How to use server-side flag evaluation on PostHog

Install the appropriate PostHog SDK for your server environment. For Python: pip install posthog, for Node.js: npm install posthog-node, for PHP: composer require posthog/posthog-php. Choose the SDK that matches your backend technology stack.

Set up your PostHog client with your project API key. In Python:

import posthog
posthog.api_key = 'your-project-api-key'
posthog.host = 'https://app.posthog.com'

For Node.js:

const PostHog = require('posthog-node')
const client = new PostHog('your-project-api-key')

Configure your PostHog client to enable local flag evaluation by setting personal_api_key parameter. This allows the SDK to fetch flag definitions and evaluate them locally:

posthog.personal_api_key = 'your-personal-api-key'
posthog.poll_interval = 30  # seconds

Navigate to Feature Flags in your PostHog dashboard. Click New Feature Flag, enter a flag key (e.g., 'new-checkout-flow'), set release conditions, and click Save. Ensure the flag is set to Active status for server-side evaluation.

Use the is_feature_enabled method to check if a feature flag is enabled for a specific user. Example:

is_enabled = posthog.is_feature_enabled(
    'new-checkout-flow',
    'user-123',
    person_properties={'email': 'user@example.com'}
)

For multivariate flags, use get_feature_flag to retrieve the specific variant value:

variant = posthog.get_feature_flag(
    'checkout-button-color',
    'user-123',
    person_properties={'plan': 'premium'}
)
if variant == 'red':
    # Show red button

Wrap flag evaluation in try-catch blocks to handle network issues or SDK errors gracefully:

try:
    is_enabled = posthog.is_feature_enabled('feature-key', user_id)
except Exception as e:
    # Log error and use default behavior
    is_enabled = False

Check the Feature Flags section in PostHog to monitor evaluation metrics, user distribution, and flag performance. Use the Insights tab to analyze how flag changes affect user behavior and key metrics.