJECTED); } orderSender.send(order); LOGGER.info("Order response sent: {}", mapper.writeValueAsString(order)); } }
The last step is configuration. It is provided inside application.yml
file. We have to properly define destinations for channels. While order-service
is assigning orders-out
destination to output channel, and orders-in
destination to input channel, account-service
and product-service
do the opposite. It is logical, because message sent by order-service
via its output destination is received by consuming services via their input destinations. But it is still the same destination on shared broker’s exchange. Here are configuration settings of order-service
.
spring: cloud: stream: bindings: output: destination: orders-out input: destination: orders-in rabbit: bindings: input: consumer: exchangeType: direct
Here’s configuration provided for account-service
and product-service
.
spring: cloud: stream: bindings: output: destination: orders-in input: destination: orders-out rabbit: bindings: output: producer: exchangeType: direct routingKeyExpression: '"#"'
Finally, you can run our sample microservice. For now, we just need to run a single instance of each microservice. You can easily generate some test requests by running JUnit test class OrderControllerTest
provided in my source code repository inside module order-service
. This case is simple. In the next we will study more advanced sample with multiple running instances of consuming services.
Scaling up
To scale up our Spring Cloud Stream applications we just need to launch additional instances of each microservice. They will still listen for the incoming messages on the same topic exchange as the currently running instances. After adding one instance of account-service
and product-service
we may send a test order. The result of that test won’t be satisfactory for us… Why? A single order is received by all the running instances of every microservice. This is exactly how topic exchanges works – the message sent to topic is received by all consumers, which are listening on that topic. Fortunately, Spring Cloud Stream is able to solve that problem by providing solution called consumer group. It is responsible for guarantee that only one of the instances is expected to handle a given message, if they are placed in a competing consumer relationship. The transformation to consumer group mechanism when running multiple instances of the service has been visualized on the following figure.
Configuration of a consumer group mechanism is not very difficult. We just have to set group
parameter with name of the group for given destination. Here’s the current binding configuration for account-service
. The orders-in
destination is a queue created for direct communication with order-service
, so only orders-out
is grouped using spring.cloud.stream.bindings..group
property.
spring: cloud: stream: bindings: output: destination: orders-in input: destination: orders-out group: account
Consumer group mechanisms is a concept taken from Apache Kafka, and implemented in Spring Cloud Stream also for RabbitMQ broker, which does not natively support it. So, I think it is pretty interesting how it is configured on RabbitMQ. If you run two instances of the service without setting group name on destination there are two bindings created for a single exchange (one binding per one instance) as shown in the picture below. Because two applications are listening on that exchange, there four bindings assigned to that exchange in total.
If you set group name for selected destination Spring Cloud Stream will create a single binding for all running instances of given service. The name of binding will be suffixed with group name.
Because, we have included spring-cloud-starter-sleuth
to the project dependencies the same traceId
header is sent between all the asynchronous requests exchanged during realization of single request incoming to the order-service
POST endpoint. Thanks to that we can easily correlate all logs using this header using Elastic Stack (Kibana).
Automated Testing
You can easily test your microservice without connecting to a message broker. To achieve it you need to include spring-cloud-stream-test-support
to your project dependencies. It contains the TestSupportBinder
bean that lets you interact with the bound channels and inspect any messages sent and received by the application.
<dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-stream-test-support</artifactId> <scope>test</scope> </dependency>
In the test class we need to declare MessageCollector
bean, which is responsible for receiving messages retained by TestSupportBinder
. Here’s my test class from account-service
. Using Processor
bean I send test order to input channel. Then MessageCollector
receives message that is sent back to order-service
via output channel. Test method testAccepted
creates order that should be accepted by account-service, while testRejected
method sets too high order price that results in rejecting the order.
@RunWith(SpringRunner.class) @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT) public class OrderReceiverTest { private static final Logger LOGGER = LoggerFactory.getLogger(OrderReceiverTest.class); @Autowired private Processor processor; @Autowired private MessageCollector messageCollector; @Test @SuppressWarnings("unchecked") public void testAccepted() { Order o = new Order(); o.setId(1L); o.setAccountId(1L); o.setCustomerId(1L); o.setPrice(500); o.setProductIds(Collections.singletonList(2L)); processor.input().send(MessageBuilder.withPayload(o).build()); Message received = (Message) messageCollector.forChannel(processor.output()).poll(); LOGGER.info("Order response received: {}", received.getPayload()); assertNotNull(received.getPayload()); assertEquals(OrderStatus.ACCEPTED, received.getPayload().getStatus()); } @Test @SuppressWarnings("unchecked") public void testRejected() { Order o = new Order(); o.setId(1L); o.setAccountId(1L); o.setCustomerId(1L); o.setPrice(100000); o.setProductIds(Collections.singletonList(2L)); processor.input().send(MessageBuilder.withPayload(o).build()); Message received = (Message) messageCollector.forChannel(processor.output()).poll(); LOGGER.info("Order response received: {}", received.getPayload()); assertNotNull(received.getPayload()); assertEquals(OrderStatus.REJECTED, received.getPayload().getStatus()); } }
Conclusion
Message-driven microservices are a good choice whenever you don’t need synchronous response from your API. In this article I have shown sample use case of publish/subscribe model in inter-service communication between your microservices. The source code is as usual available on GitHub (https://github.com/piomin/sample-message-driven-microservices.git). For more interesting examples with usage of Spring Cloud Stream library, also with Apache Kafka, you can refer to Chapter 11 in my book Mastering Spring Cloud (https://www.packtpub.com/application-development/mastering-spring-cloud).
Partitioning with Apache Kafka and Vert.x
Apache Kafka is a distributed streaming platform. It also may act as a messaging system in your architecture. Traditional message brokers provides two models of communication: queuing and publish-subscribe (topics). Queues are used for point-to-point messaging, while topics allows you broadcast data to multiple target consumers. Kafka does not provide queuing mechanism directly. However, it introduces the consumer group concept, which generalizes both queuing and publish-subscribe models. The consumer group mechanisms guarantees that a single message would be processed by the only one consumer that belongs to the given group. It is especially useful when you have more than one instance of your service, which listens for messages incoming to the topic. That feature makes your consumers to behave as queuing clients within the same group.
Eclipse Vert.x is a lightweight and fast toolkit for building reactive applications on the JVM. I have already introduced that solution is the some of my previous posts, for example Asynchronous Microservices with Vert.x. Vert.x does not force you to implement a reactive application. You may create a standard service, which processes the HTTP requests asynchronously in accordance with Asynchronous I/O concept.
The purpose of this article
The main purpose of this article is to show you the main features of Apache Kafka, that may be useful when creating applications consuming messages. The Java client’s library choice is not a key point here. However, in my opinion Vert.x that is asynchronous, high performance framework perfectly matches to Apache Kafka. It provides Vert.x Kafka client, which allows you to read and send messages from/to an Kafka cluster. Before we proceed to the sample, let’s first dive into the core abstraction of Kafka.
Kafka topic
I’m assuming you excellent know what topic is and what is its main role. The every message incoming to the topic goes to every subscriber. What is the main difference between Kafka and standard topic provided by other message brokers? Kafka topic is partitioned. Each partition is an ordered, immutable sequence of records. Every record can be uniquecly identified within the partition by a sequential id number called the offset. The Kafka cluster retains all published records according to the configured retention period.
Consumer may subscribe to the whole topic or only to the selected partition. It can also control the offset from where it starts processing data. For example, it is able to reset offset in order reprocess data from the past or just or skip ahead to the most recent record to consume only messages currently sent to the topic. Here’s the figure that illustrates a single partition structure with producer and consumers listening for the incoming data.
