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Feature flags

Note

This is currently in Beta, as we might change Store parameters in the next release.

The feature flags utility provides a simple rule engine to define when one or multiple features should be enabled depending on the input.

Terminology

Feature flags are used to modify behaviour without changing the application's code. These flags can be static or dynamic.

Static flags. Indicates something is simply on or off, for example TRACER_ENABLED=True.

Dynamic flags. Indicates something can have varying states, for example enable a list of premium features for customer X not Y.

Tip

You can use Parameters utility for static flags while this utility can do both static and dynamic feature flags.

Warning

Be mindful that feature flags can increase the complexity of your application over time; use them sparingly.

If you want to learn more about feature flags, their variations and trade-offs, check these articles:

Key features

  • Define simple feature flags to dynamically decide when to enable a feature
  • Fetch one or all feature flags enabled for a given application context
  • Support for static feature flags to simply turn on/off a feature without rules

Getting started

IAM Permissions

Your Lambda function must have appconfig:GetConfiguration IAM permission in order to fetch configuration from AWS AppConfig.

Required resources

By default, this utility provides AWS AppConfig as a configuration store.

The following sample infrastructure will be used throughout this documentation:

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AWSTemplateFormatVersion: "2010-09-09"
Description: Lambda Powertools Feature flags sample template
Resources:
  FeatureStoreApp:
    Type: AWS::AppConfig::Application
    Properties:
      Description: "AppConfig Application for feature toggles"
      Name: product-catalogue

  FeatureStoreDevEnv:
    Type: AWS::AppConfig::Environment
    Properties:
      ApplicationId: !Ref FeatureStoreApp
      Description: "Development Environment for the App Config Store"
      Name: dev

  FeatureStoreConfigProfile:
    Type: AWS::AppConfig::ConfigurationProfile
    Properties:
      ApplicationId: !Ref FeatureStoreApp
      Name: features
      LocationUri: "hosted"

  HostedConfigVersion:
    Type: AWS::AppConfig::HostedConfigurationVersion
    Properties:
      ApplicationId: !Ref FeatureStoreApp
      ConfigurationProfileId: !Ref FeatureStoreConfigProfile
      Description: 'A sample hosted configuration version'
      Content: |
        {
              "premium_features": {
                "default": false,
                "rules": {
                  "customer tier equals premium": {
                    "when_match": true,
                    "conditions": [
                      {
                        "action": "EQUALS",
                        "key": "tier",
                        "value": "premium"
                      }
                    ]
                  }
                }
              },
              "ten_percent_off_campaign": {
                "default": false
              }
          }
      ContentType: 'application/json'

  ConfigDeployment:
    Type: AWS::AppConfig::Deployment
    Properties:
      ApplicationId: !Ref FeatureStoreApp
      ConfigurationProfileId: !Ref FeatureStoreConfigProfile
      ConfigurationVersion: !Ref HostedConfigVersion
      DeploymentStrategyId: "AppConfig.AllAtOnce"
      EnvironmentId: !Ref FeatureStoreDevEnv

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import json

import aws_cdk.aws_appconfig as appconfig
from aws_cdk import core


class SampleFeatureFlagStore(core.Construct):
    def __init__(self, scope: core.Construct, id_: str) -> None:
        super().__init__(scope, id_)

        features_config = {
            "premium_features": {
                "default": False,
                "rules": {
                    "customer tier equals premium": {
                        "when_match": True,
                        "conditions": [{"action": "EQUALS", "key": "tier", "value": "premium"}],
                    }
                },
            },
            "ten_percent_off_campaign": {"default": True},
        }

        self.config_app = appconfig.CfnApplication(
            self,
            id="app",
            name="product-catalogue",
        )
        self.config_env = appconfig.CfnEnvironment(
            self,
            id="env",
            application_id=self.config_app.ref,
            name="dev-env",
        )
        self.config_profile = appconfig.CfnConfigurationProfile(
            self,
            id="profile",
            application_id=self.config_app.ref,
            location_uri="hosted",
            name="features",
        )
        self.hosted_cfg_version = appconfig.CfnHostedConfigurationVersion(
            self,
            "version",
            application_id=self.config_app.ref,
            configuration_profile_id=self.config_profile.ref,
            content=json.dumps(features_config),
            content_type="application/json",
        )
        self.app_config_deployment = appconfig.CfnDeployment(
            self,
            id="deploy",
            application_id=self.config_app.ref,
            configuration_profile_id=self.config_profile.ref,
            configuration_version=self.hosted_cfg_version.ref,
            deployment_strategy_id="AppConfig.AllAtOnce",
            environment_id=self.config_env.ref,
        )

Evaluating a single feature flag

To get started, you'd need to initialize AppConfigStore and FeatureFlags. Then call FeatureFlags evaluate method to fetch, validate, and evaluate your feature.

