Building data pipelines with dlt, from basic to advanced

This in-depth overview will take you through the main areas of pipelining with dlt. Go to the related pages you are instead looking for the quickstart.

Why build pipelines with dlt?

dlt offers functionality to support the entire extract and load process. Let's look at the high level diagram:

First, we have a pipeline function, that can infer a schema from data and load the data to the destination. We can use this pipeline with json data, dataframes, or other iterable objects such as generator functions.

This pipeline provides effortless loading via a schema discovery, versioning and evolution engine that ensures you can "just load" any data with row and column level lineage.

By utilizing a dlt pipeline, we can easily adapt and structure data as it evolves, reducing the time spent on maintenance and development.

This allows our data team to focus on leveraging the data and driving value, while ensuring effective governance through timely notifications of any changes.

For extract, dlt also provides source and resource decorators that enable defining how extracted data should be loaded, while supporting graceful, scalable extraction via micro-batching and parallelism.

The simplest pipeline: 1 liner to load data with schema evolution

import dlt

dlt.pipeline(destination='duckdb', dataset_name='mydata').run([{'id': 1, 'name': 'John'}], table_name="users")

A pipeline in the dlt library is a powerful tool that allows you to move data from your Python code to a destination with a single function call. By defining a pipeline, you can easily load, normalize, and evolve your data schemas, enabling seamless data integration and analysis.

For example, let's consider a scenario where you want to load a list of objects into a DuckDB table named "three". With dlt, you can create a pipeline and run it with just a few lines of code:

1. Create a pipeline to the destination.
2. Give this pipeline data and run it.

import dlt

pipeline = dlt.pipeline(destination="duckdb", dataset_name="country_data")

data = [
    {'country': 'USA', 'population': 331449281, 'capital': 'Washington, D.C.'},
    {'country': 'Canada', 'population': 38005238, 'capital': 'Ottawa'},
    {'country': 'Germany', 'population': 83019200, 'capital': 'Berlin'}
]

info = pipeline.run(data, table_name="countries")

print(info)

In this example, the pipeline function is used to create a pipeline with the specified destination (DuckDB) and dataset name ("country_data"). The run method is then called to load the data from a list of objects into the table named "countries". The info variable stores information about the loaded data, such as package IDs and job metadata.

The data you can pass to it should be iterable: lists of rows, generators, or dlt sources will do just fine.

If you want to configure how the data is loaded, you can choose between write_dispositions such as replace, append and merge in the pipeline function.

Here is an example where we load some data to duckdb by upserting or merging on the id column found in the data. In this example, we also run a dbt package and then load the outcomes of the load jobs into their respective tables. This will enable us to log when schema changes occurred and match them to the loaded data for lineage, granting us both column and row level lineage. We also alert the schema change to a Slack channel where hopefully the producer and consumer are subscribed.

import dlt

# have data? dlt likes data
data = [{'id': 1, 'name': 'John'}]

# open connection
pipeline = dlt.pipeline(
    destination='duckdb',
    dataset_name='raw_data'
)

# Upsert/merge: Update old records, insert new
load_info = pipeline.run(
    data,
    write_disposition="merge",
    primary_key="id",
    table_name="users"
)

Add dbt runner, optionally with venv:

venv = dlt.dbt.get_venv(pipeline)
dbt = dlt.dbt.package(
    pipeline,
    "https://github.com/dbt-labs/jaffle_shop.git",
    venv=venv
)
models_info = dbt.run_all()

# Load metadata for monitoring and load package lineage.
# This allows for both row and column level lineage,
# as it contains schema update info linked to the loaded data
pipeline.run([load_info], table_name="loading_status", write_disposition='append')
pipeline.run([models_info], table_name="transform_status", write_disposition='append')

Let's alert any schema changes:

from dlt.common.runtime.slack import send_slack_message

slack_hook = "https://hooks.slack.com/services/xxx/xxx/xxx"

for package in load_info.load_packages:
    for table_name, table in package.schema_update.items():
        for column_name, column in table["columns"].items():
            send_slack_message(
                slack_hook,
                message=f"\tTable updated: {table_name}: Column changed: {column_name}: {column['data_type']}"
            )

Extracting data with dlt

Extracting data with dlt is simple - you simply decorate your data-producing functions with loading or incremental extraction metadata, which enables dlt to extract and load by your custom logic.

Technically, two key aspects contribute to dlt's effectiveness:

• Scalability through iterators, chunking, parallelization.
• The utilization of implicit extraction DAGs that allow efficient API calls for data enrichments or transformations.

Scalability via iterators, chunking, and parallelization

dlt offers scalable data extraction by leveraging iterators, chunking, and parallelization techniques. This approach allows for efficient processing of large datasets by breaking them down into manageable chunks.

