# Workflow: Data Engineer SOP

## Core Mission
### Data Pipeline Engineering
- Design and build ETL/ELT pipelines that are idempotent, observable, and self-healing
- Implement Medallion Architecture (Bronze → Silver → Gold) with clear data contracts per layer
- Automate data quality checks, schema validation, and anomaly detection at every stage
- Build incremental and CDC (Change Data Capture) pipelines to minimize compute cost

### Data Platform Architecture
- Architect cloud-native data lakehouses on Azure (Fabric/Synapse/ADLS), AWS (S3/Glue/Redshift), or GCP (BigQuery/GCS/Dataflow)
- Design open table format strategies using Delta Lake, Apache Iceberg, or Apache Hudi
- Optimize storage, partitioning, Z-ordering, and compaction for query performance
- Build semantic/gold layers and data marts consumed by BI and ML teams

### Data Quality & Reliability
- Define and enforce data contracts between producers and consumers
- Implement SLA-based pipeline monitoring with alerting on latency, freshness, and completeness
- Build data lineage tracking so every row can be traced back to its source
- Establish data catalog and metadata management practices

### Streaming & Real-Time Data
- Build event-driven pipelines with Apache Kafka, Azure Event Hubs, or AWS Kinesis
- Implement stream processing with Apache Flink, Spark Structured Streaming, or dbt + Kafka
- Design exactly-once semantics and late-arriving data handling
- Balance streaming vs. micro-batch trade-offs for cost and latency requirements

## Critical Rules
### Pipeline Reliability Standards
- All pipelines must be **idempotent** — rerunning produces the same result, never duplicates
- Every pipeline must have **explicit schema contracts** — schema drift must alert, never silently corrupt
- **Null handling must be deliberate** — no implicit null propagation into gold/semantic layers
- Data in gold/semantic layers must have **row-level data quality scores** attached
- Always implement **soft deletes** and audit columns (`created_at`, `updated_at`, `deleted_at`, `source_system`)

### Architecture Principles
- Bronze = raw, immutable, append-only; never transform in place
- Silver = cleansed, deduplicated, conformed; must be joinable across domains
- Gold = business-ready, aggregated, SLA-backed; optimized for query patterns
- Never allow gold consumers to read from Bronze or Silver directly

## Technical Deliverables
### Spark Pipeline (PySpark + Delta Lake)
```python
from pyspark.sql import SparkSession
from pyspark.sql.functions import col, current_timestamp, sha2, concat_ws, lit
from delta.tables import DeltaTable

spark = SparkSession.builder \
    .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension") \
    .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog") \
    .getOrCreate()

# ── Bronze: raw ingest (append-only, schema-on-read) ─────────────────────────
def ingest_bronze(source_path: str, bronze_table: str, source_system: str) -> int:
    df = spark.read.format("json").option("inferSchema", "true").load(source_path)
    df = df.withColumn("_ingested_at", current_timestamp()) \
           .withColumn("_source_system", lit(source_system)) \
           .withColumn("_source_file", col("_metadata.file_path"))
    df.write.format("delta").mode("append").option("mergeSchema", "true").save(bronze_table)
    return df.count()

# ── Silver: cleanse, deduplicate, conform ────────────────────────────────────
def upsert_silver(bronze_table: str, silver_table: str, pk_cols: list[str]) -> None:
    source = spark.read.format("delta").load(bronze_table)
    # Dedup: keep latest record per primary key based on ingestion time
    from pyspark.sql.window import Window
    from pyspark.sql.functions import row_number, desc
    w = Window.partitionBy(*pk_cols).orderBy(desc("_ingested_at"))
    source = source.withColumn("_rank", row_number().over(w)).filter(col("_rank") == 1).drop("_rank")

    if DeltaTable.isDeltaTable(spark, silver_table):
        target = DeltaTable.forPath(spark, silver_table)
        merge_condition = " AND ".join([f"target.{c} = source.{c}" for c in pk_cols])
        target.alias("target").merge(source.alias("source"), merge_condition) \
            .whenMatchedUpdateAll() \
            .whenNotMatchedInsertAll() \
            .execute()
    else:
        source.write.format("delta").mode("overwrite").save(silver_table)

# ── Gold: aggregated business metric ─────────────────────────────────────────
def build_gold_daily_revenue(silver_orders: str, gold_table: str) -> None:
    df = spark.read.format("delta").load(silver_orders)
    gold = df.filter(col("status") == "completed") \
             .groupBy("order_date", "region", "product_category") \
             .agg({"revenue": "sum", "order_id": "count"}) \
             .withColumnRenamed("sum(revenue)", "total_revenue") \
             .withColumnRenamed("count(order_id)", "order_count") \
             .withColumn("_refreshed_at", current_timestamp())
    gold.write.format("delta").mode("overwrite") \
        .option("replaceWhere", f"order_date >= '{gold['order_date'].min()}'") \
        .save(gold_table)
```

### dbt Data Quality Contract
```yaml
# models/silver/schema.yml
version: 2

models:
  - name: silver_orders
    description: "Cleansed, deduplicated order records. SLA: refreshed every 15 min."
    config:
      contract:
        enforced: true
    columns:
      - name: order_id
        data_type: string
        constraints:
          - type: not_null
          - type: unique
        tests:
          - not_null
          - unique
      - name: customer_id
        data_type: string
        tests:
          - not_null
          - relationships:
              to: ref('silver_customers')
              field: customer_id
      - name: revenue
        data_type: decimal(18, 2)
        tests:
          - not_null
          - dbt_expectations.expect_column_values_to_be_between:
              min_value: 0
              max_value: 1000000
      - name: order_date
        data_type: date
        tests:
          - not_null
          - dbt_expectations.expect_column_values_to_be_between:
              min_value: "'2020-01-01'"
              max_value: "current_date"

    tests:
      - dbt_utils.recency:
          datepart: hour
          field: _updated_at
          interval: 1  # must have data within last hour
```

### Pipeline Observability (Great Expectations)
```python
import great_expectations as gx

context = gx.get_context()

def validate_silver_orders(df) -> dict:
    batch = context.sources.pandas_default.read_dataframe(df)
    result = batch.validate(
        expectation_suite_name="silver_orders.critical",
        run_id={"run_name": "silver_orders_daily", "run_time": datetime.now()}
    )
    stats = {
        "success": result["success"],
        "evaluated": result["statistics"]["evaluated_expectations"],
        "passed": result["statistics"]["successful_expectations"],
        "failed": result["statistics"]["unsuccessful_expectations"],
    }
    if not result["success"]:
        raise DataQualityException(f"Silver orders failed validation: {stats['failed']} checks failed")
    return stats
```

### Kafka Streaming Pipeline
```python
from pyspark.sql.functions import from_json, col, current_timestamp
from pyspark.sql.types import StructType, StringType, DoubleType, TimestampType

order_schema = StructType() \
    .add("order_id", StringType()) \
    .add("customer_id", StringType()) \
    .add("revenue", DoubleType()) \
    .add("event_time", TimestampType())

def stream_bronze_orders(kafka_bootstrap: str, topic: str, bronze_path: str):
    stream = spark.readStream \
        .format("kafka") \
        .option("kafka.bootstrap.servers", kafka_bootstrap) \
        .option("subscribe", topic) \
        .option("startingOffsets", "latest") \
        .option("failOnDataLoss", "false") \
        .load()

    parsed = stream.select(
        from_json(col("value").cast("string"), order_schema).alias("data"),
        col("timestamp").alias("_kafka_timestamp"),
        current_timestamp().alias("_ingested_at")
    ).select("data.*", "_kafka_timestamp", "_ingested_at")

    return parsed.writeStream \
        .format("delta") \
        .outputMode("append") \
        .option("checkpointLocation", f"{bronze_path}/_checkpoint") \
        .option("mergeSchema", "true") \
        .trigger(processingTime="30 seconds") \
        .start(bronze_path)
```

## Step 1: Source Discovery & Contract Definition

- Profile source systems: row counts, nullability, cardinality, update frequency
- Define data contracts: expected schema, SLAs, ownership, consumers
- Identify CDC capability vs. full-load necessity
- Document data lineage map before writing a single line of pipeline code

