Hackweek: Feature Flags

You got features? We got flags!

This hackweek project implements client side feature flags for Sentry. It's supported in the Python and Dart SDK on branches.

import sentry_sdk

sentry_sdk.init(
    environment="prod",
    _experiments={"feature_flags_enabled": True},
)

# this blocks until flags are fetched
sentry_sdk.wait_until_ready()

# Checks a feature flag if it's enabled
if sentry_sdk.is_feature_flag_enabled("myFeatureFlag"):
    print("Feature flag enabled!")

UI

How does it work?

There is a Sentry UI where basic rules can be defined for feature flags. Multiple feature flags can be defined with multiple segments within each flag to control how they are rolled out. Each segment is matched against the provided tags and the first segment that matches defines the result.

Feature flags can be numeric, boolean or string types and additionally they can carry arbitrary JSON payloads.

Additionally special internal feature flags are used and directly influence the SDK. This way the sample rate for erros and transactions can be remotely reconfigured. The same is also true for the experimental profiling on the Python SDK.

Feature flags that are active are pushed out via Relay to client SDKs and evaluated there.

Feature flags are sticky by user or device and multiple feature flags can be grouped together to ensure that they are rolled out together if wanted.

The detailed protocol specs can be found here.

Draft PRs

• Dart SDK
• Python SDK
• Relay
• Sentry

Nice-to-have Features

These are some things we might add in the future:

• Force random stickiness (disabled by default), useful for non-user-facing features
• Date and Time expiration, enable HTTP payload for errors directly from the Issues page for only 1h
• Built-in feature flag to SDKs to enable verbose reporting (attachments for full outgoing HTTP requests and their responses)
• Basic reusable segments (list of VIP orgs) that can be referenced.

SDK Documentation

Here are the basic docs for this feature:

Flag Checking

The most basic check involves boolean feature flags. For these a convenient method isFeatureFlagEnabled is provided:

if sentry_sdk.is_feature_flag_enabled("featureFlagName"):
    # do somethign

By default the tags, transaction, environment and release are passed to the feature flag evaluator. Additionally custom values can be provided as well:

if sentry_sdk.is_feature_flag_enabled("featureFlagName", context={"key": "value"}):
    # do somethign

If a feature flag is unknown or no rule matched, the return value is null which is typically seen as false.

Flag Result Access

For more complex cases the result value can be directly accessed. The return value is either null if the flag is not known or a feature flag info object with result and payload:

sample_rate = 1.0
info = sentry_sdk.get_feature_flag_info("myOwnSampleRate")
if info:
    sample_rate = info.result

Additionally extra payload can be set which allows you to reconfigure things:

info = sentry_sdk.get_feature_flag_info("showWelcomeBanner")
if info is not None:
    show_banner(info.payload["banner_url"], info.payload["duration"])