Understanding the microservice architecture

The microservice architecture is a way to develop a single application containing a set of smaller services. These services are independent of each other and run in their own processes. An important advantage of these services is that they can be developed and deployed independently. In other words, we can say that microservices are a way to segregate our services so they can be handled completely independent of each other in the context of design, development, deployment, and upgrades.

In a monolithic application, we have a self-contained assembly of user interface, direct sale, and inventory. In the microservice architecture, the services part of the application changes to the following depiction:

Here, business components have been segregated into individual services. These independent services now are the smaller units that existed earlier within the self-contained assembly, in the monolithic architecture. Both direct sales and inventory services are independent of each other, with the dotted lines depicting their existence in the same ecosystem yet not bound within a single scope. Refer to the following diagram:

From the preceding image, it's clear that our user interface can interact with any of the services. There is no need to intervene in any service when a UI calls a service. Both the services are independent of each other, without being aware of when the other one would be called by the user interface. Both services are liable for their own operations and not for any other part in the whole system. Although much closer to the microservice architecture, the preceding visualization is not entirely a complete visualization of the intended microservice architecture.

In the microservice architecture, services are small, independent units with their own persistent stores.

Now let's bring this final change so that each service will have its own database persisting the necessary data:

Here, User interface is interacting with those services that have their own independent storage. In this case, when a user interface calls a service for direct sales, the business flow for direct sales is executed independently of any data or logic contained within the inventory service.

The solution that the use of microservices provides has a lot of likely benefits, as discussed next:

• Smaller codebase: Each service is small, therefore, easier to develop and deploy as a unit
• Ease of independent environment: With the separation of services, all developers work independently, deploy independently, and no one is bothered about any module dependency

With the adoption of the microservice architecture, monolithic applications are now harnessing the associated benefits, as it can now be scaled easily and deployed using a service independently.

Messaging in microservices

It is very important to carefully consider the choice of the messaging mechanism when dealing with the microservice architecture. If this one aspect is ignored, then it can compromise the entire purpose of designing using the microservice architecture. In monolithic applications, this is not a concern as the business functionality of the components gets invoked through function calls. On the other hand, this happens via a loosely coupled web service level messaging feature, where services are primarily based on SOAP.  In the case of the microservice messaging mechanism, it should be simple and lightweight.

There are no set rules for making a choice between various frameworks or protocols for a microservice architecture. However, there are a few points worth consideration here. Firstly, it should be simple enough to implement, without adding any complexity to your system. Secondly, it should be lightweight enough, keeping in mind the fact that the microservice architecture could heavily rely on interservice messaging. Let's move ahead and consider our choices for both synchronous and asynchronous messaging along with the different messaging formats.

Synchronous messaging

When a timely response is expected from a service by a system and the system waits on it till a response is received from the service, it is synchronous messaging. What's left is the most sought-after choice in the case of microservices. It is simple and supports HTTP request-response, thereby leaving little room to look for an alternative. This is also one of the reasons that most implementations of microservices use HTTP (API-based styles).

Asynchronous messaging

When a system is not immediately expecting a timely response from the service and the system can continue processing without blocking on that call, it is asynchronous messaging.

Let's incorporate this messaging concept into our application and see how it would change and look:

Message formats

Over the past many years, working with MVC and the like has got me hooked on the JSON format. You could also consider XML. Both the formats would do fine on HTTP with the API style resource. There are easily available binary message formats in case you need to use one. We are not recommending any format; you can go ahead with any of the selected message formats.