PostHog Integration

Send Featureflow flag exposures to PostHog as $feature_flag_called events — the event PostHog experiments use to decide who was exposed to which variant — with Featureflow deciding the assignment. See A/B testing with your analytics tool for the whole loop.

Setup

PostHog counts a user as exposed to an experiment when it receives a $feature_flag_called event whose $feature_flag matches the experiment's flag key and whose $feature_flag_response is a variant. The JavaScript SDK's exposureIntegration sends one per user, flag and variant:

import Featureflow, { exposureIntegration } from 'featureflow-client';
import posthog from 'posthog-js';

posthog.init('YOUR-POSTHOG-PROJECT-KEY', { api_host: 'https://us.i.posthog.com' });

const featureflow = await Featureflow.init('your-js-client-key', {
  integrations: [
    exposureIntegration(({ key, variant }) => {
      posthog.capture('$feature_flag_called', {
        $feature_flag: key,
        $feature_flag_response: variant,
        [`$feature/${key}`]: variant
      });
      // Attach the variant to every later event from this user, so metric events
      // can be attributed when the experiment uses a custom exposure event.
      posthog.register({ [`$feature/${key}`]: variant });
    })
  ]
});

Exposures go through your PostHog instance, so they carry your PostHog distinct id and person — no user-id mapping needed — and are deduplicated per user, flag and variant for the page's lifetime, so calling evaluate() on every render will not inflate your PostHog event volume.

Send only the flags you're experimenting with

exposureIntegration(send, { flags: ['checkout-v2', 'pricing-test'] })
// or a naming convention
exposureIntegration(send, { flags: (key) => key.startsWith('exp-') })

In PostHog

1. Create an Experiment whose feature flag key is the same key as your Featureflow flag, with variant keys matching your Featureflow variant keys (for example control and test). The flag lives in Featureflow — you do not need to roll it out in PostHog; PostHog only needs the key to match incoming exposures.
2. Pick your goal metrics — events you already capture.
3. PostHog attributes each user to the variant in their exposure event; metric events do not need any flag property when the experiment uses the default $feature_flag_called exposure. The $feature/<key> property registered above is what PostHog requires if you switch the experiment to a custom exposure event.

Notes

• Variant keys must match on both sides — PostHog reports by the $feature_flag_response value it receives.
• Available from featureflow-client 2.6.0 / react-featureflow-client 2.6.0. On older versions, listen to the raw EVALUATION event and dedupe yourself.
• Server-side, the Node SDK emits a raw evaluation event you can forward with posthog-node's capture() using your own distinct id — see the Node SDK README.

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Any problems? Reach us via the in-app chat or at support@featureflow.io.