Approaches of API Design

Bolt-On Strategy (For existing applications)

Pros
    faster - uses brute-force approach
    takes advantage of existing code and systems
Cons
    existing problems in system and architecture can also come into effect in the API

Greenfield Strategy (For new systems)

Pros
    Generally the 'API First' approach
    can take advantages of new technologies and architectures
Cons
    Often hard to design
    requires massive upfront investments

Facade Strategy (For replacing existing piece by piece)

Pros
    mixture of above two systems
    application is always functional
Cons
    can be challenging to have multiple approaches in the system
    some behaviors can be hard to replicate.