Ruby

Eppo's Ruby SDK can be used for feature flagging, randomized experiment assignment, and contextual multi armed bandits:

• GitHub repository
• RubyGems gem

Getting Started

Installation

Install the SDK with gem:

gem install eppo-server-sdk

or add to you Gemfile:

gem 'eppo-server-sdk', '~> 3.2.7'

Usage

Begin by initializing a singleton instance of Eppo's client with an SDK key from the Eppo interface. Once initialized, the client can be used to make assignments anywhere in your app. Initialization should happen when your application starts up to generate a singleton client instance, once per application lifecycle:

Initialize once

require 'eppo_client'

config = EppoClient::Config.new('<YOUR_API_KEY>')
EppoClient::init(config)

Assign anywhere

require 'eppo_client'

client = EppoClient::Client.instance
variation = client.get_string_assignment(
  '<FLAG-KEY>',
  '<SUBJECT-KEY>',
  {
    # Mapping of any subject metadata for targeting.
  },
  '<DEFAULT-VALUE>'
)

Flags and other features become available to the SDK after initialization.

After initialization, the SDK begins polling Eppo’s CDN every 30 seconds to retrieve the most recent experiment configurations (variation values, traffic allocation, etc.). Note that polling happens independently of assignment calls and is non blocking.

The SDK stores these configurations in memory so that assignments thereafter are effectively instant. For more information, see the architecture overview page.

info

By default, the Eppo client initialization is asynchronous to ensure no critical code paths are blocked. For more information on handling non-blocking initialization, see our documentation here.

Connecting an event logger

Eppo is architected so that raw user data never leaves your system. As part of that, instead of pushing subject-level exposure events to Eppo's servers, Eppo's SDKs integrate with your existing logging system. This is done with a logging callback function defined at SDK initialization.

config = EppoClient::Config.new(
  '<YOUR_API_KEY>',
  assignment_logger: CustomAssignmentLogger.new
)
EppoClient::init(config)

This logger takes an analytic event created by Eppo, assignment, and writes in to a table in the data warehouse (Snowflake, Databricks, BigQuery, or Redshift). You can read more on the Event Logging page.

The code below illustrates an example implementation of logging with Segment, but you could also use other event-tracking systems. The only requirement is that the SDK can call a log_assignment method. Here we override Eppo's AssignmentLogger class with a function named log_assignment, then instantiate a config using an instance of the custom logger class, and finally instantiate the client:

require 'segment/analytics'

# Connect to Segment (or your own event-tracking system)
Analytics = Segment::Analytics.new({ write_key: 'SEGMENT_WRITE_KEY' })

class CustomAssignmentLogger < EppoClient::AssignmentLogger
  def log_assignment(assignment)
    Analytics.track(assignment["subject"], "Eppo Assignment", assignment)
  end
end

config = EppoClient::Config.new(
  '<YOUR_API_KEY>',
  assignment_logger: CustomAssignmentLogger.new
)
EppoClient::init(config)

See below for details on the schema of the assignment analytic event.

Getting variations

Now that the SDK is initialized and connected to your event logger, you can check what variant a specific subject (typically user) should see by calling the get_<Type>_Assignment functions. Each time this function is called, the SDK will invoke the provided logging function to record the assignment.

For example, for a string-valued flag, use get_string_assignment:

require 'eppo_client'

client = EppoClient::Client.instance
variation = client.get_string_assignment(
  '<FLAG-KEY>',
  '<SUBJECT-KEY>',
  {
    # Mapping of any subject metadata for targeting.
  },
  '<DEFAULT-VALUE>'
)

Note that Eppo uses a unified API for feature gates, experiments, and mutually exclusive layers. This makes it easy to turn a flag into an experiment or vice versa without having to do a code release.

The get_string_assignment function takes four inputs to assign a variation:

• flag_key - The key for the flag you are evaluating. This key is available on the feature flag detail page (see below).
• subject_key - A unique identifier for the subject being experimented on (e.g., user), typically represented by a UUID. This key is used to deterministically assign subjects to variants.
• subject_attributes - A map of metadata about the subject used for targeting. If targeting is not needed, pass in an empty object.
• default_value - The value that will be returned if no allocation matches the subject, if the flag is not enabled, if get_string_assignment is invoked before the SDK has finished initializing, or if the SDK was not able to retrieve the flag configuration. Its type must match the get_<Type>_assignment call.

Typed assignments

Every Eppo flag has a return type that is set on creation in the dashboard. Once a flag is created, assignments in code should be made using the corresponding typed function:

get_boolean_assignment(...)
get_numeric_assignment(...)
get_integer_assignment(...)
get_string_assignment(...)
get_json_assignment(...)

Each function has the same signature, but returns the type in the function name. The only exception is default_value, which should be the same type as the flag. To read more about when to use which flag type, see the flag types page.

