Guide to add a custom check

What you’ll learn

• Add a custom check to an existing project
• Run it in local Kubernetes with k3d

What you’ll need

• poetry
• k3d
• helm
• kubectl

Writing and configuring a custom check

We’re building on top of the spacecrafts app, which is described in more detail in this guide. You need to have it running locally in order to follow this guide.

The custom check handler is in a file named src/apps/components/checks.py and looks like follows:

# src/apps/components/checks.py
import logging

from django.core.checks import Error

from apps.components.models import Component

logger = logging.getLogger("hurricane")


def example_check_main_engine(app_configs=None, **kwargs):
    """
    Check for existence of the "Main engine" component in the database
    """

    # your check logic here
    errors = []
    logger.info("Our check has been called :]")
    # we need to wrap all sync calls to the database into a sync_to_async wrapper for Hurricane to use it in async way
    if not Component.objects.filter(title="Main engine").exists():
        errors.append(
            Error(
                "an error",
                hint="There is no main engine in the spacecraft, it need's to exist with the name 'Main engine'. "
                "Please create it in the admin or by installing the fixture.",
                id="components.E001",
            )
        )

    return errors

You can see that it is a very simple check, that checks whether a component with the title “Main engine” exists. Because how can you fly a spacecraft without a main engine, right? If it isn’t found, the check returns an error.

Next, we set a default app config in apps/components/__init__.py

# src/apps/components/__init__.py
default_app_config = 'apps.components.apps.ComponentsConfig'

Now we need to register this check, so that Django can use it in its check logic. Note that we register the check with the tag hurricane, as Hurricane only runs check with that tag. Additionally we need to set deploy=True, so that the check will not be run at every management command. Your apps/components/apps.py should have the following content:

# apps/components/apps.py
from django.apps import AppConfig


class ComponentsConfig(AppConfig):
    default_auto_field = "django.db.models.BigAutoField"
    name = "apps.components"

    def ready(self):
        from django.core.checks import register

        from apps.components.checks import example_check_main_engine

        register(example_check_main_engine, "hurricane", deploy=True)

We register our check only after the application is ready, otherwise we will run into the error of AppNotReady. This way we make sure, that this check is only registered after the application is ready, as it requires a connection to the model.

To verify whether the check works, you can just delete the Main engine component from the database. If you’re running Hurricane in a local virtualenv, you can also directly browse to /alive at the probe port to inspect its output. See the basic spacecrafts guide for how to do that.