Destination Functions

Destination functions allow you to transform and annotate your Segment events and send them to any external tool or API without worrying about setting up or maintaining any infrastructure.

All functions are scoped to your workspace, so members of other workspaces won’t be able to view or use them.

Functions is available to all customer plan types with a free allotment of usage hours. Read more about Functions usage limits, or see your workspace’s Functions usage stats.

Destination functions can’t currently accept data from Object Cloud sources.

Create a destination function

  1. From your workspace, go to the Catalog and click the Functions tab.
  2. Click New Function.
  3. Select Destination Function and click Build.

Tip: Want to see some example functions? Check out the templates available in the Functions UI, or in the open-source Segment Functions Library. (Contributions welcome!)

When you click Build, a code editor appears. Use the editor to write the code for your function, configure settings, and test the function’s behavior.

Functions Editor

Code the destination function

Segment invokes a separate part of the function (called a “handler”) for each event type that you send to your destination function.

Your function is not invoked for an event if a destination filter is set up, and the event doesn’t pass the filter.

The default source code template includes handlers for all event types. However, you do not need to implement all of them - just use the ones you need, and skip the ones you don’t.

Destination functions can define handlers for each message type in the Segment spec:

  • onIdentify
  • onTrack
  • onPage
  • onScreen
  • onGroup
  • onAlias
  • onDelete
  • onBatch

Each of the functions above accepts two arguments:

  • event - Segment event object, where fields and values depend on the type of event. For example, for “Identify” events, the object is formatted to match the Identify spec.
  • settings - Set of settings for this function.

We’ll learn more about settings later, let’s see how we can process Segment events with destination function first.

The example below shows a destination function that listens for “Track” events, and sends some details about them to an external service.

async function onTrack(event) {
  await fetch('', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    body: JSON.stringify({
      event_name: event.event,
      timestamp: event.timestamp

To change which type of event the handler listens to, you can rename it to the name of the message type. For example, if you rename this function onIdentify, it listens for “Identify” events instead.

Functions’ runtime includes a fetch() polyfill using a node-fetch package. Check out the node-fetch documentation for usage examples.

Errors and error handling

A function’s execution is considered successful if it finishes without any errors. You can also throw an error to indicate a failure on purpose. You can use these errors to validate event data before processing it, to ensure your function works as expected.

You can throw the following pre-defined error types to indicate that the function ran as expected, but that data could not be delivered:

  • EventNotSupported
  • InvalidEventPayload
  • ValidationError
  • RetryError

The examples show basic uses of these error types.

async function onGroup(event) {
  if (! {
    throw new InvalidEventPayload('Company name is required')

async function onPage(event) {
  if (! {
    throw new ValidationError('Page name is required')

async function onAlias(event) {
  throw new EventNotSupported('Alias event is not supported')

async function onTrack(event) {
  let res
  try {
    res = await fetch('', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json'
      body: JSON.stringify({ event })
  } catch (err) {
    // Retry on connection error
    throw new RetryError(err.message)
  if (res.status >= 500 || res.status === 429) {
    // Retry on 5xx and 429s (ratelimits)
    throw new RetryError(`HTTP Status ${res.status}`)

If you do not supply a function for an event type, Segment throws an EventNotSupported error by default.

You can read more about error handling below.

Runtime and dependencies

Functions use Node.js 10.x.

Functions do not currently support importing dependencies, but you can contact Segment Support to request that one be added.

The following dependencies are installed in the function environment by default.

Only the crypto Node.js module is included (exposed as crypto). Other built-in Node.js modules are not available.


Per-function global caching is available in the cache namespace. The following functions are available:

  • cache.load(key: string, ttl: number, fn: async () => any): Promise<any>
    • Obtains a cached value for the provided key, invoking the callback if the value is missing or has expired. The ttl is the maximum duration in milliseconds the value can be cached. If omitted or set to -1, the value will have no expiry. There is no guarantee that a value will be retained in the cache for the provided duration, however. The cache space is limited, so efforts to minimize the cached value size will afford a higher cache hit ratio.
  • cache.delete(key: string): void
    • Forcefully remove the value associated withe the key.

