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PedramNavid committed Feb 28, 2024
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---
title: "Dagster Embedded ELT"
description: Lightweight ELT framework for building ELT pipelines with Dagster, through helpful pre-built assets and resources
---

# Dagster Embedded ELT

This package provides a framework for building ELT pipelines with Dagster through helpful asset decorators and resources. It is in experimental development, and we'd love to hear your feedback.

This package includes a single implementation using <a href="https://slingdata.io">Sling</a>, which provides a simple way to sync data between databases and file systems.

We plan on adding additional embedded ELT tool integrations in the future.

---

## Overview

To get started with `dagster-embedded-elt` and Sling, first, familiarize yourself with <a href="https://docs.slingdata.io/sling-cli/run/configuration/replication">Sling's replication</a> configuration. The replication configuration is a YAML file that specifies the source and target connections, as well as which streams to sync from. The `dagser-embedded-elt` integration uses this configuration to build the assets for both sources and destinations.

The typical pattern for building an ELT pipeline with Sling has three steps:

1. First, define a <a href="https://docs.slingdata.io/sling-cli/run/configuration/replication"> `replication.yaml`</a> file that specifies the source and target connections, as well as which streams to sync from.

2. Next, create a <PyObject module="dagster_embedded_elt.sling" object="SlingResource" /> and pass a list of <PyObject module="dagster_embedded_elt.sling" object="SlingConnectionResource" /> for each connection to the `connection` parameter, ensuring you name the resource using the same name given to the connection in the Sling configuration.

3. Use the <PyObject module="dagster_embedded_elt.sling" object="sling_assets" /> decorator to define an asset that will run the Sling replication job and yield from the <PyObject module="dagster_embedded_elt.sling" object="SlingResource" method="replicate" /> method to run the sync.

Each step is explained in detail below:

---

## Step 1: Setting up a Sling replication configuration

Dagster's Sling integration is built around Sling's replication configuration. You may provide either a path to an existing `replication.yaml` file, or construct a dictionary that represents the configuration in Python.

This configuration is passed to the Sling CLI to run the replication job.

Here's an example of a `replication.yaml` file:

```yaml
SOURCE: MY_POSTGRES
TARGET: MY_SNOWFLAKE

defaults:
mode: full-refresh
object: "{stream_schema}_{stream_table}"

streams:
public.accounts:
public.users:
public.finance_departments:
object: "departments"
```
Or in Python:
```python file=/integrations/embedded_elt/replication_config.py
replication_config = {
"SOURCE": "MY_POSTGRES",
"TARGET": "MY_DUCKDB",
"defaults": {"mode": "full-refresh", "object": "{stream_schema}_{stream_table}"},
"streams": {
"public.accounts": None,
"public.users": None,
"public.finance_departments": {"object": "departments"},
},
}
```

## Step 2: Creating a Sling resource

Next, you will need to create a <PyObject module="dagster_embedded_elt.sling" object="SlingResource" /> object that contains references to the connections specified in the replication configuration.

A <PyObject module="dagster_embedded_elt.sling" object="SlingResource" /> takes a `connections` parameter, where each <PyObject module="dagster_embedded_elt.sling" object="SlingConnectionResource" /> represents a connection to a source or target database. You may provide as many connections to the `SlingResource` as needed.

The `name` parameter in the <PyObject module="dagster_embedded_elt.sling" object="SlingConnectionResource" /> should match the `SOURCE` and `TARGET` keys in the replication configuration.

You may pass a connection string or arbitrary keyword arguments to the <PyObject module="dagster_embedded_elt.sling" object="SlingConnectionResource" /> to specify the connection details. See the <a href="https://docs.slingdata.io/connections/database-connections">Sling connections reference</a> for the specific connection types and parameters.

```python file=/integrations/embedded_elt/sling_connection_resources.py
from dagster_embedded_elt.sling.resources import (
SlingConnectionResource,
SlingResource,
)

from dagster import EnvVar

sling_resource = SlingResource(
connections=[
# Using a connection string from an environment variable
SlingConnectionResource(
name="MY_POSTGRES",
type="postgres",
connection_string=EnvVar("POSTGRES_CONNECTION_STRING"),
),
# Using a hard-coded connection string
SlingConnectionResource(
name="MY_DUCKDB",
type="duckdb",
connection_string="duckdb:///var/tmp/duckdb.db",
),
# Using a keyword-argument constructor
SlingConnectionResource(
name="MY_SNOWFLAKE",
type="snowflake",
host=EnvVar("SNOWFLAKE_HOST"),
user=EnvVar("SNOWFLAKE_USER"),
role="REPORTING",
),
]
)
```

## Step 3: Define the Sling assets

Now you can define a Sling asset using the <PyObject module="dagster_embedded_elt.sling" object="sling_assets" /> decorator. Dagster will read the replication configuration to produce Assets.

