22.10.2025

Flagr - managing feature flags and A/B testing in applications

Flagr is an open-source service written in Go that helps deliver the right user experience to the right users and monitor the results.
It combines the functionality of feature flags, A/B testing, and dynamic configuration, providing a convenient REST API for flag management and evaluation.

Why Use Flagr

Flagr simplifies the process of deploying and controlling new features in applications.
Developers can safely enable or disable features, run experiments, and analyze their impact - all without restarting the service.

How Flagr Works

Flagr stores all flags and experiments in a database and provides an API for their evaluation.
Clients (applications, services, microservices) request a flag value for a specific user or context, and Flagr returns the appropriate configuration.

Usage Example

# Get flag value via API
curl -X POST "http://localhost:18000/api/v1/evaluation" \
-H "Content-Type: application/json" \
-d '{
"entityID": "user123",
"entityType": "user",
"flagKey": "new_ui"
}'

If the new_ui flag is active for that user, the application can display the new interface version.
Thus, Flagr allows you to test features safely and react quickly to results.

Where You Can Use Flagr

Advantages

FAQ