What is Messaging?

Messaging is a communication mechanism used for system interactions. In software development messaging enables distributed communication that is loosely coupled. A messaging client can send messages to, and receive messages from, any other client. The structure of message can be defined as follows-

RabbitMQ - Table Of Contents
What is Messaging?
Getting Started with RabbitMQ - Install and start RabbitMQ.
Spring Boot + RabbitMQ Publish Message Example
Spring Boot + RabbitMQ Tutorial - Configure Listeners to consume messages using MessageListenerContainer
Spring Boot + RabbitMQ Consume Message Example using RabbitListener
Spring Boot + RabbitMQ Tutorial - Implement Exchange Types
Spring Boot + RabbitMQ Tutorial - Retry and Error Handling Example
Spring Cloud Stream - RabbitMQ Publish Message Example
Spring Cloud Stream - RabbitMQ Consume Message Example
Pivotal Cloud Foundry Tutorial - Deploying Spring Boot + RabbitMQ Application to PCF 

Synchronous Messaging is implemented when such that the messaging client sends a message and expects the response immediately. So the sender client waits for the response before he can execute the next task. So until and unless the message is recieved the sender is blocked.

Asynchronous Messaging is implemented such that the messaging client sends a message and does not expect the response immediately. So the sender client does not for the response before he can execute the next task. So the sender is not blocked.

Real time systems usually have a combination of syschronous and ascynchronous communication

Message Broker - are responsible for establishing connections with various client systems. Let us consider role of Message Broker in a telecom system. Suppose initially there is no message broker. Then each telephone connection will have a direct line with all other telephone connections.

Suppose tomorrow if another telephone connection needs to be added, then all existing telephone connections will need to get a direct line with this new telephone connection. As more connections are added this will get more complicated.

With Message broker all connections are registered with Message Broker. So all connections only need to connect to the message broker. It will automatically route the message to the correct client based on some message configuration.