Health Checks for HMLR Flask Applications

Consistent health endpoints

Features

• Easy provision of standard /health endpoint
• Nearly easy provision of standard /health/cascade endpoint
• Helpers for web app and PostgreSQL dependencies

This package depends on:

• Flask
• Requests

Flask applications

Import HealthChecks and initialise it as a Flask extension:

Just enough to get /health working:

from landregistry.healthchecks import HealthChecks
from <somewhere> import app

health = HealthChecks()
health.init_app(app)

Adding dependencies for the cascading health check:

DEPENDENCIES = {
    "Postgres": SQLALCHEMY_DATABASE_URI,
    "some-app": 'http://some-app:8080'
}
health.add_dependencies(DEPENDENCIES)

Caution

This currently doesn't do anything clever with avoiding endpoint collisions. It'll just fail.

HealthChecks methods

On initialisation, the extension registers /health and /health/cascade/<x> endpoints without futher intervention.

The behaviour of the endpoints is documented in api/openapi.yml.

add_web_dependency(name, uri)

Add a single web-app dependency. The URI should be the base URI for the service (this extension will add /health/cascade/<num> as required).

add_dependencies(dict)

Create a set of standard dependency checks. Supply a dictionary of name/uri pairs. Compatible with the DEPENDENCIES configuration item from the skeleton application. Will accept a pair with a name of postgres and a value containing a SQLAlchemy connection URI.

add_dependency(name, callback)

Create a custom dependency check.

Adds a dependency named name. Callback is a function pointer. The supplied function should return a dictionary (any contents will be added to the healthcheck response body) on success or raise an exception to indicate failure.

Custom dependency results will appear in the 'db' field of the healthcheck response.

Custom dependency example

Somewhere, define your new health check helper method. It must return a dict on success (empty is OK) and throw an exception on failure.

def one_is_more_than_zero_healthcheck():
    if 1 <= 0:
        raise Exception('The numbers have gone wrong')
    return {}

Use this method as a callback in add_dependency:

health = HealthChecks(app)

health.add_dependency('one_and_zero', one_is_more_than_zero_healthcheck)

Now your application will check that 1 is greater than 0 as part of its cascading healthcheck.

Configuration Options

Configure the package behaviour but setting application configuration values (e.g. in config.py).

The /health and /health/cacasde/<x> routes have the following options: