We are all familiar with Hogwarts' magical postal network, the
Owl Post Office. This magical delivery system relies on owls to deliver
packages and letters. They can even pick up an item from any address and give
it to another using their magical tracking abilities. Similarly, we can
compare RabbitMQ, a message broker to the Owl Post Office. It allows applications
to interact with one another and exchange messages.
But why should we use message broker?
An e-commerce application ships thousands of items per day and sends email
notifications for each one. This is a synchronous operation. This means
that the item's ship status is saved in the database, and an email is sent.
Now, on bad days, the email server goes down or crashes due to overload. So
none of us receive the email notification.
Assume we add a service layer between the shipment service and e-mail service.
So, shipping service sends the shipment notification to the service layer,
which will route the message to the email service. If the email service goes
down, the service layer will store the messages. When the email service
goes live,
the service layer will push those messages to the e-mail service.
And this service layer is nothing but a message broker.
RabbitMQ is a distributed message and stream broker. A message broker is
software that sits among applications, allowing them to exchange messages.
RabbitMQ is useful for decoupling services, remote procedure calls (RPC),
streaming services, and IoT.
RabitMQ is most commonly used in microservice-based architectures. It
operates asynchronously. This means they do not follow a simple request-response pattern, and we
must wait for replies.
RabbitMQ, like the postal service, sends messages from producer to
consumer.
Producer
A Producer is an action or event that generates
messages. Credit card transactions, a drop or rise in stock price, or an order
dispatch are all examples of Producers.
Consumer
On the other hand, Consumers are the entities that
listen to messages. Humans are the perfect example of a consumer. We
consume everything, from news to alcohol. Jokes aside! Because a message broker might be connected to multiple producers and
consumers, communication between them is asynchronous.
Exchange
But, wait a second! How does the message broker get a message from the
producer to the consumer? The answer is Exchange. It functions as RabbitMQ's brain. Exchange helps message
broker to route messages from producer to consumer.
Queue & Binding
A message broker may have multiple exchanges. Exchanges always receive
messages from producers. Consumers are not connected directly to exchanges.
Queues connect exchanges and consumers.
Binding connects queues to exchanges. You can think of queues
like our letterbox. Exchange pushes messages into queues, from which
interested consumers can consume them.
An exchange can be bound to multiple queues, and a queue can be linked to
numerous exchanges. The consumer might also listen to messages from multiple
queues.
Connection & Channel
RabbitMQ is designed to implement the AMQP (Advance Message
Queuing Protocol). AMQP is an open messaging protocol that defines the rules
for message exchange, queueing, and routing in a messaging system.
To communicate with RabbitMQ, a client or application must first establish a
connection. A client can be either a producer or a consumer. The
connection is established using either TCP or TLS. The main purpose of
a connection is to establish a secure path between the client and
RabbitMQ.
A connection can have several channels. But, why do we need channels? We can set up multiple connections
between the client and the broker to exchange messages. Keeping multiple
TCP connections open at the same time is undesirable because it consumes
system resources and makes firewall configuration more difficult. So,
according to the AMQP protocol, channels are "lightweight connections that share a single TCP connection".
We can use the amqp-client Java library to communicate with
RabbitMQ in a Java application. This library is available through the
Maven repository. We need to install RabbitMQ on our local machine to communicate with the
Java application. You can find an installation guide
here.
Producer & Consumer using Java
Now, let's look at how to create a message and how to consume it using
RabbitMQ and Java. First, we write a program that will connect and publish a
message to RabbitMQ.
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import java.nio.charset.StandardCharsets;
public class Publisher {
private static final String HOST = "localhost";
private static final String QUEUE = "OWL-POST";
private static final String MESSAGE = "Happy birthday Hermione!";
public static void main(String[] args) {
// CREATE A CONNECTION FACTORY
ConnectionFactory connectionFactory = new ConnectionFactory();
connectionFactory.setHost(HOST);
// CREATE A CONNECTION FROM FACTORY
try (Connection connection = connectionFactory.newConnection()) {
// GET CHANNEL FROM CONNECTION
Channel channel = connection.createChannel();
// ASSIGN A QUEUE TO CHANNEL
channel.queueDeclare(QUEUE, false, false, false, null);
// PUBLISH A MESSAGE TO CHANNEL
channel.basicPublish("", QUEUE, null, MESSAGE.getBytes(StandardCharsets.UTF_8));
System.out.println(" Message sent : '" + MESSAGE + "'");
} catch (Exception e) {
System.out.println(e.getMessage() + ", " + e.getStackTrace());
}
}
}
We have used amqp-client Java client library to communicate with
RabbitMQ.
RabbitMQ is running on our local machine, so the host is localhost. The
queue name is 'OWL-POST'.
Consumer should connect to this queue to consume message from producer.
ConnectionFactory is a factory class that allows you to open a
connection to RabbitMQ.
First, we get a connection from ConnectionFactory, and then we create a
channel. We create a queue using queueDeclare() method by passing the
queue name. This queue is idempotent, which means that if it already exists under this name, it will not be
created again.
Next, we use the basicPublish() method to send our encoded message to
the queue. The first argument of the basicPublish() method is the exchange
name. Because we passed a blank string as the first argument, we are
connecting to RabbitMQ's default exchange.
Here is our consumer.
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.DeliverCallback;
import java.nio.charset.StandardCharsets;
public class Consumer {
private static final String HOST = "localhost";
private static final String QUEUE = "OWL-POST";
public static void main(String[] args) {
// CREATE A CONNECTION FACTORY
ConnectionFactory connectionFactory = new ConnectionFactory();
connectionFactory.setHost(HOST);
try {
// CREATE A CONNECTION
Connection connection = connectionFactory.newConnection();
// CREATE A CHANNEL FROM CONNECTION
Channel channel = connection.createChannel();
// ASSIGN A QUEUE TO CHANNEL
channel.queueDeclare(QUEUE, false, false, false, null);
System.out.println(" Waiting for message...");
DeliverCallback deliverCallback = (consumeMsg, delivery) -> {
String message = new String(delivery.getBody(), StandardCharsets.UTF_8);
System.out.println(" Message received : '" + message + "'");
};
channel.basicConsume(QUEUE, true, deliverCallback, consumeMsg -> {});
} catch (Exception e) {
System.out.println(e.getMessage() + ", " + e.getStackTrace());
}
}
}
So the consumer is connected to the 'OWL-POST' queue. And it receives the
message via the basicConsume() method.
DeliverCallback is a callback interface. It is notified when the
consumer receives the message and is passed as an argument to the
basicConsume() method.
Here is the message from Harry -'Happy birthday Hermione!'.
Happy coding!!! 😊
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