The following diagram shows the evolution of the application architecture from a three-tier architecture to SOA/ESB and then to microservices in terms of flexibility toward scalability and decoupling:

Microservices have evolved from being tiered and the SOA architecture and are becoming the accepted pattern for building modern applications. This is due to the following reasons:

• Extreme scalability
• Extreme decoupling
• Extreme agility

These are key points regarding the design of a distributed scalable application where developers can pick the best programming language of their choice to develop their own service.

A major differentiation between monolithic and microservices is that, with microservices, the services are loosely coupled, and they communicate using dumb pipe or low-level REST or gRPC protocols. One way to achieve loose coupling is through the use of a separate data store for each service. This helps services isolate themselves from each other since a particular service is not blocked due to another service holding a data lock. Separate data stores allow the microservices to scale up and down, along with their data stores, independently of all the other services.

It is also important to point out the early pioneers in microservices, which we will discuss in the next section.