1---2name: data-engineering3description: Data pipeline architecture, ETL/ELT patterns, data quality, batch vs stream processing, orchestration, and data governance principles.4---56# Data Engineering Principles78Guidelines for building reliable, scalable data pipelines and platforms.910## When to Invoke11- Designing data pipelines (ETL/ELT)12- Evaluating batch vs stream processing13- Data quality and governance requirements14- Data warehouse/lake architecture decisions1516## Pipeline Architecture1718### Design Principles191. **Idempotent pipelines** — re-running produces same result. Use upserts, not inserts.202. **Schema evolution** — handle new fields without breaking consumers.213. **Exactly-once processing** — deduplication at ingestion, idempotency keys.224. **Incremental processing** — process only new/changed data, not full reloads.2324### Patterns25| Pattern | When to Use |26|---|---|27| **Batch ETL** | Scheduled, high volume, latency-tolerant |28| **Streaming** | Real-time, event-driven, low latency |29| **Lambda** | Both batch and stream (complexity trade-off) |30| **Kappa** | Stream-only, reprocessing via replay |31| **Medallion** | Bronze (raw) → Silver (cleaned) → Gold (curated) |3233## Data Quality3435### Checks (Non-Negotiable)36- **Completeness** — no unexpected nulls in required fields37- **Uniqueness** — no duplicate records on primary keys38- **Referential integrity** — foreign keys resolve39- **Freshness** — data arrives within SLA window40- **Volume** — row counts within expected range (±threshold)4142### Framework43```44Source → Validate (schema, nulls, types) → Transform → Validate (business rules) → Load → Verify (counts, checksums)45```4647## Orchestration4849| Tool | Strength |50|---|---|51| Apache Airflow | Most mature, Python-native, DAG-based |52| Dagster | Type-safe, asset-oriented, modern |53| Prefect | Pythonic, flow-based, cloud-native |5455### Best Practices56- DAGs should be idempotent and retriable57- Separate orchestration from computation58- Use backfill capabilities for historical reprocessing59- Alert on SLA breaches, not just failures6061## Data Modeling6263| Model | When |64|---|---|65| **Star schema** | Analytics, BI dashboards, simple queries |66| **Data Vault** | Enterprise, auditability, multiple sources |67| **Dimensional** | Aggregated reporting, OLAP |6869## Governance70- Data lineage tracked (source → transformation → destination)71- Access controls per dataset/table72- PII identified and masked/encrypted73- Retention policies documented and automated7475## Related76- Database Design Principles @.gemini/skills/database-design-principles/SKILL.md77- SQL Idioms @.gemini/skills/sql-idioms/SKILL.md78- Logging and Observability Principles @.gemini/skills/logging-and-observability-principles/SKILL.md