Book - Enterprise Integration Patterns - Designing, Building And Deploying Messaging Solutions

The book explains various patterns required for the creation of right Microservice. 

Difference between Synchronous and Asynchronous with real world example:

A telephone call is a synchronous form of communication. We can only communicate with the other party if the other party is available at the time we place the call. 

Voice mail on the other hand, allows asynchronous communication. With voice mail, when the receiver does not answer, the caller can leave him a message; later the receiver (at his convenience) can listen to the messages queued in his mailbox. 

Voice mail enables the caller to leave a message now so that the receiver can listen to it later, which is lot easier than trying to get the caller and the receiver on the phone at the same time. Voice mail bundles (at least part of) a phone call into a message and queues it for later consumption.

What is messaging?

Messaging is a technology that enables high-speed, asynchronous, program-to-program communication with reliable delivery.

What are messages?

Programs communicate by sending packets of data called messages to each other. 

The message itself is simply some sort of data structure—such as a string, a byte array, a record, or an object. 

It can be interpreted simply as data, as the description of a command to be invoked on the receiver, or as the description of an event that occurred in the sender. 

A message actually contains two parts, 

1. a header and 

2. a body. 

The header contains meta-information about the message—who sent it, where it’s going, etc.; this information is used by the messaging system and is mostly (but not always) ignored by the applications using the messages. 

The body contains the data being transmitted and is ignored by the messaging system. In conversation, when an application developer who is using messaging talks about a message, he’s usually referring to the data in the body of the message. 

Channels, also known as queues, are logical pathways that connect the programs and convey messages. A channel behaves like a collection or array of messages, but one that is magically shared across multiple computers and can be used concurrently by multiple applications. 

A sender or producer is a program that sends a message by writing the message to a channel. 

A receiver or consumer is a program that receives a message by reading (and deleting) it from a channel.

What is a messaging System 

Messaging capabilities are typically provided by a separate software system called a messaging system or message-oriented middleware (MOM). 

A messaging system manages messaging the way a database system manages data persistence. 

An administrator configures the messaging system with the channels that define the paths of communication between the applications same like DBA creates a database schema. 

The messaging system then coordinates and manages the sending and receiving of messages. 

The reason a messaging system is needed to move messages from one computer to another is that computers and the networks that connect them are inherently unreliable. 

Just because one application is ready to send a communication does not mean that the other application is ready to receive it. 

Even if both applications are ready, the network may not be working, or may fail to transmit the data properly. 

A messaging system overcomes these limitations by repeatedly trying to transmit the message until it succeeds. 

Under ideal circumstances, the message is transmitted successfully on the first try, but circumstances are often not ideal. In essence, a message is transmitted in five steps: 

1. Create — The sender creates the message and populates it with data. 

2. Send — The sender adds the message to a channel. 

3. Deliver — The messaging system moves the message from the sender’s computer to the receiver’s computer, making it available to the receiver. 

4. Receive — The receiver reads the message from the channel. 

5. Process — The receiver extracts the data from the message. 

Messaging Concepts:

1. Send and Forget: The sending application sends the message to the message channel. Once that send is complete, the sender can go on to other work while the messaging system transmits the message in the background. The sender can be confident that the receiver will eventually receive the message and does not have to wait until that happens.

2. Send and Forward: when the sending application sends the message to the message channel, the messaging system stores the message on the sender’s computer, either in memory or on disk. In step 3, the messaging system delivers the message by forwarding it from the sender’s computer to the receiver’s computer, and then stores the message once again on the receiver’s computer. This store-and-forward process may be repeated many times, as the message is moved from one computer to another, until it reaches the receiver’s computer.