Assignments

Assignments are the mechanism through which a given Subject is assigned to a variation for a feature flag or experiment.

The Eppo SDK supports the following assignment types:

String
Boolean
JSON
Integer
Numeric

Assignment Types

String Assignments

String assignments return a string value that is set as the variation. String flags are the most common type of flags and are useful for both A/B/n tests and advanced targeting use cases.

flag_key = "flag-key-123"
subject_key = get_user_id() || "user-123"
default_value = "version-a"
subject_attributes = %{
  "country" => "US",
  "age" => 30,
  "is_returning_user" => true
}

client = EppoSdk.Server.get_instance()
variant = EppoSdk.Client.get_string_assignment(
  client,
  flag_key,
  subject_key,
  subject_attributes,
  default_value
)

case variant do
  "version-a" -> handle_version_a()
  "version-b" -> handle_version_b()
end

Boolean Assignments

Boolean flags support simple on/off toggles. They're useful for simple, binary feature switches like blue/green deployments or enabling/disabling a new feature.

variant = EppoSdk.Client.get_boolean_assignment(
  client,
  flag_key,
  subject_key,
  subject_attributes,
  false  # default value
)

if variant do
  handle_feature_enabled()
else
  handle_feature_disabled()
end

JSON Assignments

JSON flags work best for advanced configuration use cases. The JSON flag can include structured information such as:

Marketing copy for a promotional campaign
Configuration parameters for a feature
UI customization settings

default_campaign = %{
  "hero" => false,
  "hero_image" => "placeholder.png",
  "hero_title" => "Placeholder Hero Title",
  "hero_description" => "Placeholder Hero Description"
}

campaign_json = EppoSdk.Client.get_json_assignment(
  client,
  flag_key,
  subject_key,
  subject_attributes,
  default_campaign
)

Numeric Assignments

The SDK provides both integer and floating-point numeric assignments. These are useful for testing different numeric values like:

Price points
Number of items to display
Timeout durations

# Integer assignment example
num_items = EppoSdk.Client.get_integer_assignment(
  client,
  flag_key,
  subject_key,
  subject_attributes,
  10  # default value
)

# Floating point assignment example
price = EppoSdk.Client.get_numeric_assignment(
  client,
  flag_key,
  subject_key,
  subject_attributes,
  9.99  # default value
)

Assignment Logging

Assignment Logger Schema

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

timestampstrDefault: undefined

The time when the subject was assigned to the variation in ISO format. Example: "2021-06-22T17:35:12.000Z"

featureFlagstrDefault: undefined

An Eppo feature flag key. Example: "recommendation-algo"

allocationstrDefault: undefined

An Eppo allocation key. Example: "allocation-17"

experimentstrDefault: undefined

An Eppo experiment key. Example: "recommendation-algo-allocation-17"

subjectstrDefault: undefined

An identifier of the subject or user assigned to the experiment variation. Example: UUID

subjectAttributesMapDefault: {}

A free-form map of metadata about the subject. These attributes are only logged if passed to the SDK assignment function. Example: %{ "country" => "US" }

variationstrDefault: undefined

The experiment variation the subject was assigned to. Example: "control"

metaDataMapDefault: %{sdkName: 'elixir'}

A free-form map of metadata about the assignment. By default, the SDK will include the SDK name for logging.

Assignment Types​

String Assignments​

Boolean Assignments​

JSON Assignments​

Numeric Assignments​

Assignment Logging​

Assignment Logger Schema​