Note: It's worth reading the Building apps overview for a quick introduction to how to build your own app.

plugin.json file

A plugin.json file is structured as follows:

{
  "name": "<plugin_display_name>",
  "url": "<repo_url>",
  "description": "<description>",
  "main": "<entry_point>",
  "config": [
    {
      "markdown": "A Markdown block.\n[Use links](http://example.com) and other goodies!"
    },
    {
      "key": "param1",
      "name": "<param1_name>",
      "type": "<param1_type>",
      "default": "<param1_default_value>",
      "hint": "<param1_hint_value>",
      "required": true,
      "secret": true
    },
    {
      "key": "param2",
      "name": "<param2_name>",
      "type": "<param2_type>",
      "default": "<param2_default_value>",
      "required": false
    }
  ]
}

Here's an example plugin.json file from our 'Hello world app':

{
  "name": "Hello World",
  "url": "https://github.com/PostHog/helloworldplugin",
  "description": "Greet the World and Foo a Bar, JS edition!",
  "main": "index.js",
  "config": [
    {
      "markdown": "This is a sample app!"
    },
    {
      "key": "bar",
      "name": "What's in the bar?",
      "type": "string",
      "default": "baz",
      "hint": "This will be sent in a **property**",
      "required": false
    }
  ]
}

Most options in this file are self-explanatory, but there are a few worth exploring further:

main

main determines the entry point for your app, where your setupPlugin and processEvent functions are. More on these later.

config

config consists of an array of objects that each pertain to a specific configuration field or markdown explanation for your plugin.

Each object in a config can have the following properties:

Note: You can have a config field that only contains markdown. This won't be used to configure your app but can be placed anywhere in the config array and is useful for customizing the content of your app's configuration step in the PostHog UI.

PluginMeta

Check out App Types for a full spec of types for app authors.

Every plugin server function is called by the server with an object of type PluginMeta that will always contain the object cache, and can also include global, attachments, and config, which you can use in your logic.

Here's what they do:

config

Gives you access to the app config values as described in plugin.json and configured via the PostHog interface.

Example:

export async function processEvent(event, { config }) {
    event.properties['greeting'] = config.greeting
    return event
}

global

The global object is used for sharing functionality between setupPlugin and the rest of the special functions, like processEvent, onEvent, or runEveryMinute, since global scope does not work in the context of PostHog apps. global is not shared across worker threads

Example:

export function setupPlugin({ global, config }) {
    global.eventsToTrack = (config.eventsToTrack || '').split(',') 
}

export function processEvent(event, { global, config }) {
    if(global.eventsToTrack.includes(event.event)) {
        // Do something
    }
}

attachments

attachments gives access to files uploaded by the user for config parameters of type attachment. An attachment has the following type definition:

interface PluginAttachment {
    content_type: string
    file_name: string
    contents: any
}

As such, accessing the contents of an uploaded file can be done with attachments.attachmentName.contents.

Example:

export function setupPlugin({ attachments, global }: Meta) {
    if (attachments.maxmindMmdb) {
        global.ipLookup = new Reader(attachments.maxmindMmdb.contents)
    }
}

Maximizing reliability with RetryError

Since plugins generally handle data in some way, it's crucial for data integrity that each plugin is as reliable as possible. One system-level mechanism you can leverage to improve reliability is function retries.

While normally a plugin function simply fails without ceremony the moment it throws an error, select functions can be retried by throwing a special error type: RetryError – which is included in the @posthog/plugin-scaffold package.

As an example, it's safe to assume that a connection to an external service will fail eventually. Due to security considerations, setTimeout cannot be used in a plugin to wait until the network problem has passed, but with function retries the solution is even simpler! Just catch the connection error and throw new RetryError – the system will re-run the function for you:

import { RetryError } from '@posthog/plugin-scaffold'

export function setupPlugin() {
    try {
        // Some network connection
    } catch {
        throw new RetryError('Service is unavailable, but it might be back up in a moment')
    }
}

At the same time, make sure NOT to use RetryError when the problem cannot be intermittent – perhaps an invalid config, an unhandled edge case, or just a random bug in the code of the plugin. Retrying such a case would just put extra load on the system, without any benefit.

import { RetryError } from '@posthog/plugin-scaffold'

export function setupPlugin({ config }) {
    let eventsToTrack
    try {
        eventsToTrack = config.nonExistentKey.split(',')
    } catch {
        throw new RetryError('Retrying this will never help')
    }
}

The maximum number of retries is documented with each function, as it might differ across them. However, the mechanism is constant in its use of exponential backoff, that is: the wait time between retries is doubled with each attempt. For instance, if the 1st retry takes place 1 s after the initial failure, the gap between the 5th and the 6th will be 32 s (2^5).