For example, consider a scenario where you need to extract data from a massive database with millions of records. Instead of loading the entire dataset at once, dlt allows you to use iterators to fetch data in smaller, more manageable portions. This technique enables incremental processing and loading, which is particularly useful when dealing with limited memory resources.

Furthermore, dlt facilitates parallelization during the extraction process. By processing multiple data chunks simultaneously, dlt takes advantage of parallel processing capabilities, resulting in significantly reduced extraction times. This parallelization enhances performance, especially when dealing with high-volume data sources.

Implicit extraction DAGs

dlt incorporates the concept of implicit extraction DAGs to handle the dependencies between data sources and their transformations automatically. A DAG represents a directed graph without cycles, where each node represents a data source or transformation step.

When using dlt, the tool automatically generates an extraction DAG based on the dependencies identified between the data sources and their transformations. This extraction DAG determines the optimal order for extracting the resources to ensure data consistency and integrity.

For instance, imagine a pipeline where data needs to be extracted from multiple API endpoints and undergo certain transformations or enrichments via additional calls before loading it into a database. dlt analyzes the dependencies between the API endpoints and transformations and generates an extraction DAG accordingly. The extraction DAG ensures that the data is extracted in the correct order, accounting for any dependencies and transformations.

When deploying to Airflow, the internal DAG is unpacked into Airflow tasks in such a way to ensure consistency and allow granular loading.

Defining Incremental Loading

Incremental loading is a crucial concept in data pipelines that involves loading only new or changed data instead of reloading the entire dataset. This approach provides several benefits, including low-latency data transfer and cost savings.

Declarative loading

Declarative loading allows you to specify the desired state of the data in the target destination, enabling efficient incremental updates. With dlt, you can define the incremental loading behavior using the write_disposition parameter. There are three options available:

1. Full load: This option replaces the entire destination dataset with the data produced by the source on the current run. You can achieve this by setting write_disposition='replace' in your resources. It is suitable for stateless data that doesn't change, such as recorded events like page views.
2. Append: The append option adds new data to the existing destination dataset. By using write_disposition='append', you can ensure that only new records are loaded. This is suitable for stateless data that can be easily appended without any conflicts.
3. Merge: The merge option is used when you want to merge new data with the existing destination dataset while also handling deduplication or upserts. It requires the use of merge_key and/or primary_key to identify and update specific records. By setting write_disposition='merge', you can perform merge-based incremental loading.

For example, let's say you want to load GitHub events and update them in the destination, ensuring that only one instance of each event is present.

You can use the merge write disposition as follows:

@dlt.resource(primary_key="id", write_disposition="merge")
def github_repo_events():
    yield from _get_event_pages()

In this example, the github_repo_events resource uses the merge write disposition with primary_key="id". This ensures that only one copy of each event, identified by its unique ID, is present in the github_repo_events table. dlt takes care of loading the data incrementally, deduplicating it, and performing the necessary merge operations.

Advanced state management

Advanced state management in dlt allows you to store and retrieve values across pipeline runs by persisting them at the destination but accessing them in a dictionary in code. This enables you to track and manage incremental loading effectively. By leveraging the pipeline state, you can preserve information, such as last values, checkpoints or column renames, and utilize them later in the pipeline.

Transforming the Data

Data transformation plays a crucial role in the data loading process. You can perform transformations both before and after loading the data. Here's how you can achieve it:

Before Loading

Before loading the data, you have the flexibility to perform transformations using Python. You can leverage Python's extensive libraries and functions to manipulate and preprocess the data as needed. Here's an example of pseudonymizing columns before loading the data.

In the above example, the pseudonymize_name function pseudonymizes the name column by generating a deterministic hash using SHA256. It adds a salt to the column value to ensure consistent mapping. The dummy_source generates dummy data with an id and name column, and the add_map function applies the pseudonymize_name transformation to each record.

After Loading

For transformations after loading the data, you have several options available:

Using dbt

dbt is a powerful framework for transforming data. It enables you to structure your transformations into DAGs, providing cross-database compatibility and various features such as templating, backfills, testing, and troubleshooting. You can use the dbt runner in dlt to seamlessly integrate dbt into your pipeline. Here's an example of running a dbt package after loading the data:

import dlt
from pipedrive import pipedrive_source

# load to raw
pipeline = dlt.pipeline(
    pipeline_name='pipedrive',
    destination='bigquery',
    dataset_name='pipedrive_raw'
)

load_info = pipeline.run(pipedrive_source())
print(load_info)

Now transform from loaded data to dbt dataset:

pipeline = dlt.pipeline(
    pipeline_name='pipedrive',
    destination='bigquery',
    dataset_name='pipedrive_dbt'
)

# make venv and install dbt in it.
venv = dlt.dbt.get_venv(pipeline)

# get package from local or github link and run
dbt = dlt.dbt.package(pipeline, "pipedrive/dbt_pipedrive/pipedrive", venv=venv)
models = dbt.run_all()

# show outcome
for m in models:
    print(f"Model {m.model_name} materialized in {m.time} with status {m.status} and message {m.message}")

In this example, the first pipeline loads the data using pipedrive_source(). The second pipeline performs transformations using a dbt package called pipedrive after loading the data. The dbt.package function sets up the dbt runner, and dbt.run_all() executes the dbt models defined in the package.