Contextual Bandits

To leverage Eppo's contextual bandits using the Ruby SDK, there are two additional steps over regular feature flags:

1. Add a bandit action logger to the assignment logger
2. Querying the bandit for an action

Logging bandit actions

In order for the bandit to learn an optimized policy, we need to capture and log the bandit actions. This requires adding a bandit action logging callback to the AssignmentLogger class

class MyLogger < EppoClient::AssignmentLogger
    def log_assignment(assignment):
        ...

    def log_bandit_action(bandit_action):
        # implement me

We automatically log the following data:

Field	Description	Example
timestamp (Date)	The time when the action is taken in UTC	2024-03-22T14:26:55.000Z
flagKey (String)	The key of the feature flag corresponding to the bandit	"bandit-test-allocation-4"
banditKey (String)	The key (unique identifier) of the bandit	"ad-bandit-1"
subject (String)	An identifier of the subject or user assigned to the experiment variation	"ed6f85019080"
action (String)	The action assigned by the bandit	"promo-20%-off"
subjectNumericAttributes (Hash{String => Float})	Metadata about numeric attributes of the subject. Hash of the name of attributes their numeric values	{"age": 30}
subjectCategoricalAttributes (Hash{String => String})	Metadata about non-numeric attributes of the subject. Hash of the name of attributes their string values	{"loyalty_tier": "gold"}
actionNumericAttributes (Hash{String => Float})	Metadata about numeric attributes of the assigned action. Hash of the name of attributes their numeric values	{"discount": 0.1}
actionCategoricalAttributes (Hash{String => String})	Metadata about non-numeric attributes of the assigned action. Hash of the name of attributes their string values	{"promoTextColor": "white"}
actionProbability (Float)	The weight between 0 and 1 the bandit valued the assigned action	0.25
modelVersion (String)	Unique identifier for the version (iteration) of the bandit parameters used to determine the action probability	"v123"

Querying the bandit for an action

To query the bandit for an action, you can use the get_bandit_action function. This function takes the following parameters:

• flag_key (String): The key of the feature flag corresponding to the bandit
• subject_key (String): The key of the subject or user assigned to the experiment variation
• subject_attributes (Attributes): The context of the subject
• actions (Hash{String => Attributes}): A hash that maps available actions to their attributes
• default (String): The default variation to return if the bandit cannot be queried

The following code queries the bandit for an action:

require 'eppo_client'

client = EppoClient::Client.instance
bandit_result = client.get_bandit_action(
  "shoe-bandit",
  name,
  EppoClient::Attributes.new(
    numeric_attributes: { "age" => age }, categorical_attributes: { "country" => country }
  ),
  {
    "nike" => EppoClient::Attributes.new(
      numeric_attributes: { "brand_affinity" => 2.3 },
      categorical_attributes: { "image_aspect_ratio" => "16:9" }
    ),
    "adidas" => EppoClient::Attributes.new(
      numeric_attributes: { "brand_affinity" => 0.2 },
      categorical_attributes: { "image_aspect_ratio" => "16:9" }
    )
  },
  "control"
)

Subject Context

The subject context contains contextual information about the subject that is independent of bandit actions. For example, the subject's age or country.

The subject context has type Attributes which has two fields:

• numeric_attributes (Hash{String => Float}): A hash of numeric attributes (such as "age")
• categorical_attributes (Hash{String => String}): A hash of categorical attributes (such as "country")

note

The categerical_attributes are also used for targeting rules for the feature flag similar to how subject_attributes are used for that with regular feature flags.

Action Contexts

Next, supply a hash with actions and their attributes: actions: Hash{String => Attributes}. If the user is assigned to the bandit, the bandit selects one of the actions supplied here, and all actions supplied are considered to be valid; if an action should not be shown to a user, do not include it in this hash.

The action attributes are similar to the subject_attributes but hold action-specific information. Note that we can use Attributes.empty to create an empty attribute context.

Note that action contexts can contain two kinds of information:

• Action-specific context: e.g., the image aspect ratio of the image corresponding to this action
• User-action interaction context: e.g., there could be a "brand-affinity" model that computes brand affinities of users to brands, and scores of this model can be added to the action context to provide additional context for the bandit.

Result

The bandit_result is an instance of BanditResult, which has two fields:

• variation (String): The variation that was assigned to the subject
• action (Optional[String]): The action that was assigned to the subject

The variation returns the feature flag variation; this can be the bandit itself, or the "status quo" variation if the user is not assigned to the bandit. If we are unable to generate a variation, for example when the flag is turned off, then the default variation is returned. In both of those cases, the action is nil, and you should use the status-quo algorithm to select an action.

When action is not nil, the bandit has selected that action to be shown to the user.

Status quo algorithm

In order to accurately measure the performance of the bandit, we need to compare it to the status quo algorithm using an experiment. This status quo algorithm could be a complicated algorithm that selects an action according to a different model, or a simple baseline such as selecting a fixed or random action. When you create an analysis allocation for the bandit and the action in BanditResult is nil, implement the desired status quo algorithm based on the variation value.

Appendix

Debugging

You may encounter a situation where a flag assignment produces a value that you did not expect. There are functions detailed here to help you understand how flags are assigned, which will allow you to take corrective action on potential configuration issues.

Assignment event schema

The SDK will invoke the log_assignment function with an assignment object that contains the following fields:

Field	Description	Example
experiment (string)	An Eppo experiment key	"recommendation-algo-allocation-17"
subject (string)	An identifier of the subject or user assigned to the experiment variation	UUID
variation (string)	The experiment variation the subject was assigned to	"control"
timestamp (string)	The time when the subject was assigned to the variation	2021-06-22T17:35:12.000Z
subjectAttributes (map)	A Hash of metadata about the subject. These attributes are only logged if passed to the SDK assignment function	{ "country": "US" }
featureFlag (string)	An Eppo feature flag key	"recommendation-algo"
allocation (string)	An Eppo allocation key	"allocation-17"

note

More details about logging and examples (with Segment, Rudderstack, mParticle, and Snowplow) can be found in the event logging page.