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Use a listener to send a Slack notification when a dataset is updated

Airflow listeners allow you to execute custom code when certain events occur anywhere in your Airflow instance, for example when any DAG run fails or any dataset is updated.

Listeners are implemented as an Airflow plugin and can contain any code. In this tutorial, you'll use a listener to send a Slack notification whenever any dataset is updated.

info

If you only need to implement notifications for specific DAGs and tasks, consider using Airflow callbacks instead.

caution

The on_dataset_created and on_dataset_changed listeners are currently considered experimental and might be subject to breaking changes in future releases.

Time to complete​

This tutorial takes approximately 15 minutes to complete.

Assumed knowledge​

To get the most out of this tutorial, make sure you have an understanding of:

Prerequisites​

Step 1: Configure your Astro project​

  1. Create a new Astro project:

    $ mkdir astro-listener-tutorial && cd astro-listener-tutorial
    $ astro dev init

  2. Add the following line to your Astro project requirements.txt file to install the Slack Airflow provider.

    apache-airflow-providers-slack==8.4.0

  3. Add the following environment variable to your Astro project .env file to create an Airflow connection to Slack. Make sure to replace <your-slack-webhook-token> with your own Slack webhook token in the format of T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX.

    AIRFLOW_CONN_SLACK_WEBHOOK_CONN='{
        "conn_type": "slackwebhook",
        "host": "https://hooks.slack.com/services/",
        "password": "<your-slack-webhook-token>"
    }'

Step 2: Create your listener​

To define an Airflow listener, you add the code you want to execute to a relevant @hookimpl-decorated listener function. In this example, you define your code in the on_dataset_changed function to run whenever any dataset is updated.

  1. Create a new file called listeners_code.py in your plugins folder.
  2. Copy the following code into the file:
from airflow.datasets import Dataset
from airflow.listeners import hookimpl
from airflow.models.taskinstance import TaskInstance
from airflow.utils.state import TaskInstanceState
from airflow.providers.slack.hooks.slack_webhook import SlackWebhookHook
from sqlalchemy.orm.session import Session
from datetime import datetime

SLACK_CONN_ID = "slack_webhook_conn"


@hookimpl
def on_dataset_changed(dataset: Dataset):
    """Execute if a dataset is updated."""
    print("I am always listening for any Dataset changes and I heard that!")
    print("Posting to Slack...")
    hook = SlackWebhookHook(slack_webhook_conn_id=SLACK_CONN_ID)
    hook.send(text=f"A dataset was changed!")
    print("Done!")
    if dataset.uri == "file://include/bears":
        print("Oh! This is the bears dataset!")
        print("Bears are great :)")
        start_date = datetime.now().date()
        end_date = datetime(2024, 10, 4).date()
        days_until = (end_date - start_date).days
        print(f"Only approximately {days_until} days until fat bear week!")

This listener is defined using the on_dataset_changed hookspec. It posts a message to Slack whenever any dataset is updated and executes an additional print statement if the dataset that is being updated has the URI file://include/bears.

Step 3: Create the listener plugin​

For Airflow to recognize your listener, you need to create a plugin that registers it.

  1. Create a new file called listener_plugin.py in your plugins folder.

  2. Copy the following code into the file:

    from airflow.plugins_manager import AirflowPlugin
    from plugins import listener_code

    class MyListenerPlugin(AirflowPlugin):
        name = "my_listener_plugin"
        listeners = [listener_code]

  3. If your local Airflow environment is already running, restart it to apply the changes to your plugins.

Step 4: Create your DAG​

  1. In your dags folder, create a file called producer_dag.py.

  2. Copy the following code into the file.

    """
    ## DAG to produce to a Dataset showcasing the on_dataset_changed listener

    This DAG will produce to a Dataset, updating it which triggers the 
    on_dataset_changed listener define as an Airflow Plugin.

    The DAG also shows the difference between a Dataset and ObjectStoragePath.
    """

    from airflow.datasets import Dataset
    from airflow.decorators import dag, task
    from airflow.io.path import ObjectStoragePath
    from pendulum import datetime
    import requests


    URI = "file://include/bears"
    MY_DATASET = Dataset(URI)
    base_local = ObjectStoragePath(URI)


    @dag(
        start_date=datetime(2023, 12, 1),
        schedule="0 0 * * 0",
        catchup=False,
        doc_md=__doc__,
        tags=["on_dataset_changed listener", "2-8"],
    )
    def producer_dag():
        @task(
            outlets=[MY_DATASET],
        )
        def get_bear(base):
            r = requests.get("https://placebear.com/200/300")
            file_path = base / "bear.jpg"

            if r.status_code == 200:
                base.mkdir(parents=True, exist_ok=True)
                file_path.write_bytes(r.content)
                file_path.replace("bear.jpg")
            else:
                print(f"Failed to retrieve image. Status code: {r.status_code}")

        get_bear(base=base_local)


    producer_dag()

    This simple DAG contains one task that queries the placebear API and writes the image retrieved to a local .png file in the include folder using the Airflow object storage feature. The task produces an update to the file://include/bears dataset, which triggers the listener you created in Step 2.

Step 5: Run your DAG​

  1. Run astro dev start in your Astro project to start Airflow, then open the Airflow UI at localhost:8080.

  2. In the Airflow UI, run the producer_dag DAG by clicking the play button.

  3. After the DAG run completed, go to the task logs of the get_bear task to see print statements from your listener plugin.

    [2023-12-17, 14:46:51 UTC] {logging_mixin.py:188} INFO - I am always listening for any dataset changes and I heard that!
    [2023-12-17, 14:46:51 UTC] {logging_mixin.py:188} INFO - Posting to Slack...
    [2023-12-17, 14:46:51 UTC] {base.py:83} INFO - Using connection ID 'slack_webhook_conn' for task execution.
    [2023-12-17, 14:46:51 UTC] {logging_mixin.py:188} INFO - Done!
    [2023-12-17, 14:46:51 UTC] {logging_mixin.py:188} INFO - Oh! This is the bears dataset!
    [2023-12-17, 14:46:51 UTC] {logging_mixin.py:188} INFO - Bears are great :)
    [2023-12-17, 14:46:51 UTC] {logging_mixin.py:188} INFO - Only approximately 292 days until fat bear week

  4. Open your Slack workspace to see a new message from your webhook.

    Screenshot of a Slack message sent by the webhook, saying &quot;A dataset was changed!&quot;.

  5. (Optional) View your complimentary bear picture at include/bears/bear.png.

Conclusion​

Congratulations! You now know how to create an Airflow listener to run custom code whenever any dataset is updated in your whole Airflow environment. Following the same pattern you can implement listeners for other events, such as when any task has failed, any DAG starts running or a lifecycle event occurs.

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