Behavioral Patterns: Case Studies

These are synthetic design scenarios, not reports of measured production outcomes. Use the GoF book index for pattern vocabulary and the contracts for executable scope.

Pricing variants

A checkout adds promotions and carrier policies. Compare a table of fixed values, callables and objects. Choose Strategy only when policy independence, testing or ownership earns the boundary. Require integer-unit and rounding contracts plus a selection table. Failure witness: one provider returns dollars where cents are expected. More classes do not resolve ambiguous policy precedence.

Paid-order notifications

Analytics and views react to accepted payment. Compare direct calls, local Observer and durable queued delivery under explicit requirements. State who owns subscriptions and whether a callback failure affects the caller's result. Failure witness: a callback throws after payment succeeded and the caller retries the payment. A callback exception cannot automatically roll back an external charge.

Editor history

A drawing editor needs clear, move and recolor undo. Compare immutable snapshots, inverse operations and shared persistent versions under measured memory constraints. Memento captures originator-defined state; Command captures intent/execution. Failure witness: a shallow shape snapshot changes when the live shape mutates. Snapshots are valid when they satisfy the budget; they are not inherently naive.

Document lifecycle

Draft/review/published/archived has rejected commands and stale approvals. Compare a transition table with State objects. Keep public status/error observations and one state authority. Failure witness: a previous review's approval arrives after revise/submit and is accepted using only the label review. Add generation semantics as an intentional contract extension.

Ordered approvals

Every required expense check must approve before acceptance. Compare a straightforward function with a handler pipeline. A first-match allow chain is wrong when it skips mandatory checks. Failure witness: manager allow prevents finance rejection from running. Specify ordering, continuation, failure and terminal result before choosing a Chain-shaped implementation.

For each scenario submit one tradeoff memo, one trace and one simpler alternative. These can reuse practice/project evidence; no separate mandatory project count.