Creating a New Bauplan Data Pipeline

This skill guides you through creating a new bauplan data pipeline project from scratch, including the project configuration and SQL/Python transformation models.

CRITICAL: Branch Safety

NEVER run pipelines on main branch. Always use a development branch.

Branch naming convention: <username>.<branch_name> (e.g., john.feature-pipeline). Get your username with bauplan info. See Workflow Checklist for exact commands.

Prerequisites

Before creating the pipeline, verify that:

1. You have a development branch (not main)
2. Source tables exist in the bauplan lakehouse (the default namespace is bauplan)
3. You understand the schema of the source tables

Pipeline as a DAG

A bauplan pipeline is a DAG of functions (models). Key rules:

1. Models: SQL or Python functions that transform data
2. Source Tables: Existing lakehouse tables - entry points to your DAG
3. Inputs: Each model can take multiple tables via bauplan.Model() references
4. Outputs: Each model produces exactly one table:
   • SQL: output name = filename (trips.sql → trips)
   • Python: output name = function name (def clean_trips() → clean_trips)
5. Topology: Implicitly defined by input references - bauplan determines execution order

Expectations: Data quality functions that take tables as input and return a boolean.

Example DAG

[lakehouse: taxi_fhvhv] ──→ [trips.sql] ──→ [clean_trips] ──→ [daily_summary]
                                                ↑
[lakehouse: taxi_zones] ────────────────────────┘

In this example:

• taxi_fhvhv and taxi_zones are source tables (already in lakehouse)
• trips.sql reads from taxi_fhvhv (SQL model, first node)
• clean_trips takes trips and taxi_zones as inputs (Python model, multiple inputs)
• daily_summary takes clean_trips as input (Python model, single input)

Required User Input

Before writing a pipeline, you MUST gather the following information from the user:

1. Pipeline purpose (required): What transformations should the DAG perform? What is the business logic or goal?
2. Source tables (required): Which tables from the lakehouse should be used as inputs? Verify they exist with bauplan table get
3. Output tables (required): Which tables should be materialized at the end of the pipeline? These are the final outputs visible to downstream consumers
4. Materialization strategy (optional): Should output tables use REPLACE (default) or APPEND?
5. Strict mode (optional): Should the pipeline run in strict mode? If yes, all CLI commands will use --strict flag, which fails on issues like output column mismatches during dry-run, allowing immediate error detection and correction.

If the user hasn't provided this information, ask before proceeding with implementation.

Strict Mode (--strict flag)

When strict mode is enabled, append --strict to all bauplan run commands:

# Without strict mode (default)
bauplan run --dry-run
bauplan run

# With strict mode enabled
bauplan run --dry-run --strict
bauplan run --strict

Benefits of strict mode:

• Fails immediately on output column mismatches
• Fails immediately if an expectation fails
• Allows you to rectify declaration errors before pipeline completion
• Recommended when iterating on pipeline development

Project Structure

A bauplan project is a folder containing:

my-project/
  bauplan_project.yml    # Required: project configuration
  model.sql              # Optional: a single SQL model, one per file
  models.py              # Optional: Python models (one file can have >1 models, or be split into multiple files)
  expectations.py        # Optional: data quality tests (if any)

bauplan_project.yml

Every project is a separate folder which requires this configuration file:

project:
  id: <unique-uuid>       # Generate a unique UUID
  name: <project_name>    # Descriptive name for the project

When to Use SQL vs Python Models

IMPORTANT: SQL models should be LIMITED to first nodes in the pipeline graph only.

• SQL models: Use ONLY for nodes that read directly from source tables in the lakehouse (tables outside your pipeline graph)
• Python models: Preferred for ALL other transformations in the pipeline

This ensures consistency and allows for better control over transformations, output schema validation, and documentation.

SQL Models (First Nodes Only)

SQL models are .sql files where:

• The filename becomes the output table name
• The FROM clause defines input tables
• Optional: Add materialization strategy as a comment

Use SQL models only when reading from existing lakehouse tables:

-- trips.sql
-- First node: reads from taxi_fhvhv table in the lakehouse
SELECT
    pickup_datetime,
    PULocationID,
    trip_miles
FROM taxi_fhvhv
WHERE pickup_datetime >= '2022-12-01'

Output table: trips (from filename) Input table: taxi_fhvhv (from FROM clause, exists in lakehouse)

Python Models (Preferred)

Python models use decorators to define transformations. They should be used for all pipeline nodes except first nodes reading from the lakehouse.

Key Decorators

• @bauplan.model() - Registers function as a model
• @bauplan.model(columns=[...]) - Specify expected output columns for validation (Optional but recommended)
• @bauplan.model(materialization_strategy='REPLACE') - Persist output to lakehouse
• @bauplan.python('3.11', pip={'pandas': '1.5.3'}) - Specify Python version and packages

Best Practice: Output Columns Validation

IMPORTANT: whenever possible, specify the columns parameter in @bauplan.model() to define the expected output schema. This enables automatic validation of your model's output.

