impact-stack-observability 0.3.0

Observability for Impact Stack

Observability for Impact Stack

A package providing observability features for Flask apps.

Health checks

The Health Flask extension provides health checks via an HTTP API.

The list of checks is extendable per app.

How it works

When called the requested checks run and return True or False. When all checks return True the service/app is considered healthy (i.e. in the running state), otherwise it is considered to be in a degraded state.

Checks are expected to be a Python callable, which takes no arguments and returns a boolean.

Usage

Create an implementation module, e.g in a health.py file:

"""Instantiate and configure checks."""

from impact_stack.observability import health as health_

health = health_.Health()


health.register_check("true", health_.checks.check_true, add_to_defaults=False)


@health.register_check("foo", add_to_defaults=False)
def check_foo():
    """Check for availability of foo."""
    return True

In app.py:

import flask

from . import health

app = flask.Flask(__name__)
health.health.init_app(app)

No configuration is read from app.config. You are supposed to configure and optionally extend the Health extension in the implementation module.

Checking the liveliness of the app

You can do a simple liveliness check at /health/ping (default), which will just return a 200 OK text response.

Checking the service health

A more comprehensive health check is available at /health/ (default). This returns a JSON response with following structure (prettyfied):

{
  "health": "running",
  "available_checks": [
    "true",
    "foo"
  ],
  "checks": {
    "foo": true
  }
}

The health field can be of:

• running: All requested checks returned true
• degraded: At least one of the checks returned false

Specific checks can be requested with the checks parameter:

• /health/?checks=_defaults: Run all the checks registered as defaults, same as omitting the checks parameter alltogether
• /health/?checks=true,foo: Run the listed checks

Headers to prevent caching of the health responses are set.

Provided checks

Some generic checks are providing in this module.

check_true and check_false

Dummy checks which return just true resp. false

check_db

(Requires Flask-SQLAlchemy.)

Checks if the DB (using the default SQLAlchemy engine) is available by trying a SELECT 1

base_check_api_liveliness

(Needs instantiation.)

Base check for checking the liveliness of an (external) HTTP API.

Example usage:

import functools

from impact_stack.observability import health as health

check_example = functools.partial(
    health_.checks.base_check_api_liveliness,
    "GET",
    "https://api.example.com/v1/health/ping",
    200,  # expected response status code
    2.0,  # timeout
)
health.register_check("example", check_example)

Authorization

A simple mechanism to prevent unrestricted calls to the /health/ endpoint is using a shared secret in the HTTP calls.

Set the required config variable HEALTH_URL_SECRET to a (random) string and use the GET param auth in calls to the endpoint. Returns 401 if the secret does not match. Raises a RuntimeError if not set.

If the HEALTH_URL_SECRET is set to None, checking the secret is disabled.

Tracing

Tracing is implemented by using the OpenTelemetry Python API and SDK. OpenTelemetry is a standard for implementing distributed tracing and supports a variety of programming languages by providing API and SDK libraries. Multiple services exist which receive distributed traces from instrumented apps and provide users an interface to the traces.

See

• https://opentelemetry.io/ for a general introduction to Open Telemetry and tracing,
• https://opentelemetry.io/docs/languages/python/instrumentation/ for the Python API and SDK,
• https://opentelemetry-python-contrib.readthedocs.io/en/latest/ for Python specfic "autoinstrumentations".

You need to "instrument" and configure your app in your code base in order to produce traces.

Using this library you have 3 ways to achieve that, which can all be used together:

• Use the provided autoinstrument_app() function for a basic set of instrumentation,
• use the provided new_span() decorator to instrument your app for specific functions or methods, or
• just use the OpenTelemetry API to instrument your app.

Basic Usage

Configure the app:

# app.py
import flask
from impact_stack.observability import tracing

app = flask.Flask(__name__)
app.config["TRACING_ENABLED"] = True  # default
# instrumentors default: None, means using all instrumentors found in the Python
# environment
app.config["TRACING_INSTRUMENTORS"] = ["flask"]
tracing_ext = tracing.Tracing()

# Initialize tracer, create the resource used by this tracer
tracing_ext.init_app(
    app,
    attributes={"additional.attribute": "foo"}
)

Then run the app with appropriate OS environment variables set, e.g.

env OTEL_SERVICE_NAME=myapp OTEL_TRACES_EXPORTER=console OTEL_RESOURCE_ATTRIBUTES=foo.bar=baz flask run

You can also use the utility functions init_tracing() and autoinstrument_app() if you need a bit more control.

NB: The autoinstrument_app() call can happen before init_tracing as well, but then the logging autoinstrumentation is not able to pick up the service.name resource value. So you will loose some functionality in this case.

Autoinstrumentations

Tested and supported autoinstrumentations are:

• celery: https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/celery/celery.html
• flask: https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/flask/flask.html
• logging: https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/requests/requests.html
• requests: https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/requests/requests.html
• sqlalchemy: https://opentelemetry-python-contrib.readthedocs.io/en/latest/instrumentation/sqlalchemy/sqlalchemy.html

By default autoinstrument_app() reads all installed entry points named opentelemetry_instrumentor and tries to instantiate them. Special care is applied for flask and sqlalchemy (when using Flask-SQLAlchemy) autoinstrumentations.

For the record: The OpenTelemetry API in general is not bound to something like a Flask app being available before autoinstrumentation. This library on the other hand is intented to be used with Flask apps and thus mandates an Flask app object for it's use of autoinstrument_app().

Take a good look at the autoinstrumentations and check how to use them.

Forking servers (gunicorn)

For WSGI servers like gunicorn you need to use lifecycle hooks for initializing the tracing. See https://opentelemetry-python.readthedocs.io/en/latest/examples/fork-process-model/README.html.