Sample architecture
Let me say some words about the sample system architecture. Its source code is available on GitHub (https://github.com/piomin/sample-vertx-kafka-messaging.git). In accordance of the principle that one picture speaks more than a thousand words, the diagram illustrating the architecture of our system is visible below. We have one topic created on Kafka platform, that consists of two partitions. There is one client application that exposes REST API allowing to send orders into the system and then forwarding them into the topic. The target partition is calculated basing on the type of order. We may create orders with types SINGLE and MULTIPLE. There are also some applications that consumes data from topic. First of them single-order-processor reads data from partition 0, the second multiple-order-processor from partition 1, and the last all-order-processor does not choose any partition.
Running Kafka
To run Apache Kafka on the local machine we may use its Docker image. The image shared by Spotify also starts ZooKeeper server, which is used by Kafka. If you run Docker on Windows the default address of its virtual machine is 192.168.99.100.
docker run -d --name kafka -p 2181:2181 -p 9092:9092 --env ADVERTISED_HOST=192.168.99.100 --env ADVERTISED_PORT=9092 spotify/kafka
However, that option assumes the topics would be automatically created during application startup. I’ve got some problems with it while creating multi-partitions topic. There is also another image ches/kafka, which requires starting ZooKeeper separately, but provides Kafka client interface.
docker run -d --name zookeeper -p 2181:2181 zookeeper docker run -d --name kafka -p 9092:9092 -p 7203:7203 --network kafka-net --env KAFKA_ADVERTISED_HOST_NAME=192.168.99.100 --env ZOOKEEPER_IP=192.168.99.100 ches/kafka
Finally, we can run ches/kafka container in client mode and then create topic orders-out with two partitions.
docker run --rm --network kafka-net ches/kafka kafka-topics.sh --create --topic orders-out --replication-factor 1 --partitions 2 --zookeeper 192.168.99.100:2181 Created topic "orders-out".
Building producer application
First, we need to include Maven dependencies to enable Vert.x framework for the application. If the application exposes RESTful HTTP API you should include vertx-web
. Library vertx-kafka-client
has to be included to all the sample modules.
To start Vert.x as Java application we have to create verticle by extending AbstractVerticle
. Then the verticle needs to be deployed in the main method using Vertx
object. For more details about Vert.x and verticles concept you may refer to one of my previous article mentioned in the preface.
public class OrderVerticle extends AbstractVerticle { public static void main(String[] args) { Vertx vertx = Vertx.vertx(); vertx.deployVerticle(new OrderVerticle()); } }
The next step is to define producer using KafkaProducer
interface. We have to provide connection settings and serializer implementation class. You can choose between various built-in serializer implemementations. The most suitable for me was JsonObjectSerializer
, which requires JsonObject
as an input parameter.
Properties config = new Properties(); config.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "192.168.99.100:9092"); config.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class); config.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, JsonObjectSerializer.class); config.put(ProducerConfig.ACKS_CONFIG, "1"); KafkaProducer producer = KafkaProducer.create(vertx, config);
The procuder is invoked inside POST method route definition. It returnes an asynchronous response with a status after sending message to the topic. The message is created using KafkaProducerRecord
interface. It takes topic’s name, request object and partition number as the parameters. As you may see in the fragment of code below, partition number is calculated on the basis order type (o.getType().ordinal()
).
Router router = Router.router(vertx); router.route("/order/*").handler(ResponseContentTypeHandler.create()); router.route(HttpMethod.POST, "/order").handler(BodyHandler.create()); router.post("/order").produces("application/json").handler(rc -> { Order o = Json.decodeValue(rc.getBodyAsString(), Order.class); KafkaProducerRecord record = KafkaProducerRecord.create("orders", null, rc.getBodyAsJson(), o.getType().ordinal()); producer.write(record, done -> { if (done.succeeded()) { RecordMetadata recordMetadata = done.result(); LOGGER.info("Record sent: msg={}, destination={}, partition={}, offset={}", record.value(), recordMetadata.getTopic(), recordMetadata.getPartition(), recordMetadata.getOffset()); o.setId(recordMetadata.getOffset()); o.setStatus(OrderStatus.PROCESSING); } else { Throwable t = done.cause(); LOGGER.error("Error sent to topic: {}", t.getMessage()); o.setStatus(OrderStatus.REJECTED); } rc.response().end(Json.encodePrettily(o)); }); }); vertx.createHttpServer().requestHandler(router::accept).listen(8090);
Building consumer applications
The consumer configuration is very similar to that for producer. We also have to set connection settings and class using for deserialization. There is one interesting setting, which has been defined for the consumer in the fragment of code visible below. It is auto.offset.reset (ConsumerConfig.AUTO_OFFSET_RESET_CONFIG
). It sets the initial offset in Kafka for the customer during initialization. If you would like to read all records from the beginning of stream use value earliest
. If you would like to processes only the newest records (received after application startup) set that property to latest
. Because in our case Kafka acts as a message broker, it is set to latest
.
Properties config = new Properties(); config.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "192.168.99.100:9092"); config.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class); config.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class); config.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "latest"); config.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false"); KafkaConsumer consumer = KafkaConsumer.create(vertx, config);
As you probably remember we have three different application that subscribe to the topic. The first of them, implemented under the module all-order-processor
consumes all the events incoming to the the topic. This implemementation is relatively the simplest. We only need to invoke subscribe
method and pass the name of topic as a parameter. Then every incoming message is processed by handler
method.
consumer.subscribe("orders-out", ar -> { if (ar.succeeded()) { LOGGER.info("Subscribed"); } else { LOGGER.error("Could not subscribe: err={}", ar.cause().getMessage()); } }); consumer.handler(record -> { LOGGER.info("Processing: key={}, value={}, partition={}, offset={}", record.key(), record.value(), record.partition(), record.offset()); Order order = Json.decodeValue(record.value(), Order.class); order.setStatus(OrderStatus.DONE); LOGGER.info("Order processed: id={}, price={}", order.getId(), order.getPrice()); });
The implementation of consuming method for the other applications is a little more complicated. Besides defining target topic, every consumer can ask for a specific partition. The application multiple-order-processor
subscribes to partition 1, while multiple-order-processor
to partition 0.
TopicPartition tp = new TopicPartition().setPartition(1).setTopic("orders-out"); consumer.assign(tp, ar -> { if (ar.succeeded()) { LOGGER.info("Subscribed"); consumer.assignment(done1 -> { if (done1.succeeded()) { for (TopicPartition topicPartition : done1.result()) { LOGGER.info("Partition: topic={}, number={}", topicPartition.getTopic(), topicPartition.getPartition()); } } else { LOGGER.error("Could not assign partition: err={}", done1.cause().getMessage()); } }); } else { LOGGER.error("Could not subscribe: err={}", ar.cause().getMessage()); } });
The implamentation of handle method inside multiple-order-processor
is pretty interesting. If it receives order with non-empty field relatedOrderId it tries to find it in the historical records stored in topic. It may achieved by calling seek
method on KafkaConsumer
.
consumer.handler(record -> { LOGGER.info("Processing: key={}, value={}, partition={}, offset={}", record.key(), record.value(), record.partition(), record.offset()); Order order = Json.decodeValue(record.value(), Order.class); if (ordersWaiting.containsKey(record.offset())) { LOGGER.info("Related order found: id={}, price={}", order.getId(), order.getPrice()); LOGGER.info("Current price: price={}", order.getPrice() + ordersWaiting.get(record.offset()).getPrice()); consumer.seekToEnd(tp); } if (order.getRelatedOrderId() != null && !ordersWaiting.containsKey(order.getRelatedOrderId())) { ordersWaiting.put(order.getRelatedOrderId(), order); consumer.seek(tp, order.getRelatedOrderId()); } });
Testing
Now it is time to launch our applications. You may run the main classes from your IDE or build the whole project using mvn clean install
command and then run it with java -jar
. Also run two instances of all-order-processor
in order to check out how a consumer groups mechanism works in practice.
