The following challenges make the monolith design unsuitable for large applications:

• Huge code base: As we are developing the application as a single unit, everything is placed under a single code base. In our previous example, we have all the Employee-Management-related functionality in a single code base. So, even if we are making a small change, such as updating a tax slab for employee salaries, we have to take the whole code for our Employee Data Management project and build the whole application instead of just the tax-related part.
• Testing: As the application is managed as a single unit, we need to test the whole application, even if a small change is made, to make sure there are no integration or regression issues.
• Availability: Let's say that, while updating an employee data report, a developer introduced an error that caused the system to run out of memory, which will bring down the whole system. So a report that actually might not add too much value to the system and may be used rarely by users has the capability of bringing down the whole system.
• Scalability: The code is deployed as a single unit, hence we can only scale the application as a whole, making it a heavy operation. For example, if we just need to execute multiple instances of salary processing on pay day, we cannot do that in isolation; and we need to scale the whole application by providing more hardware firepower (vertical scaling) or make copies of the whole application on different machines (horizontal scaling).
• Inflexible: Say we need to create a special reporting feature and we know a tool or framework is available in a different language than we use. It is not possible in a monolith architecture to use different programming languages or technologies. We are stuck with the original choice, which might have been made years ago and is not relevant anymore.
• Difficult to upgrade: Even a small decision, such as moving from Java EE 6 to 8, would mean that the whole application, along with all the features, needs to be tested and even a small problem would hinder the upgrade, whereas if we were dealing with multiple small services, we could upgrade them independently and sequentially.
• Slow Development and time to market: For these reasons, it is difficult to make changes to an application with a monolith design. This approach does not fit well in today's agile world, where customers need to see changes and modifications as soon as possible, rather than waiting for a release that takes years.

We have talked about some of the challenges of monolith applications. As the application size grows and it becomes complex, it is not possible to manage a monolith application easily. Due to these challenges, the industry has explored different approaches to manage applications, with Microservices being a very popular solution.