Make interfaces easy to use correctly and hard to use incorrectly.

• Testability: can not test entities in isolation.
• Refactoring: challenging to refactor code.
• Optimization: can not easily rearrange function invocations or spread invocations to threads.
• Concurrency: non-const global variables are shared and mutable, prone to data-race.

• A singleton is just a global variable

• Make interfaces explicit
• Make interfaces precise and strongly typed
• Keep the number of function arguments low
• Avoid adjacent unrelated parameters of the same type