I will explain and give short hints that might help you better understand these three terms and why each is important in delivering quality products.
Reliability
A reliable application is an application trusted by users to perform its function without failing. It should be able to tolerate and minimize hardware failures and any software bug or human error.
What can you do to build a reliable application?
Provide a testing environment where the application can run with real data without affecting real users.
Test the application with all types of tests: automated testing, integration tests, end-to-end tests, etc.
Monitor your application and track all the errors that might appear in your application.
Penetration testing is mandatory if your system needs strong security and helps you to increase confidence in your product.
No system is built error-free, but having a good setup can help you identify and fix the errors quickly, minimizing the risks of failures in the production environment.
Scalability
There is no scaling formula that you can apply for all systems. You must identify the load parameters.
Your problem might be the volume of reads, the volume of writes, the response time requirements, or another. The solution is different, depending on what you need to scale.
One sure thing is that you should make improvements based on real measurements. Set up tools to monitor, measure, and identify what you want to scale.
It's important to note that scalability is not only about adding more resources to handle more traffic. It is also about improving the system's design, optimizing the code and managing the data growth.
Maintainability
As a software engineer, my job is to design applications so that new engineers can easily understand my code and it is easy to adapt to new laws/regulations or business decisions.
Keep it stupid simple!
This design principle suggests that most systems work best if they are kept simple rather than made complex. It is often used as a reminder to designers and developers to avoid unnecessary complexity and to prioritize simplicity in their work.
The phrase reminds people that simple solutions are often more effective and easier to use than complex ones.
Good abstraction layers remove complexity.
To understand this, you can think about an application where you fetched some data from a server. Imagine how it would be to implement the whole TCP/HTTPS protocol only to fetch some data. This would add too much complexity.
Your machine only runs code made of 0 and 1. Imagine writing your application only with 1 and 0. Luckily, we have abstractions that allow us to write code blocks with variables, if statements, for loops, etc.
This is the power of abstraction. Build the application architecture in blocks with single responsibilities. Not only that everything will be much easier, but it will also help during bug investigations.
Final thoughts
It is essential to know at what stage the application is in the development process and what are the critical parts to be solved. If you are building only an MVP, you might not need to invest the time in scaling the application but in adding all the required features. Otherwise, you would waste time optimizing a product that is not yet tested on the market.
All the mentioned things have a direct impact on your business and should be treated with full responsibility by the engineering team at its proper time:
Reliability keeps your users/customers on your platform.
Scalability helps you to expand, handle more users/customers and data.
Maintainance is where your major part of the budget is going: fixing bugs, paying technical debt, adding new features, etc.