Each stream will render two assets, one for the source stream and one for the target destination. You may override how assets are named by passing in a custom <PyObject module="dagster_embedded_elt.sling" object="DagsterSlingTranslator" /> object.

```python file=/integrations/embedded_elt/sling_dagster_translator.py
from dagster_embedded_elt import sling
from dagster_embedded_elt.sling import (
DagsterSlingTranslator,
SlingResource,
sling_assets,
)

from dagster import Definitions, file_relative_path

replication_config = file_relative_path(__file__, "../sling_replication.yaml")
sling_resource = SlingResource(connections=[...]) # Add connections here


@sling_assets(replication_config=replication_config)
def my_assets(context, sling: SlingResource):
yield from sling.replicate(
replication_config=replication_config,
dagster_sling_translator=DagsterSlingTranslator(),
)
for row in sling.stream_raw_logs():
context.log.info(row)


defs = Definitions(
assets=[
my_assets,
],
resources={
"sling": sling_resource,
},
)
```

That's it! You should now be able to view your assets in Dagster and run the replication job.

## Examples

This is an example of how to setup a Sling sync between Postgres and Snowflake:

```python file=/integrations/embedded_elt/postgres_snowflake.py
from dagster_embedded_elt.sling import (
DagsterSlingTranslator,
SlingConnectionResource,
SlingResource,
sling_assets,
)

from dagster import EnvVar

source = SlingConnectionResource()(
name="MY_PG",
type="postgres",
host="localhost",
port=5432,
database="my_database",
user="my_user",
password=EnvVar("PG_PASS"),
)

target = SlingConnectionResource(
name="MY_SF",
type="snowflake",
host="hostname.snowflake",
user="username",
database="database",
password=EnvVar("SF_PASSWORD"),
role="role",
)

sling = SlingResource(
connections=[
source,
target,
]
)
replication_config = {
"SOURCE": "MY_PG",
"TARGET": "MY_SF",
"defaults": {"mode": "full-refresh", "object": "{stream_schema}_{stream_table}"},
"streams": {
"public.accounts": None,
"public.users": None,
"public.finance_departments": {"object": "departments"},
},
}


@sling_assets(replication_config=replication_config)
def my_assets(context, sling: SlingResource):
yield from sling.replicate(
replication_config=replication_config,
dagster_sling_translator=DagsterSlingTranslator(),
)
```

Similarily, you can define file/storage connections:

```python startafter=start_storage_config endbefore=end_storage_config file=/integrations/embedded_elt/s3_snowflake.py
source = SlingConnectionResource()(
name="MY_S3",
type="s3",
bucket="sling-bucket",
access_key_id=EnvVar("AWS_ACCESS_KEY_ID"),
secret_access_key=EnvVar("AWS_SECRET_ACCESS_KEY"),
)

sling = SlingResource(connections=[source, target])

replication_config = {
"SOURCE": "MY_S3",
"TARGET": "MY_SF",
"defaults": {"mode": "full-refresh", "object": "{stream_schema}_{stream_table}"},
"streams": {
"s3://my-bucket/my_file.parquet": {
"object": "marts.my_table",
"primary_key": "id",
},
},
}


@sling_assets
def my_assets(context, sling: SlingResource):
yield from sling.replicate(
replication_config=replication_config,
dagster_sling_translator=DagsterSlingTranslator(),
)
```

## Relevant APIs

| Name | Description |
| --------------------------------------------------------------------------------- | ------------------------------------------------------------------------------ |
| <PyObject module="dagster_embedded_elt.sling" object="sling_assets" /> | The core Sling asset factory for building syncs |
| <PyObject module="dagster_embedded_elt.sling" object="SlingResource" /> | The Sling Resource used for handing credentials to databases and object stores |
| <PyObject module="dagster_embedded_elt.sling" object="DagsterSlingTranslator" /> | A translator for specifying how to map between Sling and Dagster types |
| <PyObject module="dagster_embedded_elt.sling" object="SlingConnectionResource" /> | A Sling connection resource for specifying the connection details |

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