The summary.
PATTERNS ARE A VOCABULARY, NOT A GOAL
Applying them because they are known produces complexity with no benefit and code harder to follow than the direct version.
Write the straightforward solution first, and refactor toward a pattern when the need actually appears.
LEARN THE PROBLEMS THEY ADDRESS, NOT THE IMPLEMENTATIONS
The implementation varies by language; the problem does not.
THE MOST VALUABLE IDEA IN THE CREATIONAL PATTERNS
Not creating dependencies inside the things that use them. That is what makes code testable, because they can be substituted.
Singletons introduce global state, hide dependencies and make testing difficult — pass the instance in instead.
SEVERAL BEHAVIOURAL PATTERNS REPLACE CONDITIONAL CHAINS WITH POLYMORPHISM
APPLY TIMEOUTS TO EVERYTHING EXTERNAL, AND MAKE OPERATIONS SAFE TO REPEAT
Retries are inevitable, and a hanging dependency without a timeout holds resources indefinitely.
Backoff must increase and be randomised, or many clients retry simultaneously and worsen the problem.
ANTI-PATTERNS ARISE GRADUALLY, EACH STEP SEEMING REASONABLE
One conceptual change requiring edits in many places means a missing abstraction, or boundaries in the wrong place.