The evaluate method supports two optional parameters:

  • context: Value to be evaluated against each rule defined for the given feature
  • default: Sentinel value to use in case we experience any issues with our store, or feature doesn't exist
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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="features"
)

feature_flags = FeatureFlags(store=app_config)

def lambda_handler(event, context):
    # Get customer's tier from incoming request
    ctx = { "tier": event.get("tier", "standard") }

    # Evaluate whether customer's tier has access to premium features
    # based on `has_premium_features` rules
    has_premium_features: bool = feature_flags.evaluate(name="premium_features",
                                                        context=ctx, default=False)
    if has_premium_features:
        # enable premium features
        ...

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{
    "username": "lessa",
    "tier": "premium",
    "basked_id": "random_id"
}

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{
    "premium_features": {
        "default": false,
        "rules": {
            "customer tier equals premium": {
                "when_match": true,
                "conditions": [
                    {
                        "action": "EQUALS",
                        "key": "tier",
                        "value": "premium"
                    }
                ]
            }
        }
    },
    "ten_percent_off_campaign": {
        "default": false
    }
}

Static flags

We have a static flag named ten_percent_off_campaign. Meaning, there are no conditional rules, it's either ON or OFF for all customers.

In this case, we could omit the context parameter and simply evaluate whether we should apply the 10% discount.

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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="features"
)

feature_flags = FeatureFlags(store=app_config)

def lambda_handler(event, context):
    apply_discount: bool = feature_flags.evaluate(name="ten_percent_off_campaign",
                                                            default=False)

    if apply_discount:
        # apply 10% discount to product
        ...

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{
    "ten_percent_off_campaign": {
        "default": false
    }
}

Getting all enabled features

As you might have noticed, each evaluate call means an API call to the Store and the more features you have the more costly this becomes.

You can use get_enabled_features method for scenarios where you need a list of all enabled features according to the input context.

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from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app = APIGatewayRestResolver()

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="features"
)

feature_flags = FeatureFlags(store=app_config)

@app.get("/products")
def list_products():
    ctx = {
        **app.current_event.headers,
        **app.current_event.json_body
    }

    # all_features is evaluated to ["geo_customer_campaign", "ten_percent_off_campaign"]
    all_features: list[str] = feature_flags.get_enabled_features(context=ctx)

    if "geo_customer_campaign" in all_features:
        # apply discounts based on geo
        ...

    if "ten_percent_off_campaign" in all_features:
        # apply additional 10% for all customers
        ...

def lambda_handler(event, context):
    return app.resolve(event, context)

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{
    "body": "{\"username\": \"lessa\", \"tier\": \"premium\", \"basked_id\": \"random_id\"}",
    "resource": "/products",
    "path": "/products",
    "httpMethod": "GET",
    "isBase64Encoded": false,
    "headers": {
        "CloudFront-Viewer-Country": "NL"
    }
}

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{
    "premium_features": {
        "default": false,
        "rules": {
            "customer tier equals premium": {
                "when_match": true,
                "conditions": [
                    {
                        "action": "EQUALS",
                        "key": "tier",
                        "value": "premium"
                    }
                ]
            }
        }
    },
    "ten_percent_off_campaign": {
        "default": true
    },
    "geo_customer_campaign": {
        "default": false,
        "rules": {
            "customer in temporary discount geo": {
                "when_match": true,
                "conditions": [
                    {
                        "action": "KEY_IN_VALUE",
                        "key": "CloudFront-Viewer-Country",
                        "value": ["NL", "IE", "UK", "PL", "PT"]
                    }
                ]
            }
        }
    }
}

Beyond boolean feature flags

When is this useful?

You might have a list of features to unlock for premium customers, unlock a specific set of features for admin users, etc.

Feature flags can return any JSON values when boolean_type parameter is set to false. These can be dictionaries, list, string, integers, etc.

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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="features"
)

feature_flags = FeatureFlags(store=app_config)

def lambda_handler(event, context):
    # Get customer's tier from incoming request
    ctx = { "tier": event.get("tier", "standard") }

    # Evaluate `has_premium_features` base don customer's tier
    premium_features: list[str] = feature_flags.evaluate(name="premium_features",
                                                        context=ctx, default=False)
    for feature in premium_features:
        # enable premium features
        ...

