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This document was generated from the README in the Rust GitHub repository.


To connect to Unleash, you'll need your Unleash API url (e.g. https://<your-unleash>/api) and a server-side API token (how do I create an API token?).

Unleash API client for Rustlang

Unleash is a feature flag API system. This is a client for it to facilitate using the API to control features in Rust programs.

Client overview

The client is written using async rust. For communicating with the Unleash API surf or reqwest support is built in, or any async HTTP client can be provided by the user if they implement the thin trait used to abstract over the actual client.

Examples with async-std and tokio are in the examples/ in the source tree.

To use it in a sync program, run an async executor and block_on() the relevant calls. As the client specification requires sending background metrics to the API, you will need to arrange to call the poll_for_updates method from a thread as demonstrated in examples/

The unleash defined strategies are included, to support custom strategies use the ClientBuilder and call the strategy method to register your custom strategy memoization function.

The crate documentation should be consulted for more detail.


The easiest way to get started with the Client is using the ClientBuilder. A simple example is provided:

let config = EnvironmentConfig::from_env()?;
let client = client::ClientBuilder::default()
.into_client::<UserFeatures, reqwest::Client>(

The values required for the into_client method are described as follows (in order, as seen above):

  • api_url - The server URL to fetch toggles from.
  • app_name - The name of your application.
  • instance_id - A unique ID, ideally per run. A runtime generated UUID would be a sensible choice here.
  • authorization - An Unleash client secret, if set this will be passed as the authorization header.

While the above code shows the usage of the EnvironmentConfig, this isn't required and is provided as a convenient way of reading a data from the system environment variables.

EnvironmentConfig PropertyEnvironment VariableRequired?

Note that if you do use the EnvironmentConfig as a way of accessing the system variables, you'll need to ensure that all the environment variables marked as required in the above table are set, or a panic will be raised.

The ClientBuilder also has a few builder methods for setting properties which are assumed to have good defaults and generally do not require changing. If you do need to alter these properties you can invoke the following methods on the builder (as seen above with the interval).

intervalu64Sets the polling interval to the Unleash server, in milliseconds15000ms
disable_metric_submissionN/ATurns off the metrics submission to UnleashOn
enable_string_featuresN/ABy default the Rust SDK requires you to define an enum for feature resolution, turning this on will allow you to resolve your features by string types instead, through the use of the is_enabled_str method. Be warned that this is enforced by asserts and calling is_enabled_str without turning this on with result in a panicOff


Core Unleash API features work, with Rust 1.59 or above. The MSRV for this project is weakly enforced: when a hard dependency raises its version, so will the minimum version tested against, but if older rust versions work for a user, that is not prevented. time in particular is known to enforce a 6-month compiler age, so regular increases with the minimum version tested against are expected.

Unimplemented Unleash specified features:

  • local serialised copy of toggles to survive restarts without network traffic.

Code of conduct

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.


PR's on Github as normal please. Cargo test to run the test suite, rustfmt code before submitting. To run the functional test suite you need an Unleash API to execute against.

For instance, one way:

docker-compose up -d

Visit http://localhost:4242/ and log in with admin + unleash4all, then create a new API token at http://localhost:4242/admin/api/create-token for user admin, type Client.

Then run the test suite:

UNLEASH_CLIENT_SECRET="<tokenvalue>" \
cargo test --features functional -- --nocapture

or similar. The functional test suite looks for a manually setup set of features. E.g. log into the Unleash UI on port 4242 and create a feature called default.

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