2nd/10_Wiki/Topics/Other/decisions.md

---
id: wiki-2026-0508-decisions
title: Architecture Decision Records (ADR)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [ADR, Decision Log, Architecture Decisions]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [adr, architecture, decisions, documentation, engineering]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: markdown
  framework: adr-tools
---

# Architecture Decision Records (ADR)

## 매 한 줄
> **"매 immutable record of architectural choices, written in the moment of decision"**. Michael Nygard 가 2011 년 originally proposed; 2026 modern engineering org 의 standard practice — single ADR file per decision, append-only, version-controlled in repo alongside code.

## 매 핵심

### 매 ADR file 구조
- **Title**: short noun phrase (e.g., "Use PostgreSQL for transactional store").
- **Status**: Proposed → Accepted → Deprecated → Superseded.
- **Context**: forces 와 constraints 의 background.
- **Decision**: what we will do — active voice, declarative.
- **Consequences**: positive / negative / neutral trade-offs.

### 매 lifecycle
- ADR 는 immutable — supersede 하려면 new ADR 작성, old 의 status 를 `Superseded by ADR-N` 로 update.
- PR review 의 part — engineering team 의 collective signoff.
- ADR-001 부터 sequential numbering, 절대 renumbering 안 함.

### 매 응용
1. Microservices boundary 의 결정 (e.g., service split rationale).
2. Data store / message queue 의 선택 (Postgres vs DynamoDB).
3. Auth flow / API style (REST vs GraphQL vs gRPC).
4. Build tooling / CI 의 stack lock-in.

## 💻 패턴

### Nygard ADR template (md)
```markdown
# ADR-007: Adopt PostgreSQL for primary OLTP

## Status
Accepted (2026-05-08)

## Context
We need an ACID-compliant store for orders.
Read/write ratio is 3:1, peak 2k QPS.
Team has Postgres ops experience; Aurora / RDS managed offering available.

## Decision
We will use Amazon RDS for PostgreSQL 16 as the primary OLTP store.

## Consequences
+ Strong consistency, mature ecosystem.
+ Existing in-house expertise reduces ramp.
- Vendor lock-in to AWS RDS.
- Need to manage VACUUM tuning at >10M rows/table.
```

### MADR template (richer variant)
```markdown
---
status: accepted
date: 2026-05-08
deciders: [alice, bob, carol]
consulted: [security-team]
informed: [eng-all]
---
# Use Kafka for event bus

## Context and Problem Statement
How do we propagate domain events across 8 microservices?

## Considered Options
- Kafka
- RabbitMQ
- AWS SNS/SQS

## Decision Outcome
Chosen: Kafka, because durable replay + partition ordering matter.

### Positive Consequences
- Replayable history (compaction).
- High throughput (>1M msg/s).

### Negative Consequences
- Operational complexity (ZK / KRaft).
```

### adr-tools CLI workflow
```bash
# Init in repo
adr init doc/adr

# Create new ADR
adr new "Use Postgres for OLTP"
# -> doc/adr/0007-use-postgres-for-oltp.md

# Supersede old decision
adr new -s 3 "Replace MongoDB with Postgres"
# -> auto-marks ADR-0003 as Superseded
```

### Lightweight in-PR ADR (modern variant)
```markdown
<!-- .github/PULL_REQUEST_TEMPLATE.md -->
## Decision Record
- **Choice**: <one sentence>
- **Why now**: <trigger>
- **Alternatives considered**: <list>
- **Trade-offs accepted**: <list>
```

### ADR index generation (CI script)
```python
# scripts/build_adr_index.py
import re, pathlib
adrs = sorted(pathlib.Path("doc/adr").glob("[0-9]*.md"))
lines = ["# ADR Index\n"]
for f in adrs:
    title = next(l[2:].strip() for l in f.read_text().splitlines() if l.startswith("# "))
    status = re.search(r"## Status\n(\w+)", f.read_text()).group(1)
    lines.append(f"- [{f.stem}]({f.name}) — {title} ({status})")
pathlib.Path("doc/adr/README.md").write_text("\n".join(lines))
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Solo / prototype | Skip ADR — overhead > benefit |
| Team ≥ 3, multi-quarter project | Nygard ADR mandatory |
| Regulated env (FDA, SOC2) | MADR + sign-off metadata |
| Very fast-moving startup | In-PR lightweight ADR |
| Architecture review board | MADR with `consulted` / `informed` |

**기본값**: Nygard ADR in `doc/adr/`, accepted via PR review, supersession over deletion.

## 🔗 Graph
- 부모: [[Software_Architecture_Patterns]] · [[소프트웨어_설계_원칙_및_디자인_패턴|Engineering_Principles]]
- 응용: [[Microservices_Architecture]] · [[Codebase_Onboarding]]
- Adjacent: [[Pull_Request_and_Issue_Tracking]]

## 🤖 LLM 활용
**언제**: Engineering team ≥ 3, decisions have multi-month consequences, onboarding context value matters.
**언제 X**: Personal project, throwaway prototype, decisions reversible in <1 day.

## ❌ 안티패턴
- **Edit history erasure**: ADR 를 mutate — supersession chain 의 의도 가 lost.
- **Decision-by-Slack**: ADR 없이 채팅 만 의 합의 — 6 개월 후 누구 도 rationale 모름.
- **Over-documentation**: 매 trivial choice 의 ADR — signal-to-noise drops.
- **Status drift**: Accepted ADR 가 production 의 actual state 와 diverge — periodic audit 필요.

## 🧪 검증 / 중복
- Verified (Michael Nygard 2011 original post; ThoughtWorks Tech Radar "Adopt"; AWS / Spotify / GitHub public ADR repos).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full ADR taxonomy + Nygard/MADR templates + adr-tools workflow |