Let’s send some test requests to the order-service
in the following sequence.
curl -H "Content-Type: application/json" -X POST -d '{"type":"SINGLE","status":"NEW","price":200}' http://localhost:8090/order {"id":0,"type":"SINGLE","status":"PROCESSING","price":200} curl -H "Content-Type: application/json" -X POST -d '{"type":"SINGLE","status":"NEW","price":300}' http://localhost:8090/order {"id":1,"type":"SINGLE","status":"PROCESSING","price":300} curl -H "Content-Type: application/json" -X POST -d '{"type":"MULTIPLE","status":"NEW","price":400}' http://localhost:8090/order {"id":0,"type":"MULTIPLE","status":"PROCESSING","price":400} curl -H "Content-Type: application/json" -X POST -d '{"type":"MULTIPLE","status":"NEW","price":500,"relatedOrderId" :0}' http://localhost:8090/order {"id":1,"type":"MULTIPLE","status":"PROCESSING","price":500}
Here’s log from producer application.
2018-01-30 11:08:48 [INFO ] Record sent: msg={"type":"SINGLE","status":"NEW","price":200}, destination=orders-out, partition=0, offset=0 2018-01-30 11:08:57 [INFO ] Record sent: msg={"type":"SINGLE","status":"NEW","price":300}, destination=orders-out, partition=0, offset=1 2018-01-30 11:09:08 [INFO ] Record sent: msg={"type":"MULTIPLE","status":"NEW","price":400}, destination=orders-out, partition=1, offset=0 2018-01-30 11:09:27 [INFO ] Record sent: msg={"type":"MULTIPLE","status":"NEW","price":500,"relatedOrderId":0}, destination=orders-out, partition=1, offset=1
Here’s log from single-order-processor. It has processed only messages from partition 0.
2018-01-30 11:08:48 [INFO ] Processing: key=null, value={"type":"SINGLE","status":"NEW","price":200}, partition=0, offset=0 2018-01-30 11:08:57 [INFO ] Processing: key=null, value={"type":"SINGLE","status":"NEW","price":300}, partition=0, offset=1
Here’s log from multiple-order-processor. It has processed only messages from partition 1.
2018-01-30 11:09:08 [INFO ] Processing: key=null, value={"type":"MULTIPLE","status":"NEW","price":400}, partition=1, offset=0 2018-01-30 11:09:27 [INFO ] Processing: key=null, value={"type":"MULTIPLE","status":"NEW","price":500,"relatedOrderId":0}, partition=1, offset=1
Here’s log from first instance of all-order-processor.
2018-01-30 11:08:48 [INFO ] Processing: key=null, value={"type":"SINGLE","status":"NEW","price":200}, partition=0, offset=0 2018-01-30 11:08:57 [INFO ] Processing: key=null, value={"type":"SINGLE","status":"NEW","price":300}, partition=0, offset=1
Here’s log from second instance of all-order-processor. It may be a little bit surprising for you. But, if you run two instances of consumer, which listens for the whole topic each instance would process message from the single partition.
2018-01-30 11:09:08 [INFO ] Processing: key=null, value={"type":"MULTIPLE","status":"NEW","price":400}, partition=1, offset=0 2018-01-30 11:09:27 [INFO ] Processing: key=null, value={"type":"MULTIPLE","status":"NEW","price":500,"relatedOrderId":0}, partition=1, offset=1
Summary
In this article I was trying to give you a little bit of messaging with Apache Kafka. Such concepts like consumer groups or partitioning are something what makes it different from traditional messaging solutions. Kafka is widely adopted product, which can acts as storage, messaging system or stream processor. Together with popular JVM based toolkit Vert.x it may be really powerful, fast and lightweight solution for your applications that exchanges messages between each other. The key concepts introduced by Kafka has been adopted by Spring Cloud Stream, which makes them as an opinionated choice for creating messaging microservices.