Modern architectures look like this: https://github.com/donnemartin/system-design-primer https://github.com/donnemartin/system-design-primer All of the pieces

Modern architectures look like this: https://github.com/donnemartin/system-design-primer https://github.com/donnemartin/system-design-primer All of the pieces have complex and emergent behaviors as they interact with each other. Many (most?) are hosted, not even fully in control of the team using them. Recreating this environment in a hermetic way for testing is either actually impossible, or so cost prohibitive as to be practically infeasible.