We've built an open-source feature flag server for 11 years

2026-11-06T11:40:06+00:00

What is Unleash?

Unleash is a powerful open-source solution for feature management. It streamlines your development workflow, accelerates software delivery, and empowers teams to control how and when they roll out new features to end users. With Unleash, you can deploy code to production in smaller, more manageable releases at your own pace.

Feature flags in Unleash let you test your code with real production data, reducing the risk of negatively impacting your users' experience. It also enables your team to work on multiple features simultaneously without the need for separate feature branches.

Unleash is the most popular open-source solution for feature flagging on GitHub. It supports 15 official client and server SDKs and over 15 community SDKs. You can even create your own SDK if you wish. Unleash is compatible with any language and framework.

Get started with Unleash

Set up Unleash

To get started with Unleash, you can either explore Unleash Enterprise with a free trial or get started locally with our open-source solution.

Unleash Enterprise

To start with Unleash Enterprise, request a free trial. This gives you access to a hosted instance with unlimited projects and environments and features such as role-based access control, change requests, single sign-on, and SCIM for automatic user provisioning.

Unleash Open Source

To set up Unleash locally, you'll need git and docker installed on your machine.

Execute the following commands:

git clone git@github.com:Unleash/unleash.git
cd unleash
docker compose up -d

Then point your browser to localhost:4242 and log in using:

username: admin
password: unleash4all

If you'd rather run the source code in this repo directly via Node.js, see the step-by-step instructions to get up and running in the contributing guide.

Connect your SDK

Find your preferred SDK in our list of official SDKs and import it into your project. Follow the setup guides for your specific SDK.

If you use the docker compose file from the previous step, here's the configuration details you'll need to get going:

For frontend SDKs, use:
- URL: http://localhost:4242/api/frontend/
- clientKey: default:development.unleash-insecure-frontend-api-token
For backend SDKs, use:
- Unleash API URL: http://localhost:4242/api/
- API token: default:development.unleash-insecure-api-token

If you use a different setup, your configuration details will most likely also be different.

Check a feature flag

Checking the state of a feature flag in your code is easy! The syntax will vary depending on your language, but all you need is a simple function call to check whether a flag is available. Here's how it might look in Java:

if (unleash.isEnabled("AwesomeFeature")) {
  // do new, flashy thing
} else {
  // do old, boring stuff
}

Try Unleash in the cloud

Want to explore Unleash without setting it up locally? Here are two easy ways to see it in action.

Live demo

Open a running Unleash environment in your browser with no signup or setup required. This shared demo instance includes the core functionality so you can quickly get a feel for how Unleash works.

→ Launch the live demo

Free Enterprise trial

Get your own dedicated instance with a free 14-day cloud trial. This includes the full Unleash Enterprise feature set; ideal when evaluating Unleash for production use.

→ Start your free trial

Deploying to a production server

To self-host Unleash in a production environment, you need to deploy it to a persistent server.

We provide guides for deploying to services like Heroku and DigitalOcean:

For more advanced configurations options, check out our documentation on:

Community and help

We know that learning a new tool can be hard and time-consuming. We have a growing community that loves to help out. Please don't hesitate to reach out for help.

💬 Join Unleash on Slack if you want ask open questions about Unleash, feature toggling or discuss these topics in general.

💻 Create a GitHub issue if you have found a bug or have ideas on how to improve Unleash.

📚 Visit the documentation for more in-depth descriptions, how-to guides, and more.

📖 Learn more about the principles of building and scaling feature flag solutions.

Contribute to Unleash

Unleash is the largest open-source feature flag solution on GitHub. Building Unleash is a collaborative effort, and we owe a lot of gratitude to many smart and talented individuals. Building it together with the community ensures that we build a product that solves real problems for real people. We'd love to have your help too: Please feel free to open issues or provide pull requests.

Check out the CONTRIBUTING.md file for contribution guidelines and the Unleash developer guide for tips on environment setup, running the tests, and running Unleash from source. Before a pull request can be merged, contributors must read and agree to the Unleash Contributor License Agreement.

Contributors

Documentation

You can visit the Unleash documentation at https://docs.getunleash.io. As of 2 February 2026, the Unleash documentation source code lives in its own repository: Unleash/unleash-documentation.

Features our users love

Flexibility and adaptability

Get an overview of all feature flags across all your environments, applications and services
Targeted releases using activation strategies to enable and disable features for certain users or segments without having to redeploy your application.
Canary releases / gradual rollouts
Kill switches
A/B testing
2 environments
Organize feature flags using tags
Out-of-the-box integrations with popular tools (Slack, Microsoft Teams, Datadog) + integrate with anything with webhooks
Insights for managing technical debt and stale flags
API-first: everything can be automated. No exceptions.
12 official SDKs, and 10 community-contributed SDKs
Run it via Docker with the official Docker image or as a pure Node.js application

Security and performance

Privacy by design (GDPR and Schrems II). End-user data never leaves your application.
Audit logs
Enforce OWASP's secure headers via the strict HTTPS-only mode
Flexible hosting options: host it on premise or in the cloud (any cloud)
Scale with Unleash Edge independently of the Unleash server to support any number of frontend clients without overloading your Unleash instance

Looking for more features?

If you're looking for one of the following features, please take a look at our Pro and Enterprise plans:

role-based access control (RBAC)
single sign-on (SSO)
more environments
feature flags project support
advanced segmentation
additional strategy constraints
tighter security
more hosting options (we can even host it for you!)

Architecture

Unleash SDKs

To connect your application to Unleash you'll need to use a client SDK for your programming language.

Official backend SDKs:

Official frontend SDKs:

Community SDKs:

If none of the official SDKs fit your need, there's also a number of community-developed SDKs where you might find an implementation for your preferred language (such as Elixir, Dart, Clojure, and more).

Users of Unleash

Unleash is trusted by thousands of companies all over the world.

Proud Open-Source users: (send us a message if you want to add your logo here)

Migration guides

Unleash has evolved significantly over the past few years, and we know how hard it can be to keep software up to date. If you're using the current major version, upgrading shouldn't be an issue. If you're on a previous major version, check out the Unleash migration guide!

Want to know more about Unleash?

Videos and podcasts

Articles and more

The Unleash Blog
Designing the Rust Unleash API client — Medium
FeatureToggle by Martin Fowler
Feature toggling transient errors in load tests — nrkbeta
An Interview with Ivar of Unleash — Console
Unleash your features gradually, slideshow/presentation by Ivar, the creator of Unleash

github.com/Unleash