docs(10_Wiki): 위키 전체 재구성 — Topic_* 폴더를 4개 카테고리로 통합 + 대규모 중복 제거

Topic_Agent/Topic_Blog/Topics/Topics_Biz/Topics_Meeting/Topics_Rag의 마크다운 지식 문서를 Topic_General/Topic_Programming/Topic_Graphic/Topic_Business 4개 카테고리로 재분류. - 중복 제거: frontmatter의 status:duplicate/merged + duplicate_of/redirect_to 필드로 자기 자신을 중복으로 선언한 리다이렉트 stub 1032개 제거, 완전 동일 내용 파일 472개 제거, 동일 파일명·다른 내용 충돌 시 더 큰(완전한) 버전만 유지(162개 제거) — 총 1639개 중복 제거. - 분류: 폴더 단위로 명확한 항목(AI_and_ML/Coding/Architecture 등 → Programming, Comfyui/Visual_Effects → Graphic, Topics_Biz/Topics_Meeting/사업 등 → Business, Poetic_Blog_Writing/창의성/Game_Design 등 → General)은 폴더 우선순위로, 나머지 혼재 폴더(Topic_Agent/Topic_Blog/Topics 루트/Thinking & Reasoning/Other/UI_UX_Assets)는 title/tags 키워드 스코어링으로 파일 단위 분류(불명확한 경우 General로 폴백). 원본 폴더명은 "From_*" 서브폴더로 보존해 추적 가능성 유지. - 최종 배치: Programming 2784 / General 1608 / Graphic 285 / Business 249 = 4926개 문서. - 에이전트 운영 상태(.astra/.agent/.obsidian/sessions/memory/_company/docs/lessons/_shared/src)는 지식 콘텐츠가 아니므로 재분류 대상에서 제외하고 원위치 유지. - Topics/Topic_email(상위 보호 폴더 Topic_email과 파일명 100% 중복) 삭제 — 보호 폴더 자체는 미변경. - 완전히 비게 된 Topic_Agent/Topic_Blog/Topics_Biz/Topics_Rag 폴더 제거.
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+---
+id: mlops-feature-store
+title: Feature Store — Feast / Tecton / online & offline
+category: Coding
+status: draft
+source_trust_level: B
+verification_status: conceptual
+created_at: 2026-05-09
+updated_at: 2026-05-09
+tags: [mlops, feature-store, vibe-coding]
+tech_stack: { language: "Python", applicable_to: ["AI", "Backend"] }
+applied_in: []
+aliases: [feature store, Feast, Tecton, online store, offline store, feature reuse]
+---
+
+# Feature Store
+
+> ML feature 의 central registry. **Train / serve consistency, low-latency online, time-correct offline**. Feast (open) / Tecton (managed).
+
+## 📖 핵심 개념
+- Online store: 빠른 조회 (Redis / DynamoDB).
+- Offline store: 학습용 (Parquet / Snowflake).
+- Time-travel: 과거 시점 feature.
+- Reuse: 한 번 정의, 여러 model.
+
+## 💻 코드 패턴
+
+### Feast 정의
+```python
+# features.py
+from feast import Entity, Feature, FeatureView, ValueType
+from datetime import timedelta
+
+user = Entity(name='user_id', value_type=ValueType.INT64)
+
+user_features = FeatureView(
+    name='user_features',
+    entities=['user_id'],
+    ttl=timedelta(days=1),
+    features=[
+        Feature(name='age', dtype=ValueType.INT32),
+        Feature(name='total_spent', dtype=ValueType.FLOAT),
+        Feature(name='days_active', dtype=ValueType.INT32),
+    ],
+    source=parquet_source,
+)
+```
+
+### 등록
+```bash
+feast apply
+# → Online + offline schema 생성
+```
+
+### Materialize (offline → online)
+```bash
+feast materialize-incremental $(date -u +"%Y-%m-%dT%H:%M:%S")
+# → 최신 feature → online store (Redis)
+```
+
+→ Cron / Airflow 가 매일 실행.
+
+### Online get (serving)
+```python
+from feast import FeatureStore
+store = FeatureStore(repo_path='.')
+
+features = store.get_online_features(
+    features=['user_features:age', 'user_features:total_spent'],
+    entity_rows=[{'user_id': 123}],
+).to_dict()
+# {'age': [25], 'total_spent': [100.5]}
+```
+
+→ Redis 가 backend = ms latency.
+
+### Historical get (training)
+```python
+import pandas as pd
+entity_df = pd.DataFrame({
+    'user_id': [123, 456, 789],
+    'event_timestamp': [t1, t2, t3],
+})
+
+train_df = store.get_historical_features(
+    entity_df=entity_df,
+    features=['user_features:age', 'user_features:total_spent'],
+).to_df()
+```
+
+→ Time-correct: t1 시점의 user 123 feature.
+
+### Train / serve consistency
+```python
+# Train (offline)
+df = store.get_historical_features(...).to_df()
+model.fit(df)
+
+# Serve (online)
+features = store.get_online_features(...).to_dict()
+pred = model.predict([features])
+
+# → 같은 transformation, 같은 schema = 일관.
+```
+
+→ 가장 큰 가치.
+
+### Time-travel join
+```
+Feature: user_total_spent (시간 따라 변경)
+Event: 2026-05-01 user 123 click
+
+→ get historical = "2026-05-01 시점의 user 123 spent" (그 후 변경 X)
+```
+
+→ Data leakage 방지.
