Ruby

The posthog-ruby library provides tracking functionality on the server-side for applications built in Ruby.

It uses an internal queue to make calls fast and non-blocking. It also batches requests and flushes asynchronously, making it perfect to use in any part of your web app or other server-side application that needs performance.

Installation

Add this to your Gemfile:

gem "posthog-ruby"

In your app, set your API key before making any calls. If setting a custom host, make sure to include the protocol (e.g. https://).

posthog = PostHog::Client.new({
  api_key: "<ph_project_api_key>",
  host: "<ph_instance_address>", # You can remove this line if you're using https://app.posthog.com
  on_error: Proc.new { |status, msg| print msg }
})

You can find your key in the 'Project Settings' page in PostHog.

Debug logging

The log level by default is set to WARN. You can change it to DEBUG if you want to debug the client by running posthog.logger.level = Logger::DEBUG, where posthog is your initialized PostHog::Client instance.

Making calls

Capture

Capture allows you to capture anything a user does within your system, which you can later use in PostHog to find patterns in usage, work out which features to improve or where people are giving up.

A capture call requires:

• distinct id which uniquely identifies your user
• event name to specify the event

• We recommend naming events with "[noun][verb]", such as movie played or movie updated, in order to easily identify what your events mean later on (we know this from experience).

Optionally you can submit:

• properties, which is a dictionary with any information you'd like to add
• timestamp, a datetime object for when the event happened. If this isn't submitted, it'll be set to the current time
• send_feature_flags, a boolean that determines whether to send current known feature flags with this event. This is useful when running experiments which depends on this event. However, when this is set to true, we will make an additional request to fetch the current feature flags and thus slow things down. A workaround is to manually compute feature flag information so that we can avoid making the request.

For example:

posthog.capture({
    distinct_id: 'distinct id',
    event: 'movie played',
    properties: {
        movie_id: '123',
        category: 'romcom'
    }
})

Setting user properties

To set user properties, include the properties you'd like to set when capturing an event:

posthog.capture({
    distinct_id: 'distinct_id',
    event: 'event_name',
    properties: {
        '$set': { name: 'Max Hedgehog' },
        '$set_once': { initial_url: '/blog' }
    }
})

For more details on the difference between $set and $set_once, see our user properties docs.

Alias

Sometimes, you may want to assign multiple distinct IDs to a single user. This is helpful in scenarios where your primary distinct ID may be inaccessible. For example, if a distinct ID which is typically used on the frontend is not available in certain parts of your backend code. In this case, you can use alias to assign another distinct ID to the same user.

We strongly recommend reading our docs on alias to best understand how to correctly use this method.

Sending page views

If you're aiming for a backend-only implementation of PostHog and won't be capturing events from your frontend, you can send pageviews from your backend like so:

posthog.capture({
    distinct_id: 'distinct id',
    event: '$pageview',
    properties: {
        '$current_url': 'https://example.com'
    }
})

Feature flags

PostHog's feature flags enable you to safely deploy and roll back new features.

When using them with one of libraries, you should check if a feature flag is enabled and use the result to toggle functionality on and off in you application.

How to check if a flag is enabled

Note: Whenever we face an error computing the flag, the library returns Nil, instead of true or false.

posthog.is_feature_enabled('beta-feature', 'distinct id')
# returns True or False or Nil

Example use case

Here's how you might send different users a different version of your homepage, for example:

def homepage(request):
    template = "new.html" if posthog.is_feature_enabled('new_ui', 'distinct id') else "old.html"
    return render_template(template, request=request)

Note: Feature flags are persistent for users across sessions. Read more about feature flag persistence on our dedicated page.

Get a flag value

If you're using multivariate feature flags, you can also get not just whether the flag is enabled, but what value its enabled to.

Note: Whenever we face an error computing the flag, the library returns Nil, instead of true or false or a string variant value.

posthog.get_feature_flag('beta-feature', 'distinct id')
# returns string or True or False or Nil

Overriding server properties

Sometimes, you might want to evaluate feature flags using properties that haven't been ingested yet, or were set incorrectly earlier. You can do so by setting properties the flag depends on with these calls.

For example, if the beta-feature depends on the is_authorized property, and you know the value of the property, you can tell PostHog to use this property, like so:

posthog.get_feature_flag('beta-feature', 'distinct id', person_properties: {'is_authorized': True})

You can pass symbols or strings, the library treats them the same.

