Decision Classification

Not all architecture decisions carry the same weight. A decision about the primary database technology affects the system for years. A decision about the logging format affects a single configuration file. Writing the same level of ADR for both wastes effort on the small decision and underserves the large one. Classification — sorting decisions by impact and reversibility — ensures that analysis effort is proportional to decision weight.

1. Type 1: Irreversible and High-Impact Database technology, cloud provider, programming language, architectural pattern. These decisions shape the system for years and cost months to reverse. Require a full ADR with options analysis, evaluation matrix, proof of concept results, and review trigger. Budget two to four hours for the analysis. The cost of a wrong Type 1 decision dwarfs the cost of thorough analysis.
2. Type 2: Reversible and High-Impact API design patterns, caching strategy, CI/CD pipeline design. Important but changeable without rebuilding the system. Require a standard ADR with options, rationale, and review trigger. Budget thirty to sixty minutes. The analysis should be thorough but not exhaustive because the decision can be revised.
3. Type 3: Low-Impact Logging library, code formatting rules, test framework selection. Minimal blast radius. A brief decision log entry — one paragraph noting the choice and the primary reason — is sufficient. Do not write a full ADR for a decision that can be changed in an afternoon. The overhead of the documentation should not exceed the cost of reversing the decision.