There are many real-world reasons why the industry is very optimistic about MSA for modernizing legacy applications. There are several valid differences between monolithic and microservices-centric applications, as represented in the following diagram:
Monolithic applications follow a heavily-centralized architecture, whereas Microservices go for a distributed architecture. Due to the exponential increase in data size and the usage of commodity servers, the IT world is leaning toward distributed computing. Also, services are being developed and deployed by worldwide software developers to geographically-distributed servers. Thus, distributed computing can't be taken lightly anymore. MSA intrinsically supports the distributed computing characteristics and hence, is flourishing.
As previously explained, each microservice fulfills one or more business functionalities. That is, different business capabilities are implemented through separate microservices running on different processes. For an e-commerce application, we can have microservices for various modules, such as the shopping cart, customer analytics, the payment gateway, warehousing, shipping, replenishment, inventory management, and email notifications. Web-scale applications are embracing the MSA in order to be business-friendly.
For applications with frequently varying loads (where the number of users changes quickly and there can be sudden spikes in the number of messages with different data sizes), there is an insistence for mechanisms that innately support dynamic, horizontal, and real-time scalability. Through containerized microservices, we can achieve that kind of scalability, as containers are lightweight and can boot up in a matter of seconds.
Microservices eliminate the problems associated with the tightly-coupled application components of monolithic applications. Microservices can be integrated through various middleware products (synchronous and asynchronous communication). Due to the decoupling nature of microservices, microservices can be independently designed, developed, and deployed in production environments. That is, updating and upgrading microservices dooesn't affect other components of a microservices-centric application. There are a few novel testing and deployment methods emerging and evolving in order to bring the utmost dynamism to microservices environments.