flagsmith-common

Flagsmith's common library

Development Setup

This project uses Poetry for dependency management and includes a Makefile to simplify common development tasks.

Prerequisites

• Python >= 3.11
• Make

Installation

You can set up your development environment using the provided Makefile:

# Install everything (pip, poetry, and project dependencies)
make install

# Individual installation steps are also available
make install-pip       # Upgrade pip
make install-poetry    # Install Poetry
make install-packages  # Install project dependencies

Development

Run linting checks using pre-commit:

make lint

Additional options can be passed to the install-packages target:

# Install with development dependencies
make install-packages opts="--with dev"

# Install with specific extras
make install-packages opts="--extras 'feature1 feature2'"

Usage

Installation

1. poetry add flagsmith-common

2. poetry add --G dev flagsmith-common[test-tools] — this will enable the Pytest fixtures. Skipping this step will make Pytest collection fail due to missing dependencies.

3. Make sure "common.core" is in the INSTALLED_APPS of your settings module. This enables the manage.py flagsmith commands.

4. Add "common.gunicorn.middleware.RouteLoggerMiddleware" to MIDDLEWARE in your settings module. This enables the route label for Prometheus HTTP metrics.

5. To enable the /metrics endpoint, set the PROMETHEUS_ENABLED setting to True.

Test tools

Fixtures

assert_metric

To test your metrics using the assert_metric fixture:

from common.test_tools import AssertMetricFixture

def test_my_code__expected_metrics(assert_metric: AssertMetricFixture) -> None:
    # When
    my_code()

    # Then
    assert_metric(
        name="flagsmith_distance_from_earth_au_sum",
        labels={"engine_type": "solar_sail"},
        value=1.0,
    )

saas_mode

The saas_mode fixture makes all common.core.utils.is_saas calls return True.

enterprise_mode

The enterprise_mode fixture makes all common.core.utils.is_enterprise calls return True.

Markers

pytest.mark.saas_mode

Use this mark to auto-use the saas_mode fixture.

pytest.mark.enterprise_mode

Use this mark to auto-use the enterprise_mode fixture.

Metrics

Flagsmith uses Prometheus to track performance metrics.

The following default metrics are exposed:

Common metrics

• flagsmith_build_info: Has the labels version and ci_commit_sha.
• flagsmith_http_server_request_duration_seconds: Histogram labeled with method, route, and response_status.
• flagsmith_http_server_requests_total: Counter labeled with method, route, and response_status.
• flagsmith_http_server_response_size_bytes:Histogram labeled with method, route, and response_status.
• flagsmith_task_processor_enqueued_tasks_total: Counter labeled with task_identifier.

Task Processor metrics

• flagsmith_task_processor_finished_tasks_total: Counter labeled with task_identifier, task_type ("recurring", "standard") and result ("success", "failure").
• flagsmith_task_processor_task_duration_seconds: Histogram labeled with task_identifier, task_type ("recurring", "standard") and result ("success", "failure").

Guidelines

Try to come up with meaningful metrics to cover your feature with when developing it. Refer to Prometheus best practices when naming your metric and labels.

As a reasonable default, Flagsmith metrics are expected to be namespaced with the "flagsmith_" prefix.

Define your metrics in a metrics.py module of your Django application — see example. Contrary to Prometheus Python client examples and documentation, please name a metric variable exactly as your metric name.

It's generally a good idea to allow users to define histogram buckets of their own. Flagsmith accepts a PROMETHEUS_HISTOGRAM_BUCKETS setting so users can customise their buckets. To honour the setting, use the common.prometheus.Histogram class when defining your histograms. When using prometheus_client.Histogram directly, please expose a dedicated setting like so:

import prometheus_client
from django.conf import settings

flagsmith_distance_from_earth_au = prometheus_client.Histogram(
    "flagsmith_distance_from_earth_au",
    "Distance from Earth in astronomical units",
    labels=["engine_type"],
    buckets=settings.DISTANCE_FROM_EARTH_AU_HISTOGRAM_BUCKETS,
)

For testing your metrics, refer to assert_metric documentation.