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Data Pipelines

A senior data engineer's decision-making framework for building production data pipelines. This skill covers the five pillars of data engineering - ingestion patterns (ETL vs ELT), orchestration (Airflow), transformation (dbt), large-scale processing (Spark), and architecture choices (streaming vs batch) - with emphasis on when to use each pattern and the trade-offs involved. Designed for engineers who need opinionated guidance on building reliable, observable, and maintainable data infrastructure.

When to use this skill

Trigger this skill when the user:

• Designs an ETL or ELT pipeline from scratch
• Writes or debugs an Airflow DAG
• Creates dbt models, tests, or macros
• Optimizes a Spark job (shuffles, partitioning, memory tuning)
• Decides between streaming and batch processing
• Implements incremental loads or change data capture (CDC)
• Plans a data warehouse or lakehouse architecture
• Needs data quality checks, schema evolution, or pipeline monitoring

Do NOT trigger this skill for:

• BI/analytics dashboard design or visualization (use an analytics skill)
• ML model training or feature engineering (use an ML/data-science skill)

Key principles

1. Idempotency is non-negotiable - Every pipeline run with the same input must produce the same output. Design for safe re-runs from day one. Use date partitions, merge keys, or upsert logic so that retries never corrupt data.

2. Prefer ELT over ETL in modern stacks - Load raw data first, transform in the warehouse. This preserves the source of truth, enables schema-on-read, and lets analysts iterate on transformations without re-ingesting. ETL still wins when you need to filter sensitive data before it lands.

3. Partition and increment, never full-reload - Full table scans on every run do not scale. Use incremental models (dbt), date-partitioned loads, and watermarks to process only what changed. Fall back to full reload only for small reference tables or disaster recovery.

4. Orchestrate, don't script - A cron job calling a Python script is not a pipeline. Use a proper orchestrator (Airflow, Dagster, Prefect) for retries, dependency management, backfills, and observability. The orchestrator should own scheduling and state, not your application code.

5. Test data like code - Schema tests, row count checks, uniqueness constraints, and freshness SLAs are not optional. dbt tests, Great Expectations, or custom assertions should gate every pipeline stage. Bad data downstream is more expensive than a failed pipeline.

Core concepts

Data pipelines move data from sources (databases, APIs, event streams) through transformations to destinations (warehouses, lakes, serving layers). The two dominant patterns are ETL (extract-transform-load) and ELT (extract-load-transform). ETL transforms data in-flight before loading; ELT loads raw data first and transforms inside the destination.

The pipeline lifecycle has four stages: ingestion (getting data in), orchestration (scheduling and dependency management), transformation (cleaning, joining, aggregating), and serving (making data available to consumers). Each stage has specialized tools: Fivetran/Airbyte for ingestion, Airflow/Dagster for orchestration, dbt for transformation, and the warehouse itself (BigQuery, Snowflake, Redshift) for serving.

Streaming vs batch is an architecture decision, not a tool choice. Batch processes data in time-windowed chunks (hourly, daily). Streaming processes events continuously as they arrive. Most organizations need both - batch for historical aggregations and streaming for real-time dashboards or alerting. The Lambda architecture runs both in parallel; the Kappa architecture uses a single streaming layer for everything.

Common tasks

Design an ETL/ELT pipeline

Decide the pattern based on your constraints:

Need to filter PII before landing?       -> ETL (transform before load)
Want analysts to iterate on transforms?   -> ELT (load raw, transform in warehouse)
Source data volume > 1TB per load?        -> ELT with Spark for heavy transforms
Small reference data < 100MB?             -> Direct load, skip the framework

Standard ELT flow:

1. Extract from source (API, database CDC, file drop)
2. Load raw data to staging layer (preserve original schema)
3. Transform in warehouse using dbt (staging -> intermediate -> mart)
4. Test data quality at each layer boundary
5. Serve from mart layer to downstream consumers

Always land raw data in an immutable staging layer. Transformations should read from staging, never modify it. This gives you a re-playable source of truth.

Write an Airflow DAG

A well-structured DAG separates orchestration from business logic:

from airflow import DAG
from airflow.operators.python import PythonOperator
from airflow.providers.google.cloud.operators.bigquery import BigQueryInsertJobOperator
from datetime import datetime, timedelta

default_args = {
    "owner": "data-team",
    "retries": 2,
    "retry_delay": timedelta(minutes=5),
    "execution_timeout": timedelta(hours=2),
}

with DAG(
    dag_id="daily_orders_pipeline",
    schedule="0 6 * * *",
    start_date=datetime(2024, 1, 1),
    catchup=False,
    default_args=default_args,
    tags=["production", "orders"],
) as dag:

    extract = PythonOperator(
        task_id="extract_orders",
        python_callable=extract_orders_fn,
        op_kwargs={"ds": "{{ ds }}"},
    )

    transform = BigQueryInsertJobOperator(
        task_id="transform_orders",
        configuration={"query": {"query": "{% include 'sql/transform_orders.sql' %}"}},
    )

    test = PythonOperator(
        task_id="test_row_counts",
        python_callable=assert_row_counts,
    )

    extract >> transform >> test

Use catchup=False for most production DAGs unless you explicitly need backfill behavior. Set execution_timeout to prevent zombie tasks.

Build dbt models

Structure dbt projects in three layers:

models/
  staging/          -- 1:1 with source tables, light renaming/casting
    stg_orders.sql
    stg_customers.sql
  intermediate/     -- business logic joins, deduplication
    int_orders_enriched.sql
  marts/            -- final consumer-facing tables
    fct_daily_revenue.sql
    dim_customers.sql

Example incremental model:

-- models/staging/stg_orders.sql
{{
  config(
    materialized='incremental',
    unique_key='order_id',
    on_schema_change='append_new_columns'
  )
}}

select
    order_id,
    customer_id,
    order_total,
    cast(created_at as timestamp) as ordered_at
from {{ source('raw', 'orders') }}

{% if is_incremental() %}
where created_at > (select max(ordered_at) from {{ this }})
{% endif %}

Always define unique_key for incremental models. Without it, dbt appends instead of merging, causing duplicates on re-runs.

Optimize a Spark job

The three most common Spark performance killers and their fixes:

from pyspark.sql import SparkSession
from pyspark.sql.functions import broadcast

spark = SparkSession.builder.appName("optimize_example").getOrCreate()

# Broadcast small dimension table to avoid shuffle
orders = spark.read.parquet("s3://data/orders/")
products = spark.read.parquet("s3://data/products/")  # < 100MB

enriched = orders.join(broadcast(products), "product_id", "left")

# Repartition by date before writing to avoid small files
enriched.repartition("order_date").writ