unleash.unleash/website/docs/how-to/how-to-send-impression-data-to-a-sink.md

title
How to send impression data to a sink

Unleash allows you to gather impression data from your feature toggles, giving you complete visibility into who checked what toggles and when. What you do with this data is entirely up to you, but a common use case is to send it off to an aggregation and analytics service such as Posthog or Google Analytics.

This guide will take you through everything you need to do in Unleash to facilitate such a workflow. It will show you how to send data to Posthog as an example sink, but the exact same principles will apply to any other service of the same kind.

Prerequisites

We will assume that you have the necessary details for your third-party service:

• where to send the data to. We'll refer to this in the code samples below as <sink-url>.
• what format the data needs to be in. This will determine how you transform the event data before you send it.

In addition you'll need to have a toggle to record impression data for and an Unleash client SDK that supports impression data. This guide will use the JavaScript proxy SDK.

When you have those things sorted, follow the below steps.

Enable impression data for your feature toggle

Because impression data is an opt-in feature, the first step is to enable it for the feature you want to gather data from. You can use both the UI and the API.

Enabling impression data for new feature toggles

When you create a new feature toggle via the UI, there's an option at the end of the form that you must enable.

Enabling impression data for existing feature toggles

Capture impression events in your client

Initialize your analytics service

posthog.identify(userId);

Set up a listener

unleash.on("impression", (event) => {
    posthog.capture(event.eventType, {
        ...event.context,
        distinct_id: event.context?.userId,
        featureName: event.featureName,
        enabled: event.enabled,
        variant: event.variant,
    });
});

option:

unleash.on("impression", (event) => {
    posthog.capture(event.eventType, transform(event));
});

Transform and record the data

Posthog requires the distinct_id property. For the rest, we'll just spread everything into a flat object.

const transform = (event) => ({
    ...event.context,
    distinct_id: event.context?.userId,
    featureName: event.featureName,
    enabled: event.enabled,
    variant: event.variant,
})

Full example

import posthog from "posthog-js";

const unleash = new UnleashClient({
  url: 'https://eu.unleash-hosted.com/hosted/proxy',
  clientKey: 'your-proxy-key',
  appName: 'my-webapp',
});

posthog.identify(userId);

unleash.start();

unleash.on("ready", () => {
  unleash.isEnabled("my-feature-toggle");
})

unleash.on("impression", (event) => {
    posthog.capture(event.eventType, {
        ...event.context,
        distinct_id: event.context?.userId,
        featureName: event.featureName,
        enabled: event.enabled,
        variant: event.variant,
    });
})