The following example gets a JSON value through the cache, only invoking the callback as needed:

const ttl = 5 * 60 * 1000 // 5 minutes
const val = await cache.load("mycachekey", ttl, () => {
    const res = await fetch("")
    const data = await res.json()
    return data

Create settings and secrets

Settings allow you to pass configurable variables to your function, which is the best way to pass sensitive information such as security tokens. For example, you might use settings as placeholders to use information such as an API endpoint and API key. This way, you can use the same code with different settings for different purposes. When you deploy a function in your workspace, you are prompted to fill out these settings to configure the function.

First, add a setting in Settings tab in the code editor:

Settings Tab

Click Add Setting to add your new setting.

Add Setting Dialog

You can configure the details about this setting, which change how it’s displayed to anyone using your function:

  • Label - Name of the setting, which users see when configuring the function.
  • Name - Auto-generated name of the setting to use in function’s source code.
  • Type - Type of the setting’s value.
  • Description - Optional description, which appears below the setting name.
  • Required - Enable this to ensure that the setting cannot be saved without a value.
  • Encrypted - Enable to encrypt the value of this setting. Use this setting for sensitive data, like API keys.

As you change the values, a preview to the right updates to show how your setting will look and work.

Click Add Setting to save the new setting.

Once you save a setting, it appears in the Settings tab for the function. You can edit or delete settings from this tab.

Settings Tab

Next, fill out this setting’s value in the Test tab, so you can run the function and verify that the correct setting value is passed. (This value is only for testing your function.)

Test Value For Setting

Now that you’ve configured a setting and entered a test value, you can add code to read its value and run the function, as in the example below:

async function onTrack(request, settings) {
  const apiKey = settings.apiKey
  //=> "super_secret_string"

When you deploy your destination function in your workspace, you fill out the settings on the destination configuration page, similar to how you would configure a normal destination.

Destination Function Settings

Test the destination function

You can test your code directly from the editor in two ways:

Use sample events for testing

Click Use Sample Event and select the source to use events from.

Capture events to test your function

Click Run to test your function with the event you selected.

Test using manual input

You can also manually include your own JSON payload of a Segment event, instead of fetching a sample from one of your workspace sources.

Functions Editor Event Tester

If your function fails, you can check the error details and logs in the Output section.

  • Error Message - This shows the error surfaced from your function.
  • Logs - This section displays any messages to console.log() from the function.

Batching the destination function (Beta)

Batch handling for Functions is currently available as an early access beta release. By enabling batch handlers for your function, you acknowledge that your use of batch handlers is subject to Segment’s Beta Terms and Conditions, or the applicable terms governing Beta Releases found in your subscription agreement with Segment.

If you notice any bugs or have any general feedback on this new feature, contact

Batch handlers are an extension of destination functions. When you define an onBatch handler alongside the handler functions for single events (for example: onTrack or onIdentity), you’re telling Segment that the destination function can accept and handle batches of events.

Batching is available to destination functions only.

When to use batching

Consider creating a batch handler if:

  • Your function sends data to a service that has a batch endpoint. Batch endpoints may allow you both to send more data downstream and stay within the rate limits imposed by the service. Batch handlers that use one or more batch endpoints improve the efficiency of the function, and enable it to scale more easily. Specifically, you can use batch handlers to build list-based Personas destinations.
  • You have a high-throughput function and want to reduce cost. When you define a batch handler, Segment invokes the function once per batch, rather than once per event. As long as the function’s execution time isn’t adversely affected, the reduction in invocations should lead to a reduction in cost.

If a batched function receives too low a volume of events (under one event per second) to be worth batching, Segment may not invoke the batch handler.

Define the batch handler

Segment collects the events over a short period of time and combines them into a batch. The system flushes them when the batch reaches a certain number of events, or when the batch has been waiting for a specified wait time.

To create a batch handler, define an onBatch function within your destination function. You can also use the “Default Batch” template found in the Functions editor to get started quickly.

async function onBatch(events, settings){
  // handle the batch of events

The onBatch handler is an optional extension. Destination functions must still contain single event handlers as a fallback, in cases where Segment does not receive enough events to execute the batch.