First, check the schema of your source tables to understand input columns. Then specify the output columns based on your transformation:

# If input has columns: [id, name, age, city]
# And transformation drops 'city' column
# Then output columns should be: [id, name, age]

@bauplan.model(columns=['id', 'name', 'age'])

Best Practice: Docstrings with Output Schema

IMPORTANT: Every Python model should have a docstring describing the transformation and showing the output table structure as an ASCII table (if the table is too wide, show only key columns, if values are too large, truncate them in the cells).

@bauplan.model(columns=['id', 'name', 'age'])
@bauplan.python('3.11')
def clean_users(data=bauplan.Model('raw_users')):
    """
    Cleans user data by removing invalid entries and dropping the city column.

    | id  | name    | age |
    |-----|---------|-----|
    | 1   | Alice   | 30  |
    | 2   | Bob     | 25  |
    """
    # transformation logic
    return data.drop_columns(['city'])

I/O Pushdown with columns and filter

IMPORTANT: whenever possible, use columns and filter parameters in bauplan.Model() to restrict the data read. This enables I/O pushdown, dramatically reducing the amount of data transferred and improving performance. Do not read columns you don't need.

bauplan.Model(
    'table_name',
    columns=['col1', 'col2', 'col3'],   # Only read these columns
    filter="date >= '2022-01-01'"       # Pre-filter at storage level
)

Whenever possible, specify:

• columns: List only the columns your model actually needs
• filter: SQL-like filter expression to restrict rows at the storage level, if appropriate

Basic Python Model

import bauplan

@bauplan.model(
    columns=['pickup_datetime', 'PULocationID', 'trip_miles'],
    materialization_strategy='REPLACE'
)
@bauplan.python('3.11', pip={'polars': '1.15.0'})
def clean_trips(
    # Use columns and filter for I/O pushdown
    data=bauplan.Model(
        'trips',
        columns=['pickup_datetime', 'PULocationID', 'trip_miles'],
        filter="trip_miles > 0"
    )
):
    """
    Filters trips to include only those with positive mileage.

    | pickup_datetime     | PULocationID | trip_miles |
    |---------------------|--------------|------------|
    | 2022-12-01 08:00:00 | 123          | 5.2        |
    """
    import polars as pl

    df = pl.from_arrow(data)
    df = df.filter(pl.col('trip_miles') > 0.0)

    return df.to_arrow()

Python Model with Multiple Inputs

Models can take multiple tables as input - just add more bauplan.Model() parameters:

def model_with_joins(
    table_a=bauplan.Model('source_a', columns=['id', 'value']),
    table_b=bauplan.Model('source_b', columns=['id', 'name'])
):
    # Join, transform, return Arrow table
    return table_a.join(table_b, 'id', 'id')

See examples.md for complete multi-input examples with Polars.

Workflow Checklist

Copy this checklist and track your progress:

Pipeline Creation Progress:
- [ ] Step 1: Get username → bauplan info
- [ ] Step 2: Checkout main → bauplan branch checkout main
- [ ] Step 3: Create dev branch → bauplan branch create <username>.<branch_name>
- [ ] Step 4: Checkout dev branch → bauplan branch checkout <username>.<branch_name>
- [ ] Step 5: Verify source tables → bauplan table get <namespace>.<table_name>, Optional for data preview: bauplan query "SELECT * FROM <namespace>.<table_name> LIMIT 3"
- [ ] Step 6: Create project folder with bauplan_project.yml
- [ ] Step 7: Write SQL model(s) / Python model(s) for transformations respecting the guidelines
- [ ] Step 8: Verify materialization decorators (see Materialization Checklist below)
- [ ] Step 9: Dry run → bauplan run --dry-run [--strict if strict mode]
- [ ] Step 10: Run pipeline → bauplan run [--strict if strict mode]

CRITICAL: Never run on main branch. Steps 2-4 ensure you're on a development branch.

Materialization Checklist

After writing models, verify that each model has the correct materialization_strategy based on user requirements:

Model Type	No Materialization (intermediate)	Materialized Output
Python	@bauplan.model() (no strategy)	@bauplan.model(materialization_strategy='REPLACE') or 'APPEND'
SQL	No comment needed	Add comment: -- bauplan: materialization_strategy=REPLACE or APPEND

Verify for each model:

• Intermediate tables (not final outputs): NO materialization_strategy specified
• Final output tables requested by user: materialization_strategy='REPLACE' (default) or 'APPEND'
• If user specified APPEND for any table: confirm materialization_strategy='APPEND' is set

Example Python decorator for materialized output:

@bauplan.model(materialization_strategy='REPLACE', columns=['col1', 'col2'])

Example SQL comment for materialized output:

-- bauplan: materialization_strategy=REPLACE
SELECT * FROM source_table

Advanced Examples

See examples.md for:

• APPEND materialization strategy
• DuckDB queries in Python models
• Data quality expectations
• Multi-stage pipelines