Adding retrack destination function example

Readme.md

@@ -35,7 +35,8 @@ this repo as the set of examples.
 - [Slack](./destinations/slack) - Adds a Gravatar icon to events with an email and sends messages to Slack
 - [Zendesk](./destinations/zendesk) - Create new Zendesk tickets triggered by events that you send
 - [Datadog](./destinations/datadog) - Sends a metric to datadog with high level message/event type as tags
-- [Optimizely](./destinations/optimizely) - Sends conversion metrix to optimizely.
+- [Optimizely](./destinations/optimizely) - Sends conversion metrix to optimizely
+- [Retrack](./destinations/retrack) - Intercepts track event, updates user identity and enriches said event
 
 ## Development
 

destinations/retrack/Readme.md

@@ -0,0 +1,11 @@
+# Re-track Custom Destination Function
+
+Example of destination function which:
+* Intercepts track event from Segment source attached to it,
+* modifies userId and posts [alias](https://segment.com/docs/connections/spec/alias/) request, so Segment will know anticipated identity,
+* enriches event properties and posts [track](https://segment.com/docs/connections/spec/track/) request, so Segment will know enriched event,
+* all requests are being forwarded to another [Segment HTTP Tracking API Source](https://segment.com/docs/connections/sources/catalog/libraries/server/http-api/), given `apiKey` from function settings.
+
+## Settings
+
+- `apiKey` {String} ([write key of Segment HTTP Tracking API Source](https://segment.com/docs/connections/sources/catalog/libraries/server/http-api/))

destinations/retrack/handler.js

@@ -0,0 +1,81 @@
+/**
+ * @typedef {Object} SegmentTrackEvent
+ * @see {@link https://segment.com/docs/connections/spec/track/}  for full track event spec
+ * @see {@link https://segment.com/docs/connections/spec/common/} for full common fields spec
+ * @property {String} event             Track event name
+ * @property {String} messageId         Original API message identifier
+ * @property {String} userId            Tracked user id that we want to modify and re-track
+ * @property {String} timestamp         Timestamp of event in ISO 8601 format
+ * @property {Object} properties        Track event arbitrary properties, we want to add 3 own
+ */
+
+/**
+ * @typedef {Object} FunctionSettings
+ * @property {String} apiKey            Write key of source to forward alias and track events to
+ */
+
+/**
+ * Call Segment V1 API, using HTTP API Segment Source by write key
+ * @see {@link https://segment.com/docs/connections/sources/catalog/libraries/server/http-api/} for source doc
+ * @internal
+ * @param {String} apiKey
+ * @param {String<'alias'|'track'>} route
+ * @param {Object} body
+ * @return {Promise<void>}
+ */
+async function callSegmentAPI(apiKey, route, body) {
+  let response;
+
+  try {
+    response = await fetch(`https://api.segment.io/v1/${route}`, {
+      method: 'POST',
+      headers: {
+        Authorization: `Basic ${btoa(`${apiKey}:`)}`,
+        'Content-Type': 'application/json',
+      },
+      body: JSON.stringify(body),
+    });
+  } catch (error) {
+    throw new RetryError(error.message);
+  }
+
+  if (response.status >= 500 || response.status === 429) {
+    throw new RetryError(`Failed with ${response.status}`);
+  }
+}
+
+/**
+ * Handle track event
+ * @param {SegmentTrackEvent} event
+ * @param {FunctionSettings} settings
+ * @return {Promise<void>}
+ */
+async function onTrack(event, settings) {
+  const { apiKey } = settings;
+
+  if (!apiKey) {
+    throw new ValidationError('Forward source write key is required');
+  }
+
+  const anticipatedId = crypto.createHash('md5').update(event.userId).digest('hex');
+
+  const [slug1, slug2, slug3] = event.userId.split('-');
+
+  await Promise.all([
+    callSegmentAPI(apiKey, 'alias', {
+      previousId: event.userId,
+      userId: anticipatedId,
+      timestamp: event.timestamp,
+    }),
+    callSegmentAPI(apiKey, 'track', {
+      event: event.event,
+      userId: anticipatedId,
+      properties: {
+        ...event.properties,
+        slug1,
+        slug2,
+        slug3,
+      },
+    }),
+  ]);
+}