+
+### Tecton (managed)
+```python
+@stream_feature_view(
+    source=kafka_source,
+    entities=[user],
+    mode='spark_sql',
+    aggregations=[
+        Aggregation(column='amount', function='sum', time_window=timedelta(days=1)),
+    ],
+)
+def user_daily_spend(events):
+    return f"SELECT user_id, amount, ts FROM {events}"
+```
+
+→ Streaming + windowed aggregation 지원.
+
+### Real-time aggregation
+```python
+# Streaming feature
+@stream_feature_view(
+    source=kafka,
+    aggregations=[
+        Aggregation(column='clicks', function='count', time_window=timedelta(hours=1)),
+        Aggregation(column='clicks', function='count', time_window=timedelta(days=1)),
+    ],
+)
+def user_clicks(events): ...
+```
+
+→ "지난 1시간 click 수" 가 자동 maintain.
+
+### Composition
+```python
+# Combine
+@feature_view(...)
+def user_combined(user_features, item_features):
+    return user_features.join(item_features, on='user_id')
+```
+
+### Feature versioning
+```python
+@feature_view(version='v2')
+def user_features(...): ...
+
+# v1 + v2 동시 — model 별로 사용.
+```
+
+### Push (real-time)
+```python
+# Event 발생 직후
+store.push('user_clicks', {'user_id': 123, 'clicks': 5, 'event_timestamp': now})
+```
+
+→ Online store 즉시 update.
+
+### Drift (data validation)
+```python
+# Great Expectations + Feast
+from feast.data_quality import expectation
+
+@feature_view(...)
+class UserFeatures:
+    age = Feature(
+        dtype=ValueType.INT32,
+        expectations=[expect_column_values_to_be_between('age', 0, 120)],
+    )
+```
+
+### Cost
+```
+Online: Redis / DynamoDB — pay per Read.
+Offline: Parquet on S3 — cheap.
+
+Tecton: managed — $$$, 큰 팀.
+Feast: open — infra 직접.
+```
+
+### Hopsworks (alternative)
+```
+- Free + open
+- Streaming + batch
+- Built-in model registry
+```
+
+### Vertex AI Feature Store
+```python
+from google.cloud import aiplatform_v1
+client = aiplatform_v1.FeaturestoreOnlineServingServiceClient()
+
+response = client.read_feature_values(
+    entity_type='projects/.../entityTypes/user',
+    entity_id='123',
+    feature_selector={'ids': ['age', 'total_spent']},
+)
+```
+
+### SageMaker Feature Store
+```python
+from sagemaker.feature_store.feature_group import FeatureGroup
+
+fg = FeatureGroup(name='user-features', sagemaker_session=session)
+fg.create(record_identifier_name='user_id', event_time_feature_name='ts', ...)
+
+# Online get
+client.get_record(
+    FeatureGroupName='user-features',
+    RecordIdentifierValueAsString='123',
+)
+```
+
+### Direct DB (no Feast)
+```sql
+-- Materialized view 가 single source.
+CREATE MATERIALIZED VIEW user_features AS
+SELECT
+  user_id,
+  age,
+  COUNT(orders) as order_count,
+  SUM(amount) as total_spent
+FROM users LEFT JOIN orders USING (user_id)
+GROUP BY user_id;
+
+-- Train: SELECT * FROM user_features WHERE ts < ?
+-- Serve: SELECT * FROM user_features WHERE user_id = ?
+```
+
+→ 작은 ML system 가 충분.
+
+### Feature 가 reused
+```
+3 model 가 같은 'user_total_spent' 사용.
+- 정의 1번
+- 매 model 가 reference
+
+→ 변경 한 곳, 전체 효과.
+```
+
+### Naming convention
+```
+{entity}_{aggregation}_{time}
+
+user_clicks_1h
+user_avg_session_7d
+item_views_30d
+```
+
+### Consistency checks
+```python
+# Train data 와 prod data 의 분포 비교
+train_age = pd.read_parquet('train.parquet')['age']
+prod_age = client.fetch_recent_features('age', n=10000)
+
+assert ks_2samp(train_age, prod_age).pvalue > 0.01
+```
+
+### When 안 필요
+```
+- 1 model + 1 simple feature
+- POC / 작은 demo
+- Real-time stateless feature 만 (input → pred)
+```
+
+## 🤔 의사결정 기준
+| 상황 | 추천 |
+|---|---|
+| 작은 / 1-2 model | Direct DB / materialized view |
+| Open / self-host | Feast |
+| Streaming + windowed | Tecton / Hopsworks |
+| GCP | Vertex AI |
+| AWS | SageMaker |
+| Minute-level real-time | Streaming (Tecton / Hopsworks) |
+| Daily batch | Feast + cron |
+
+## ❌ 안티패턴
+- **Train / serve schema 다름**: silent error.
+- **No time-travel**: data leakage.
+- **Online TTL 없음**: stale.
+- **Materialize 안 함**: latency 큰.
+- **Feature 정의 흩어짐**: drift.
+- **Push + batch + 다른 logic**: 의도 X.
+- **Privacy 무시**: PII 가 store 에.
+
+## 🤖 LLM 활용 힌트
+- Feature store 가 train/serve consistency 의 답.
+- Time-travel = data leakage 방지.
+- 작은 system 가 materialized view 충분.
+- Streaming + window 가 필요 시 Tecton.
+
+## 🔗 관련 문서
+- [[MLOps_Model_Registry]]
+- [[Data_Eng_Streaming_ETL]]
+- [[DB_Time_Series_Patterns]]