What is Distributed Messaging Middleware?

For distributed message middleware, we must first understand two basic concepts, that is, what is a distributed system and what is middleware.

Distributed Systems

“A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messasges.”——《Distributed Systems Concepts and Design》

From the above explanation, two characteristics of distributed systems can be obtained:

Components are distributed on network computers
Coordinate actions between components through messages

middleware

Middleware is computer software that provides services to software applications beyond those available from the operating system. It can be described as "software glue". Middleware makes it easier for software developers to implement communication and input/output, so they can focus on the specific purpose of their application.——维基百科

Middleware is described as providing applications with services other than those provided by the operating system, simplifying application communication, input and output development, and allowing them to focus on their own business logic.

The explanation of middleware on Wikipedia feels a bit confusing. In fact, middleware can be understood from the perspective of "space", that is, middleware is a component in the "middle layer", which is between the upper-layer application and the underlying service. Bridge (for example, the upper layer of DB middleware is the application program, and the bottom layer is the DB service), and it is also the bridge between applications (such as distributed service components).

Distributed Messaging Middleware

“Message-oriented middleware (MOM) is software or hardware infrastructure supporting sending and receiving messages between distributed systems.”——维基百科

Wikipedia defines messaging middleware as hardware or software infrastructure (and certainly software for the scope of our discussion here) that supports sending and receiving messages in a distributed system.

Then the distributed message middleware actually means that the message middleware itself is also a distributed system.

What can message middleware do?

Any middleware must solve a problem in a specific field, and message middleware solves the problem of message passing between distributed systems. Message passing is a problem that distributed systems must face.

Assuming an e-commerce transaction scenario, the user calls the inventory system to reduce inventory after placing an order, and then needs to call the logistics system for delivery. If the transaction, inventory, and logistics belong to one system, then it is an interface call. However, with the development of the system, each module becomes larger and larger, and the business logic becomes more and more complex, so it must be service-oriented and business split. At this time, it is necessary to consider how these systems interact. The first reaction is RPC (Remote Procedure Call). As the system continues to develop, a transaction may need to call dozens of interfaces to execute the business, such as risk control systems, SMS services, and so on. At this time, the message middleware is needed to solve the problem.

The author believes that the difference between RPC and message middleware scenarios largely lies in "dependency" and "quantity". For example, the SMS notification service is not necessary for the transaction process, does not affect the ordering process, and is not strongly dependent, so the transaction system should not rely on the SMS service. For example, some data analysis programs may need to obtain the total sales volume of a day, which only requires the sales center to provide an interface to call when needed.

After the message middleware appears, the transaction scenario may be to call the inventory center and other strongly dependent systems to execute the business, and then publish a message (this message is stored in the message middleware). For example, the SMS notification service, the data statistics service, etc. all rely on the message middleware to consume this message to complete their own business logic.

From the above scenarios, it can be seen that the message middleware is actually decoupling the system, and at the same time brings benefits such as asynchrony.

简单概括一下消息中间件的应用场景大致如下：