This trace follows the actual state transitions behind the companion Design Patterns as Named Tradeoffs. It describes a common execution path; implementation details can vary, so keep the contract separate from the mechanism.
Step 1: Identify a real variation point
A design pattern names a recurring structure for solving a family of design problems. It is useful when it communicates roles and tradeoffs faster than a bespoke explanation. A pattern is not a rule to apply whenever its name sounds familiar.
Step 2: Name the shared operation
The Strategy pattern lets a caller choose among interchangeable algorithms behind a common interface. For example, a shipping quote service can select a carrier strategy based on destination and package characteristics. Tests can exercise each strategy independently and verify the selection policy separately.
Step 3: Select a strategy implementation
A strategy interface is useful when the caller must choose among genuinely interchangeable algorithms; if there is only one fixed behavior, the extra dispatch layer may add no value.
At this point, record the state that changed and check the invariant before advancing. If the operation repeats, make clear which values persist and which are recomputed.
Step 4: Execute through the interface
An abstraction becomes costly when it hides one simple behavior behind many interfaces or forces callers to understand irrelevant extension points. Prefer the smallest seam that supports a real variation. Patterns can guide a design review, but local constraints decide whether their structure helps.
Step 5: Check abstraction cost
You have two pricing rules and expect a third soon. Compare a conditional statement with a strategy interface, identifying what new requirement would make the interface worthwhile.
The trace is complete when the result satisfies the stated contract. Compare this model with the concrete runtime or system you are studying before making a performance claim.