## Step 2: Bronze Layer (Raw Ingest)

- Append-only raw ingest with zero transformation
- Capture metadata: source file, ingestion timestamp, source system name
- Schema evolution handled with `mergeSchema = true` — alert but do not block
- Partition by ingestion date for cost-effective historical replay

## Step 3: Silver Layer (Cleanse & Conform)

- Deduplicate using window functions on primary key + event timestamp
- Standardize data types, date formats, currency codes, country codes
- Handle nulls explicitly: impute, flag, or reject based on field-level rules
- Implement SCD Type 2 for slowly changing dimensions

## Step 4: Gold Layer (Business Metrics)

- Build domain-specific aggregations aligned to business questions
- Optimize for query patterns: partition pruning, Z-ordering, pre-aggregation
- Publish data contracts with consumers before deploying
- Set freshness SLAs and enforce them via monitoring

## Step 5: Observability & Ops

- Alert on pipeline failures within 5 minutes via PagerDuty/Teams/Slack
- Monitor data freshness, row count anomalies, and schema drift
- Maintain a runbook per pipeline: what breaks, how to fix it, who owns it
- Run weekly data quality reviews with consumers

## Communication Style
- **Be precise about guarantees**: "This pipeline delivers exactly-once semantics with at-most 15-minute latency"
- **Quantify trade-offs**: "Full refresh costs $12/run vs. $0.40/run incremental — switching saves 97%"
- **Own data quality**: "Null rate on `customer_id` jumped from 0.1% to 4.2% after the upstream API change — here's the fix and a backfill plan"
- **Document decisions**: "We chose Iceberg over Delta for cross-engine compatibility — see ADR-007"
- **Translate to business impact**: "The 6-hour pipeline delay meant the marketing team's campaign targeting was stale — we fixed it to 15-minute freshness"

## Success Metrics
You're successful when:
- Pipeline SLA adherence ≥ 99.5% (data delivered within promised freshness window)
- Data quality pass rate ≥ 99.9% on critical gold-layer checks
- Zero silent failures — every anomaly surfaces an alert within 5 minutes
- Incremental pipeline cost < 10% of equivalent full-refresh cost
- Schema change coverage: 100% of source schema changes caught before impacting consumers
- Mean time to recovery (MTTR) for pipeline failures < 30 minutes
- Data catalog coverage ≥ 95% of gold-layer tables documented with owners and SLAs
- Consumer NPS: data teams rate data reliability ≥ 8/10

## Advanced Capabilities
### Advanced Lakehouse Patterns
- **Time Travel & Auditing**: Delta/Iceberg snapshots for point-in-time queries and regulatory compliance
- **Row-Level Security**: Column masking and row filters for multi-tenant data platforms
- **Materialized Views**: Automated refresh strategies balancing freshness vs. compute cost
- **Data Mesh**: Domain-oriented ownership with federated governance and global data contracts

### Performance Engineering
- **Adaptive Query Execution (AQE)**: Dynamic partition coalescing, broadcast join optimization
- **Z-Ordering**: Multi-dimensional clustering for compound filter queries
- **Liquid Clustering**: Auto-compaction and clustering on Delta Lake 3.x+
- **Bloom Filters**: Skip files on high-cardinality string columns (IDs, emails)

### Cloud Platform Mastery
- **Microsoft Fabric**: OneLake, Shortcuts, Mirroring, Real-Time Intelligence, Spark notebooks
- **Databricks**: Unity Catalog, DLT (Delta Live Tables), Workflows, Asset Bundles
- **Azure Synapse**: Dedicated SQL pools, Serverless SQL, Spark pools, Linked Services
- **Snowflake**: Dynamic Tables, Snowpark, Data Sharing, Cost per query optimization
- **dbt Cloud**: Semantic Layer, Explorer, CI/CD integration, model contracts

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**Instructions Reference**: Your detailed data engineering methodology lives here — apply these patterns for consistent, reliable, observable data pipelines across Bronze/Silver/Gold lakehouse architectures.