The handler function receives an array of events. The events can be of any supported type, and a single batch may contain more than one event type. Handler functions also receive function settings.

For example, you could send the array of events to an external services batch endpoint:

async function onBatch(events, settings) {
  await fetch('', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    body: JSON.stringify(events) // send a JSON array

Configure the event types within a batch

Segment batches together any event of any type that it sees over a short period of time to increase batching efficiency and give you the flexibility to decide how batches are created. If you want to split batches by event type, you can implement this in your functions code by writing a handler.

If your downstream endpoint requires events of a single type, you can write a handler that groups events by type, and then handles the events.

async function onBatch(events, settings) {
  // group events by type
  const eventsByType = {}
  for (const event of events) {
    if (!(event.type in eventsByType)) {
      eventsByType[event.type] = []

  // concurrently process sub-batches of a specific event type
  const promises = Object.entries(eventsByType).map(([type, events]) => {
    switch (type) {
    case 'track':
      return onTrackBatch(events, settings)
    case 'identify':
      return onIdentifyBatch(events, settings)
    // ...handle other event types here...
  return Promise.all(promises)

async function onTrackBatch(events, settings) {
  // handle a batch of track events

async function onIdentifyBatch(events, settings) {
  // handle a batch of identify events

Configure your batch parameters

You cannot yet configure batch parameters (either in the code or UI) in this version of the beta. If you would like to change your batch parameters, contact with information about your specific use case, the reason you need to adjust parameters, and the URL to your destination function.

Avoid writing batch and single event handlers

Your function might not get enough event traffic to create a batch of short windows of time. In this case, your function invokes the single event handler. If you need to consolidate your code into an onBatch handler, you can define single event handlers that call onBatch.

async function onTrack(event, settings) {
  return onBatch([event], settings)// defer to onBatch

async function onIdentify(event, settings) {
  return onBatch([event], settings) // defer to onBatch

// do the same for onAlias, onGroup, onPage, onScreen, onDelete

async function onBatch(events, settings) {
  // handle batch of events

Test the batch handler

The Functions editing environment supports testing batch handlers. In the right panel of the Functions editor, click customize the event yourself to enter Manual Mode. Add events as a JSON array, with one event per element. Click Run to preview the batch handler with the specified events.

Batch handler testing

The Sample Event option tests single events only. You must use Manual Mode to add more than one event so you can test batch handlers.

The editor displays logs and request traces from the batch handler.

The Config API Functions/Preview endpoint also supports testing batch handlers. The payload must be a batch of events as a JSON array.

Handling batching errors

Standard function error types apply to batch handlers. Segment attempts to retry the batch in the case of Timeout or Retry errors. For all other error types, Segment discards the batch. If only part of a batch succeeds, Segment does not retry the failing part of the batch.

Error Type Result
Bad Request Discard
Invalid Settings Discard
Message Rejected Discard
RetryError Retry
Timeout Retry
Unsupported Event Type Discard

Save and deploy the function

Once you finish building your destination function, click Configure to name it, then click Create Function to save it.

Once you do that, the destination function appears on the Functions page in your workspace’s catalog.

If you’re editing an existing function, you can Save changes without updating instances of the function that are already deployed and running.

You can also choose to Save & Deploy to save the changes, and then choose which of the already-deployed functions to update with your changes. You might need additional permissions to update existing functions.

Destination functions logs and errors

A function can throw errors, or Segment might encounter errors while invoking your function. You can view these errors in the Event Delivery tab for your Destination as in the example below.

Destination Function Event Delivery tab

Destination functions error types

  • Bad Request - Any error thrown by the function code that is not covered by the other errors.
  • Invalid Settings - A configuration error prevented Segment from executing your code. If this error persists for more than an hour, contact Segment Support.
  • Message Rejected - Your code threw InvalidEventPayload or ValidationError due to invalid input.
  • Unsupported Event Type - Your code does not implement a specific event type (onTrack(), etc.) or threw a EventNotSupported error.
  • Retry - Your code threw RetryError indicating that the function should be retried.

