In respect to our monolithic application, where we may have an Order module, a change in the module Orders affects the module Stock and so on. It is the absence of modularity that has resulted in such a condition.
This also means that we can't reuse the functionality of a module within another module. The code is not decomposed into structured pieces that could be reused to save time and effort. There is no segregation within the code modules, and hence, no common code is available.
Business is growing and its customers are growing by leaps and bounds. New or existing customers from different regions have different preferences when it comes to the use of the application. Some like to visit the website, but others prefer to use mobile apps. The system is structured in a way that we can't share the components across a website and a mobile app. This makes introducing a mobile/device app for the business a challenging task. Business is affected as in such scenarios the company loses out on customers who prefer mobile apps.
The difficulty in replacing the component's application is using third-party libraries, an external system such as payment gateways, and an external order-tracking system. It is a tedious job to replace the old components in the currently styled monolithic architectural application. For example, if we consider upgrading the library of our module that is consuming an external order-tracking system, then the whole change would prove to be very difficult. Also, it would be an intricate task to replace our payment gateway with another one.
In any of the preceding scenarios, whenever we upgraded the components, we upgraded everything within the application, which called for a complete testing of the system and required a lot of downtime. Apart from this, the upgrade would possibly result in the form of production bugs, which would require you to repeat the whole cycle of development, testing, and deployment.