pragmatic-programmer

DRY applies to knowledge, not textual similarity. Two identical code blocks serving different business rules are not duplication; merge only when a change to one must change the other. The reference guide classifies four duplication types with mitigations for each.

A tracer bullet is thin but production-quality end-to-end code you keep and extend, while a prototype is a disposable exploration of one risky question. Use tracer bullets for integration risk and vague requirements; use prototypes for feasibility questions.

Use assertions for conditions that should never happen, such as violated invariants, and crash immediately. Use error handling for conditions that might happen, like network failures or invalid user input, and recover gracefully.

Place the dependency behind your own interface using the repository or adapter pattern so business logic never calls vendor APIs directly. Apply the forking-road test: if you could not swap the dependency in a week, add an abstraction layer.

Use PERT: compute (Optimistic + 4 x Most Likely + Pessimistic) / 6 and communicate ranges with confidence levels instead of single-point commitments. Decompose work into tasks of one day or less and keep an estimation log to calibrate your bias over time.

Skip abstraction for throwaway prototypes, trivially replaceable dependencies, or decisions with strong evidence of permanence. If the abstraction costs more than the future switch and change is unlikely, YAGNI applies.