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{
    "username": "lessa",
    "tier": "premium",
    "basked_id": "random_id"
}

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{
    "premium_features": {
        "boolean_type": false,
        "default": [],
        "rules": {
            "customer tier equals premium": {
                "when_match": ["no_ads", "no_limits", "chat"],
                "conditions": [
                    {
                        "action": "EQUALS",
                        "key": "tier",
                        "value": "premium"
                    }
                ]
            }
        }
    }
}

Advanced

Adjusting in-memory cache

By default, we cache configuration retrieved from the Store for 5 seconds for performance and reliability reasons.

You can override max_age parameter when instantiating the store.

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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="features",
    max_age=300
)

Getting fetched configuration

When is this useful?

You might have application configuration in addition to feature flags in your store.

This means you don't need to make another call only to fetch app configuration.

You can access the configuration fetched from the store via get_raw_configuration property within the store instance.

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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="configuration",
    envelope = "feature_flags"
)

feature_flags = FeatureFlags(store=app_config)

config = app_config.get_raw_configuration

Schema

This utility expects a certain schema to be stored as JSON within AWS AppConfig.

Features

A feature can simply have its name and a default value. This is either on or off, also known as a static flag.

minimal_schema.json
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{
    "global_feature": {
        "default": true
    },
    "non_boolean_global_feature": {
        "default": {"group": "read-only"},
        "boolean_type": false
    },
}

If you need more control and want to provide context such as user group, permissions, location, etc., you need to add rules to your feature flag configuration.

Rules

When adding rules to a feature, they must contain:

  1. A rule name as a key
  2. when_match boolean or JSON value that should be used when conditions match
  3. A list of conditions for evaluation
feature_with_rules.json
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{
    "premium_feature": {
        "default": false,
        "rules": {
            "customer tier equals premium": {
                "when_match": true,
                "conditions": [
                    {
                        "action": "EQUALS",
                        "key": "tier",
                        "value": "premium"
                    }
                ]
            }
        }
    },
    "non_boolean_premium_feature": {
        "default": [],
        "rules": {
            "customer tier equals premium": {
                "when_match": ["remove_limits", "remove_ads"],
                "conditions": [
                    {
                        "action": "EQUALS",
                        "key": "tier",
                        "value": "premium"
                    }
                ]
            }
        }
    }
}

You can have multiple rules with different names. The rule engine will return the first result when_match of the matching rule configuration, or default value when none of the rules apply.

Conditions

The conditions block is a list of conditions that contain action, key, and value keys:

conditions.json
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{
    ...
    "conditions": [
        {
            "action": "EQUALS",
            "key": "tier",
            "value": "premium"
        }
    ]
}

The action configuration can have the following values, where the expressions a is the key and b is the value above:

Action Equivalent expression
EQUALS lambda a, b: a == b
NOT_EQUALS lambda a, b: a != b
KEY_GREATER_THAN_VALUE lambda a, b: a > b
KEY_GREATER_THAN_OR_EQUAL_VALUE lambda a, b: a >= b
KEY_LESS_THAN_VALUE lambda a, b: a < b
KEY_LESS_THAN_OR_EQUAL_VALUE lambda a, b: a <= b
STARTSWITH lambda a, b: a.startswith(b)
ENDSWITH lambda a, b: a.endswith(b)
KEY_IN_VALUE lambda a, b: a in b
KEY_NOT_IN_VALUE lambda a, b: a not in b
VALUE_IN_KEY lambda a, b: b in a
VALUE_NOT_IN_KEY lambda a, b: b not in a
Info

The **key** and **value** will be compared to the input from the **context** parameter.

For multiple conditions, we will evaluate the list of conditions as a logical AND, so all conditions needs to match to return when_match value.

Rule engine flowchart

Now that you've seen all properties of a feature flag schema, this flowchart describes how the rule engine decides what value to return.

Rule engine

Envelope

There are scenarios where you might want to include feature flags as part of an existing application configuration.

For this to work, you need to use a JMESPath expression via the envelope parameter to extract that key as the feature flags configuration.

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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore

app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="configuration",
    envelope = "feature_flags"
)

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{
    "logging": {
        "level": "INFO",
        "sampling_rate": 0.1
    },
    "feature_flags": {
        "premium_feature": {
            "default": false,
            "rules": {
                "customer tier equals premium": {
                    "when_match": true,
                    "conditions": [
                        {
                            "action": "EQUALS",
                            "key": "tier",
                            "value": "premium"
                        }
                    ]
                }
            }
        },
        "feature2": {
            "default": false
        }
    }
}

Built-in store provider

Info

For GA, you'll be able to bring your own store.