Segment only attempts to send the event to your destination function again if a Retry error occurs.

Destination functions logs

If your function throws an error, execution halts immediately. Segment captures the event, any outgoing requests/responses, any logs the function might have printed, as well as the error itself.

Segment then displays the captured error information in the Event Delivery page for your destination function. You can use this information to find and fix unexpected errors.

Destination Function error logs

You can throw an error or a custom error and you can also add helpful context in logs using the console API. For example:

async function onTrack(event, settings) {
  const userId = event.userId

  console.log('User ID is', userId)

  if (typeof userId !== 'string' || userId.length < 8) {
    throw new ValidationError('User ID is invalid')

  console.log('User ID is valid')

Warning: Do not log sensitive data, such as personally-identifying information (PII), authentication tokens, or other secrets. You should especially avoid logging entire request/response payloads. The Function Logs tab may be visible to other workspace members if they have the necessary permissions.

Caching in destination functions

Functions execute only in response to incoming data, but the environments that functions run in are generally long-running. Because of this, you can use global variables to cache small amounts of information between invocations. For example, you can reduce the number of access tokens you generate by caching a token, and regenerating it only after it expires. Segment cannot make any guarantees about the longevity of environments, but by using this strategy, you can significantly improve the performance and reliability of your Functions by reducing the need for redundant API requests.

This example code fetches an access token from an external API and refreshes it every hour:

const TOKEN_EXPIRE_MS = 60 * 60 * 1000 // 1 hour
let token = null
async function getAccessToken () {
  const now = new Date().getTime()
  if (!token || now - token.ts > TOKEN_EXPIRE_MS) {
    const resp = await fetch('', {
      method: 'POST'
    }).then(resp => resp.json())
    token = {
      ts: now,
      value: resp.token
  return token.value

Managing destination functions

Functions permissions

Functions have specific roles which can be used for access management in your Segment workspace.

Access to functions is controlled by two permissions roles:

  • Functions Admin: Create, edit and delete all functions, or a subset of specified functions.
  • Functions Read-only: View all functions, or a subset of specified functions.

You also need additional Source Admin permissions to enable source functions, connect destination functions to a source, or to deploy changes to existing functions.

Editing and deleting functions

If you are a Workspace Owner or Functions Admin, you can manage your function from the Functions page.

Monitoring destination functions

You can use Destination Event Delivery to understand if Segment encounters any issues delivering your source data to destinations. Errors that the Function throws appear here.

If any of your deployed function instances are failing consistently, they will also appear in Connection Health.

Data control

You can use Destination Filters or Privacy Portal to manage what events and, of those events, which event properties are sent to your destination function.

Destination functions FAQs

Can I see who made changes to a function?

Yes, Functions access is logged in the Audit Trail, so user activity related to functions appears in the logs.

Does Segment retry failed function invocations?

Segment retries 9 times over the course of 4 hours. This increases the number of attempts for messages, so we try to re-deliver them another 4 times after some backoff. Segment doesn’t retry if your function throws a non-recoverable error.

Are events guaranteed to send data in order?

No, Segment can’t guarantee the order in which the events are delivered to an endpoint.

Can I create a device-mode destination?

No, destination functions are currently available as cloud-mode destinations only. We’re in the early phases of exploration and discovery for supporting customer “web plugins” for custom device-mode destinations and other use cases, but this is unsupported today.

How do I publish a destination to the public Segment catalog?

If you are a partner, looking to publish your destination and distribute your app through Segment catalog, visit the Developer Center and check out our partner docs.

How does batching affect visibility?

The Event Delivery tab continues to show metrics for individual events, even if they are batched by your function code. For more information, see Destination functions logs and errors.

How does batching impact function use and cost?

A function’s use depends on the number of times it is invoked, and the amount of time it takes to execute. When you enable batching, Segment invokes your function once per batch rather than once per event. The volume of events flowing through the function determines the number of batches, which determines the number of invocations.

If you’re sending your batch to an external service, the execution time of the function depends on the end-to-end latency of that service’s batch endpoint, which may be higher than an endpoint that receives a single event.

This page was last modified: 08 Feb 2021

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