Microservices are an architecture style used by many organizations today as a game changer to achieve a high degree of agility, speed of delivery, and scale. Microservices give us a way to develop more physically separated modular applications.

Microservices are not invented. Many organizations such as Netflix, Amazon, and eBay successfully used the divide-and-conquer technique to functionally partition their monolithic applications into smaller atomic units, each performing a single function. These organizations solved a number of prevailing issues they were experiencing with their monolithic applications.

Following the success of these organizations, many other organizations started adopting this as a common pattern to refactor their monolithic applications. Later, evangelists termed this pattern as the microservices architecture.

Microservices originated from the idea of hexagonal architecture coined by Alistair Cockburn. Hexagonal architecture is also known as the Ports and Adapters pattern.

Microservices are an architectural style or an approach to building IT systems as a set of business capabilities that are autonomous, self-contained, and loosely coupled:

The preceding diagram depicts a traditional N-tier application architecture having a presentation layer, business layer, and database layer. The modules A, B, and C represent three different business capabilities. The layers in the diagram represent a separation of architecture concerns. Each layer holds all three business capabilities pertaining to this layer. The presentation layer has web components of all the three modules, the business layer has business components of all the three modules, and the database hosts tables of all the three modules. In most cases, layers are physically spreadable, whereas modules within a layer are hardwired.

Let's now examine a microservices-based architecture.

As we can note in the preceding diagram, the boundaries are inversed in the microservices architecture. Each vertical slice represents a microservice. Each microservice has its own presentation layer, business layer, and database layer. Microservices are aligned towards business capabilities. By doing so, changes to one microservice do not impact others.

There is no standard for communication or transport mechanisms for microservices. In general, microservices communicate with each other using widely adopted lightweight protocols, such as HTTP and REST, or messaging protocols, such as JMS or AMQP. In specific cases, one might choose more optimized communication protocols, such as Thrift, ZeroMQ, Protocol Buffers, or Avro.

As microservices are more aligned to business capabilities and have independently manageable life cycles, they are the ideal choice for enterprises embarking on DevOps and cloud. DevOps and cloud are two facets of microservices.