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

1. Install the SDK

In your go.mod, add the SDK package as a dependency:

require (

Or you can install the SDK from the command line with:

go get

2. Initialize the SDK

Initialize the SDK with a SDK key, which can be generated in the Eppo interface. Initialization should happen when your application starts up to generate a singleton client instance, once per application lifecycle:

import (

var eppoClient = &eppoclient.EppoClient{}

func main() {
assignmentLogger := NewExampleAssignmentLogger()

eppoClient = eppoclient.InitClient(eppoclient.Config{
ApiKey: "<your_sdk_key>",
AssignmentLogger: assignmentLogger,

After initialization, the SDK begins polling Eppo's API at regular intervals to retrieve the most recent experiment configurations such as variation values and traffic allocation. The SDK stores these configurations in memory so that assignments thereafter are effectively instant. For more information, see the architecture overview page.

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 a callback logging function to the InitClient function on SDK initialization. The SDK invokes the callback 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. Here we define an implementation of the Eppo IAssignmentLogger interface containing a single function named LogAssignment:

import (

func main() {
// Connect to Segment (or your own event-tracking system)
client := analytics.New("YOUR_WRITE_KEY")
defer client.Close()

type ExampleAssignmentLogger struct {

func NewExampleAssignmentLogger() *ExampleAssignmentLogger {
return &ExampleAssignmentLogger{}

func (al *ExampleAssignmentLogger) LogAssignment(event eppoclient.AssignmentEvent) {
UserId: event.Subject,
Event: "Eppo Randomization Event",
Properties: event

The SDK will invoke the LogAssignment function with an event object that contains the following fields:

experiment (string)An Eppo experiment key"recommendation-algo-allocation-17"
subject (string)An identifier of the subject or user assigned to the experiment variationUUID
variation (string)The experiment variation the subject was assigned to"control"
timestamp (string)The time when the subject was assigned to the variation2021-06-22T17:35:12.000Z
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" }
featureFlag (string)An Eppo feature flag key"recommendation-algo"
allocation (string)An Eppo allocation key"allocation-17"

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

3. Assign variations

Assigning users to flags or experiments with a single GetStringAssignment function:

import (

var eppoClient = &eppoclient.EppoClient{} // in global scope
variation := eppoClient.GetStringAssignment("<SUBJECT-KEY>", "<FLAG-KEY>", <TARGETING_ATTRIBUTES>);

The GetStringAssignment function takes two required and one optional input to assign a variation:

  • subjectKey - The ID of the entity that is being experimented on, typically represented by a uuid.
  • flagOrExperimentKey - This key is available on the detail page for both flags and experiments.
  • targetingAttributes - 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.

Typed assignments

Additional functions are available:


Handling the empty assignment

We recommend always handling the empty assignment case, when the SDK returns "". Here are some examples illustrating when the SDK returns "":

  1. The Traffic Exposure setting on experiments/allocations determines the percentage of subjects the SDK will assign to that experiment/allocation. For example, if Traffic Exposure is 25%, the SDK will assign a variation for 25% of subjects and "" for the remaining 75% (unless the subject is part of an allow list).

  2. Assignments occur within the environments of feature flags. You must enable the environment corresponding to the feature flag's allocation in the user interface before getStringAssignment returns variations. It will return "" if the environment is not enabled.

Toggle to enable environment

  1. If getStringAssignment is invoked before the SDK has finished initializing, the SDK may not have access to the most recent experiment configurations. In this case, the SDK will assign a variation based on any previously downloaded experiment configurations stored in local storage, or return "" if no configurations have been downloaded.


It may take up to 10 seconds for changes to Eppo experiments to be reflected by the SDK assignments.