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

Microservices are not invented. Many organizations, such as Netflix, Amazon, and eBay had successfully used the divide and conquer technique for functionally partitioning 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 application. Following the success of these organizations, many other organizations started adopting this as a common pattern for refactoring their monolithic applications. Later, evangelists termed this pattern microservices architecture.

Microservices originated from the idea of Hexagonal Architecture, which was coined by Alister Cockburn back in 2005. Hexagonal Architecture, or Hexagonal pattern, is also known as the Ports and Adapters pattern.

In simple terms, Hexagonal architecture advocates to encapsulate business functions from the rest of the world. These encapsulated business functions are unaware of their surroundings. For example, these business functions are not even aware of input devices or channels and message formats used by those devices. Ports and adapters at the edge of these business functions convert messages coming from different input devices and channels to a format that is known to the business function. When new devices are introduced, developers can keep adding more and more ports and adapters to support those channels without touching business functions. One may have as many ports and adapters to support their needs. Similarly, external entities are not aware of business functions behind these ports and adapters. They will always interface with these ports and adapters. By doing so, developers enjoy the flexibility to change channels and business functions without worrying too much about future proofing interface designs.

The following diagram shows the conceptual view of Hexagonal Architecture:

In the preceding diagram, the application is completely isolated and exposed through a set of frontend adapters, as well as a set of backend adapters. Frontend adaptors are generally used for integrating UI and other APIs, whereas backend adapters are used for connecting to various data sources. Ports and adapters on both sides are responsible for converting messages coming in and going out to appropriate formats expected by external entities. Hexagonal architecture was the inspiration for microservices.

When we look for a definition for microservices, there is no single standard way of describing them. Martin Fowler defines microservices as follows:

The definition used in this book is as follows:

The preceding diagram depicts a traditional n-tier application architecture, having a Presentation Layer, Business Layer, and Database Layer. Modules A, B, and C represent three different business capabilities. The layers in the diagram represent separation of architecture concerns. Each layer holds all three business capabilities pertaining to that layer. The presentation layer has web components of all three modules, the business layer has business components of all three modules, and the database host tables of all 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 see in the preceding diagram, the boundaries are inversed in the microservices architecture. Each vertical slice represents a microservice. Each microservice will have its own presentation layer, business layer, and database layer. Microservices are aligned towards business capabilities. By doing so, changes to one microservice does 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.

Since 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.