The same holds for groups. If you have a group name organisation, you can add properties like so:

posthog.get_feature_flag('beta-feature', 'distinct id', groups: {'organisation': 'google'}, group_properties: {'organisation': {'is_authorized': True})
# returns string or Nil

Getting all flag values

You can also get all known flag values as well. This is useful when you want to seed a frontend client with initial known flags. Like all methods above, this also takes optional person and group properties, if known.

posthog.get_all_flags('distinct id', groups: {}, person_properties: {'is_authorized': True}, group_properties: {})
# returns hash of flag key and value pairs.

Feature Flag Payloads

Payloads allow you to retrieve a value that is associated with the matched flag. The value can be a string, boolean, number, dictionary, or array. This allows for custom configurations based on values defined in the posthog app.

posthog.get_feature_flag_payload('feature-flag-key', 'distinct id')

Local Evaluation

Note: To enable local evaluation of feature flags you must also set a personal_api_key when configuring the integration, as described in the Installation section.

Note: This feature requires version 2.0 of the library, which in turn requires a minimum PostHog version of 1.38

All feature flag evaluation requires an API request to your PostHog servers to get a response. However, where latency matters, you can evaluate flags locally. This is much faster, and requires two things to work:

1. The library must be initialised with a personal API key
2. You must know all person or group properties the flag depends on.

Then, the flag can be evaluated locally. The method signature looks exactly like above

posthog.get_feature_flag('beta-feature', 'distinct id', person_properties: {'is_authorized': True})
# returns string or Nil

Note: New feature flag definitions are polled every 30 seconds by default, which means there will be up to a 30 second delay between you changing the flag definition, and it reflecting on your servers. You can change this default on the client by setting feature_flags_polling_interval = <value in seconds>.

This works for getAllFlags as well. It evaluates all flags locally if possible. If even one flag isn't locally evaluable, it falls back to decide.

posthog.get_all_flags('distinct id', groups: {}, person_properties: {'is_authorized': True}, group_properties: {})
# returns hash of flag key and value pairs.

Restricting evaluation to local only

Sometimes, performance might matter to you so much that you never want an HTTP request roundtrip delay when computing flags. In this case, you can set the only_evaluate_locally parameter to true, which tries to compute flags only with the properties it has. If it fails to compute a flag, it returns None, instead of going to PostHog's servers to get the value.

Cohort expansion

To support feature flags that depend on cohorts locally as well, we translate the cohort definition into person properties, so that the person properties you set can be used to evaluate cohorts as well.

However, there are a few constraints here and we don't support doing this for arbitrary cohorts. Cohorts won't be evaluated locally if:

1. They have non-person properties
2. There's more than one cohort in the feature flag definition.
3. The cohort in the feature flag is in the same group as another condition.
4. The cohort has nested AND-OR filters. Only simple cohorts that have a top level OR group, and inner level ANDs will be evaluated locally.

Note that this restriction is for local evaluation only. If you're hitting PostHog's servers, all of these cohorts will be evaluated as expected. Further, posthog-node v2.6.0 onwards, and posthog-python v2.4.0 onwards do not face this issue and can evaluate all cohorts locally.

Evaluating feature flags locally in unicorn server

If you have preload_app true in your unicorn config, you can use after_fork hook which is part of the unicorn's configuration so that the feature flag cache will receive the updates from posthog dashboard.

after_fork do |server, worker|
  $posthog = PostHog::Client.new(
    api_key: '<ph_project_api_key>',
    personal_api_key: '<ph_personal_api_key>'
    host: '<ph_instance_address>',
    on_error: Proc.new { |status, msg| print msg }
  )
end

Reloading feature flags

When initializing PostHog, you can configure the interval at which feature flags are polled (fetched from the server). However, if you need to force a reload, you can use reloadFeatureFlags:

posthog.reload_feature_flags()

// Do something with feature flags here

Group analytics

Group analytics allows you to associate an event with a group (e.g. teams, organizations, etc.). Read the Group Analytics guide for more information.

Note: This is a paid feature and is not available on the open-source or free cloud plan. Learn more here.

• Capture an event and associate it with a group

posthog.capture({
    distinct_id: 'distinct id',
    event: 'movie played',
    properties: {
        movie_id: '123',
        category: 'romcom'
    }
    groups: {
        'company': '42dlsfj23f'
    }
})

• Update properties on a group

posthog.group_identify(
  {
    group_type: "organization",
    group_key: "42dlsfj23f",
    properties: {
      name: "Awesome Inc."
    }
  }
)

The name is a special property which is used in the PostHog UI for the name of the Group. If you don't specify a name property, the group ID will be used instead.

Thank you

This library is largely based on the analytics-ruby package.