Using the dlt SQL client

Another option is to leverage the dlt SQL client to query the loaded data and perform transformations using SQL statements. You can execute SQL statements that change the database schema or manipulate data within tables. Here's an example of inserting a row into the customers table using the dlt SQL client:

pipeline = dlt.pipeline(destination="bigquery", dataset_name="crm")

with pipeline.sql_client() as client:
    client.execute_sql(
        "INSERT INTO customers VALUES (%s, %s, %s)", 10, "Fred", "fred@fred.com"
    )

In this example, the execute_sql method of the SQL client allows you to execute SQL statements. The statement inserts a row with values into the customers table.

Using Pandas

You can fetch query results as Pandas data frames and perform transformations using Pandas functionalities. Here's an example of reading data from the issues table in DuckDB and counting reaction types using Pandas:

pipeline = dlt.pipeline(
    pipeline_name="github_pipeline",
    destination="duckdb",
    dataset_name="github_reactions",
    dev_mode=True
)

with pipeline.sql_client() as client:
    with client.execute_query(
        'SELECT "reactions__+1", "reactions__-1", reactions__laugh, reactions__hooray, reactions__rocket FROM issues'
    ) as table:
        reactions = table.df()

counts = reactions.sum(0).sort_values(0, ascending=False)

By leveraging these transformation options, you can shape and manipulate the data before or after loading it, allowing you to meet specific requirements and ensure data quality and consistency.

Adjusting the automated normalisation

To streamline the process, dlt recommends attaching schemas to sources implicitly instead of creating them explicitly. You can provide a few global schema settings and let the table and column schemas be generated from the resource hints and the data itself. The dlt.source decorator accepts a schema instance that you can create and modify within the source function. Additionally, you can store schema files with the source Python module and have them automatically loaded and used as the schema for the source.

By adjusting the automated normalization process in dlt, you can ensure that the generated database schema meets your specific requirements and aligns with your preferred naming conventions, data types, and other customization needs.

Customizing the Normalization Process

Customizing the normalization process in dlt allows you to adapt it to your specific requirements.

You can adjust table and column names, configure column properties, define data type autodetectors, apply performance hints, specify preferred data types, or change how ids are propagated in the unpacking process.

These customization options enable you to create a schema that aligns with your desired naming conventions, data types, and overall data structure. With dlt, you have the flexibility to tailor the normalization process to meet your unique needs and achieve optimal results.

Read more about how to configure schema generation.

Exporting and Importing Schema Files

dlt allows you to export and import schema files, which contain the structure and instructions for processing and loading the data. Exporting schema files enables you to modify them directly, making adjustments to the schema as needed. You can then import the modified schema files back into dlt to use them in your pipeline.

Read more: Adjust a schema docs.

Governance Support in dlt Pipelines

dlt pipelines offer robust governance support through three key mechanisms: pipeline metadata utilization, schema enforcement and curation, and schema change alerts.

Pipeline Metadata

dlt pipelines leverage metadata to provide governance capabilities. This metadata includes load IDs, which consist of a timestamp and pipeline name. Load IDs enable incremental transformations and data vaulting by tracking data loads and facilitating data lineage and traceability.

Read more about lineage.

Schema Enforcement and Curation

dlt empowers users to enforce and curate schemas, ensuring data consistency and quality. Schemas define the structure of normalized data and guide the processing and loading of data. By adhering to predefined schemas, pipelines maintain data integrity and facilitate standardized data handling practices.

Read more: Adjust a schema docs.

Schema evolution

dlt enables proactive governance by alerting users to schema changes. When modifications occur in the source data’s schema, such as table or column alterations, dlt notifies stakeholders, allowing them to take necessary actions, such as reviewing and validating the changes, updating downstream processes, or performing impact analysis.

These governance features in dlt pipelines contribute to better data management practices, compliance adherence, and overall data governance, promoting data consistency, traceability, and control throughout the data processing lifecycle.

Scaling and finetuning

dlt offers several mechanism and configuration options to scale up and finetune pipelines:

• Running extraction, normalization and load in parallel.
• Writing sources and resources that are run in parallel via thread pools and async execution.
• Finetune the memory buffers, intermediary file sizes and compression options.

Read more about performance.

Other advanced topics

dlt is a constantly growing library that supports many features and use cases needed by the community. Join our Slack to find recent releases or discuss what you can build with dlt.