AppConfig

AppConfig store provider fetches any JSON document from AWS AppConfig.

These are the available options for further customization.

Parameter Default Description
environment "" AWS AppConfig Environment, e.g. test
application "" AWS AppConfig Application
name "" AWS AppConfig Configuration name
envelope None JMESPath expression to use to extract feature flags configuration from AWS AppConfig configuration
max_age 5 Number of seconds to cache feature flags configuration fetched from AWS AppConfig
sdk_config None Botocore Config object
jmespath_options None For advanced use cases when you want to bring your own JMESPath functions
logger logging.Logger Logger to use for debug. You can optionally supply an instance of Powertools Logger.
AppConfigStore sample
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from botocore.config import Config

import jmespath

from aws_lambda_powertools.utilities.feature_flags import AppConfigStore

boto_config = Config(read_timeout=10, retries={"total_max_attempts": 2})

# Custom JMESPath functions
class CustomFunctions(jmespath.functions.Functions):

    @jmespath.functions.signature({'types': ['string']})
    def _func_special_decoder(self, s):
        return my_custom_decoder_logic(s)


custom_jmespath_options = {"custom_functions": CustomFunctions()}


app_config = AppConfigStore(
    environment="dev",
    application="product-catalogue",
    name="configuration",
    max_age=120,
    envelope = "features",
    sdk_config=boto_config,
    jmespath_options=custom_jmespath_options
)

Testing your code

You can unit test your feature flags locally and independently without setting up AWS AppConfig.

AppConfigStore only fetches a JSON document with a specific schema. This allows you to mock the response and use it to verify the rule evaluation.

Warning

This excerpt relies on pytest and pytest-mock dependencies.

Unit testing feature flags
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from aws_lambda_powertools.utilities.feature_flags import FeatureFlags, AppConfigStore, RuleAction


def init_feature_flags(mocker, mock_schema, envelope="") -> FeatureFlags:
    """Mock AppConfig Store get_configuration method to use mock schema instead"""

    method_to_mock = "aws_lambda_powertools.utilities.feature_flags.AppConfigStore.get_configuration"
    mocked_get_conf = mocker.patch(method_to_mock)
    mocked_get_conf.return_value = mock_schema

    app_conf_store = AppConfigStore(
        environment="test_env",
        application="test_app",
        name="test_conf_name",
        envelope=envelope,
    )

    return FeatureFlags(store=app_conf_store)


def test_flags_condition_match(mocker):
    # GIVEN
    expected_value = True
    mocked_app_config_schema = {
        "my_feature": {
            "default": expected_value,
            "rules": {
                "tenant id equals 12345": {
                    "when_match": True,
                    "conditions": [
                        {
                            "action": RuleAction.EQUALS.value,
                            "key": "tenant_id",
                            "value": "12345",
                        }
                    ],
                }
            },
            }
    }

    # WHEN
    ctx = {"tenant_id": "12345", "username": "a"}
    feature_flags = init_feature_flags(mocker=mocker, mock_schema=mocked_app_config_schema)
    flag = feature_flags.evaluate(name="my_feature", context=ctx, default=False)

    # THEN
    assert flag == expected_value

Feature flags vs Parameters vs env vars

Method When to use Requires new deployment on changes Supported services
Environment variables Simple configuration that will rarely if ever change, because changing it requires a Lambda function deployment. Yes Lambda
Parameters utility Access to secrets, or fetch parameters in different formats from AWS System Manager Parameter Store or Amazon DynamoDB. No Parameter Store, DynamoDB, Secrets Manager, AppConfig
Feature flags utility Rule engine to define when one or multiple features should be enabled depending on the input. No AppConfig

Deprecation list when GA

Breaking change Recommendation
IN RuleAction Use KEY_IN_VALUE instead
NOT_IN RuleAction Use KEY_NOT_IN_VALUE instead
get_enabled_features Return type changes from List[str] to Dict[str, Any]. New return will contain a list of features enabled and their values. List of enabled features will be in enabled_features key to keep ease of assertion we have in Beta.
boolean_type Schema This might not be necessary anymore before we go GA. We will return either the default value when there are no rules as well as when_match value. This will simplify on-boarding if we can keep the same set of validations already offered.
Last update: 2022-06-23