As of PostHog 1.37+, the following functions are retriable:

• setupPlugin
• onEvent

setupPlugin function

setupPlugin is a function you can use to dynamically set app configuration based on the user's inputs at the configuration step.

You could, for example, check if an API Key inputted by the user is valid and throw an error if it isn't, prompting PostHog to ask for a new key.

It takes only an object of type PluginMeta as a parameter and does not return anything.

Example (from the PostHog MaxMind app):

export function setupPlugin({ attachments, global }) {
    if (attachments.maxmindMmdb) {
        global.ipLookup = new Reader(attachments.maxmindMmdb.contents)
    }
}

setupPlugin can be retried up to 5 times (first retry after 5 s, then 10 s after that, 20 s, 40 s, lastly 80 s) by throwing RetryError. Attempting to retry more than 5 times disables the plugin. The plugin is disabled immediately if any error other than RetryError is thrown in setupPlugin.

On PostHog Cloud and email-enabled instances of PostHog, project members are notified by email of the plugin being disabled automatically. This is to ensure that action is taken if the plugin is important for data integrity.

teardownPlugin function

teardownPlugin is ran when an app VM is destroyed, because of, for example, a app server shutdown or an update to the app. It can be used to flush/complete any operations that may still be pending, like exporting events to a third-party service.

async function teardownPlugin({ global }) {
  await global.buffer.flush()
}

processEvent function

processEvent is the juice of your app.

In essence, it takes an event as a parameter and returns an event as a result. In the process, this event can be:

• Modified
• Sent somewhere else
• Not returned (preventing ingestion)

It takes an event and an object of type PluginMeta as parameters and returns an event.

Here's an example (from the 'Hello World App'):

async function processEvent(event, { config, cache }) {
    const counter = await cache.get('counter', 0)
    cache.set('counter', counter + 1)

    if (event.properties) {
        event.properties['hello'] = 'world'
        event.properties['bar'] = config.bar
        event.properties['$counter'] = counter
    }

    return event
}

As you can see, the function receives the event before it is ingested by PostHog, adds properties to it (or modifies them), and returns the enriched event, which will then be ingested by PostHog (after all apps run).

onEvent function

Minimum PostHog version: 1.25.0

onEvent works similarly to processEvent, except any returned value is ignored by the app server. In other words, onEvent can read an event but not modify it.

In addition, onEvent functions will run after all enabled apps have run processEvent. This ensures you will be receiving an event following all possible modifications to it.

This was originally built for and is particularly useful for export apps. These apps need to receive the "final form" of an event and send it out of PostHog, without having to modify it.

Here's a quick example:

async function onEvent(event) {
    // do something to the event
    sendEventToSalesforce(event)

    // no need to return anything
}

onEvent can be retried up to 5 times (first retry after 5 s, then 10 s after that, 20 s, 40 s, lastly 80 s) by throwing RetryError. Attempting to retry more than 5 times is ignored.

Available packages and imports

Global

fetch

⚠️ Be very careful when using fetch to send events to a PostHog instance from processEvent or onEvent! The event captured will also be run through all the installed apps and could potentially lead to an infinite loop of event generation.

Equivalent to node-fetch.

Available imports

For example:

import { Plugin, PluginMeta } from '@posthog/plugin-scaffold'

You can also use require for imports.

Testing

In order to ensure apps are stable and work as expected for all their users, we highly recommend writing tests for every app you build.

Adding testing capabilities to your app

You will need to add jest and our app testing scaffold to your project in your package.json file:

"jest": {
    "testEnvironment": "node"
},
"scripts": {
    "test": "jest ."
},
"devDependencies": {
    "@posthog/plugin-scaffold": "*",
    "jest": "^27.0.4"
}

Create your test files e.g. index.test.js or index.test.ts for testing your index.js or index.ts file

Writing tests

Write tests in jest, you can learn more about the syntax and best practices in the jest documentation. We recommend writing tests to cover the primary functions of your app (e.g. does it create events in the expected format) and also for edge cases (e.g. does it crash if no data is sent).

For more information on how to setup testing, take a look at this guide.

Logs

Apps can make use of the console for logging and debugging. console.log, console.warn, console.error, console.debug, console.info are all supported.

These logs can be seen on the 'Logs' page of each app, which can be accessed on the 'Apps' page of the PostHog UI.