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Eppo's open source Android SDK can be used for both feature flagging and experiment assignment:

1. Install the SDK

You can install the SDK using Gradle by adding to your build.gradle file:

implementation 'cloud.eppo:android-sdk:3.2.0'

2. Initialize the SDK

Initialize the SDK with an SDK key, which can be created in the Eppo web interface:


EppoClient eppoClient = EppoClient.init("YOUR_SDK_KEY");

During initialization, the SDK sends an API request to a CDN to retrieve the most recent experiment configurations from Eppo, such as variation values and traffic allocation. The SDK stores these configurations in memory so that assignments are effectively instant. For more information, see the architecture overview page.

This SDK also leverages cached configurations from previous fetches. During initialization, if a previously-cached configuration is successfully loaded, it will complete initialization with that configuration. Updates will take effect once the fetch from CDN completes. When initialization completes from either source, it will call the optionally-provided InitializationCallback.

If you are using the SDK for experiment assignments, make sure to pass in an assignment logging callback (see section below).

Define an assignment logger (experiment assignment only)

If you are using the Eppo SDK for experiment assignment (i.e., randomization), pass in an AssignmentLogger to the init function on SDK initialization. The SDK will invoke this AssignmentLogger to capture assignment data whenever a variation is assigned.

The code below illustrates an example implementation of a logging callback using Segment. You could also use your own logging system, the only requirement is that the SDK receives a logAssignment function which sends data into a table in your warehouse which Eppo has read access to. Here we create an instance of an AssignmentLogger and configure the EppoClient to use this logger with the EppoClient.Builder helper class:

AssignmentLogger assignmentLogger = new AssignmentLogger() {
public void logAssignment(Assignment assignment) {
analytics.enqueue(TrackMessage.builder("Eppo Randomized Assignment")
.put("timestamp", assignment.getTimestamp())
.put("experiment", assignment.getExperiment())
.put("subject", assignment.getSubject())
.put("variation", assignment.getVariation())

CountDownLatch lock = new CountDownLatch(1);

InitializationCallback initializationCallback = new InitializationCallback() {
public void onCompleted() {
Log.w(TAG, "Eppo client successfully initialized");

public void onError(String errorMessage) {
Log.w(TAG, "Eppo client encountered an error initializing: " + errorMessage);
Log.w(TAG, "Eppo client will serve default values for assignments");

EppoClient eppoClient = new EppoClient.Builder()

lock.await(5000, TimeUnit.MILLISECONDS);

The SDK will invoke the logAssignment function with an Assignment object that contains the following fields:

timestamp (string)The time when the subject was assigned to the variation2021-06-22T17:35:12.000Z
featureFlag (string)An Eppo feature flag key"recommendation-algo"
allocation (string)An Eppo allocation key"allocation-17"
experiment (string)An Eppo experiment key"recommendation-algo-allocation-17"
subject (string)An identifier of the subject or user assigned to the experiment variationUUID
subjectAttributes (map)A free-form map of metadata about the subject. These attributes are only logged if passed to the SDK assignment function{ "country": "US" }
variation (string)The experiment variation the subject was assigned to"control"
extraLogging (map)A map of string logging fields to string values of any extra information to be logged by the assignment{ "allocationvalue_type": "rollout" }
metaData (map)A map of string meta data fields to string values of any meta data associated with the assignment{ "sdkLibVersion": "3.0.0" }

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

3. Assign variations

Assign users to flags or experiments using the assignment function that corresponds with the value type of the flag's variations as set up in the Eppo web application.


EppoClient eppoClient = EppoClient.getInstance(); // requires the SDK to have already been initialized
String variation = eppoClient.getStringAssignment("<FLAG KEY>", "<SUBJECT KEY>", "<DEFAULT VALUE>");

The assignment functions take three required and one optional parameter to assign a variation:

  • flagKey - This key is available on the detail page for both flags and experiments. Can also be an experiment key.
  • subjectKey - The entity ID that is being experimented on, such as a user identifier, typically represented by an uuid.
  • subjectAttributes - An optional map of metadata about the subject used for targeting. If you create rules based on attributes on a flag/experiment, those attributes should be passed in on every assignment call.
  • defaultValue - The value to return if no variation is assigned, or an error is encountered making an assignment. Often this is the same as the control variation.

The following typed functions are available:


Changes in the Eppo web application may take up to 30 seconds to propagate to the CDN, where the SDK fetches updated assignments upon initialization.