OpenFeature Rust SDK

OpenFeature is an open standard that provides a vendor-agnostic, community-driven API for feature flagging that works with your favorite feature flag management tool.

🚀 Quick start

Requirements

This package was built with Rust version 1.70.0. Earlier versions might work, but is not guaranteed.

Install

Add the following content to the Cargo.toml file:

open-feature = "0.2.5"

Usage

async fn example() -> Result<(), Error> {
    // Acquire an OpenFeature API instance.
    // Note the `await` call here because asynchronous lock is used to
    // guarantee thread safety.
    let mut api = OpenFeature::singleton_mut().await;

    // Configure a provider.
    // By default [`NoOpProvider`] is used.
    api.set_provider(NoOpProvider::default()).await;

    // create a client
    let client = api.get_client();

    // get a bool flag value
    let is_feature_enabled = client
        .get_bool_value("v2_enabled", None, None)
        .unwrap_or(false)
        .await;

    Ok(())
}

Note that the default NoOpProvider always returns Err for any given input.

Extended Example

#[tokio::test]
async fn extended_example() {
    // Acquire an OpenFeature API instance.
    let mut api = OpenFeature::singleton_mut().await;

    // Set the default (unnamed) provider.
    api.set_provider(NoOpProvider::default()).await;

    // Create an unnamed client.
    let client = api.create_client();

    // Create an evaluation context.
    // It supports types mentioned in the specification.
    let evaluation_context = EvaluationContext::default()
        .with_targeting_key("Targeting")
        .with_custom_field("bool_key", true)
        .with_custom_field("int_key", 100)
        .with_custom_field("float_key", 3.14)
        .with_custom_field("string_key", "Hello".to_string())
        .with_custom_field("datetime_key", time::OffsetDateTime::now_utc())
        .with_custom_field(
            "struct_key",
            EvaluationContextFieldValue::Struct(Arc::new(MyStruct::default())),
        )
        .with_custom_field("another_struct_key", Arc::new(MyStruct::default()))
        .with_custom_field(
            "yet_another_struct_key",
            EvaluationContextFieldValue::new_struct(MyStruct::default()),
        );

    // This function returns a `Result`.
    // You can process it with functions provided by std.
    let is_feature_enabled = client
        .get_bool_value("SomeFlagEnabled", Some(&evaluation_context), None)
        .await
        .unwrap_or(false);

    if is_feature_enabled {
        // Let's get evaluation details.
        let _result = client
            .get_int_details("key", Some(&evaluation_context), None)
            .await;
    }
}

Getting a Struct from a Provider

It is possible to extract a struct from the provider. Internally, this SDK defines a type StructValue to store any structure value. The client.get_struct_value() functions takes a type parameter T. It will try to parse StructValue resolved by the provider to T, as long as T implements trait TryFrom<StructValue>.

You can pass in a type that satisfies this trait bound. When the conversion fails, it returns an Err with EvaluationReason::TypeMismatch.

API Reference

See here for the API docs.

🌟 Features

Status	Features	Description
✅	Providers	Integrate with a commercial, open source, or in-house feature management tool.
✅	Targeting	Contextually-aware flag evaluation using evaluation context.
✅	Hooks	Add functionality to various stages of the flag evaluation life-cycle.
✅	Logging	Integrate with popular logging packages.
✅	Named clients	Utilize multiple providers in a single application.
❌	Eventing	React to state changes in the provider or flag management system.
✅	Shutdown	Gracefully clean up a provider during application shutdown.
✅	Extending	Extend OpenFeature with custom providers and hooks.

_{Implemented: ✅ | In-progress: ⚠️ | Not implemented yet: ❌}

Providers

Providers are an abstraction between a flag management system and the OpenFeature SDK. Look here for a complete list of available providers. If the provider you're looking for hasn't been created yet, see the develop a provider section to learn how to build it yourself.

Once you've added a provider as a dependency, it can be registered with OpenFeature like this:

// Set the default feature provider. Please replace the `NoOpProvider` with the one you want.
// If you do not do that, [`NoOpProvider`] will be used by default.
//
// [`NoOpProvider`] always returns `Err` despite any input. You can use functions like 
// `unwrap_or()` to specify default values.
//
// If you set a new provider after creating some clients, the existing clients will pick up
// the new provider you just set.
//
// You must `await` it to let the provider's initialization to finish.
let mut api = OpenFeature::singleton_mut().await;
api.set_provider(NoOpProvider::default()).await;

In some situations, it may be beneficial to register multiple providers in the same application. This is possible using named clients, which is covered in more detail below.

Targeting

Sometimes, the value of a flag must consider some dynamic criteria about the application or user, such as the user's location, IP, email address, or the server's location. In OpenFeature, we refer to this as targeting. If the flag management system you're using supports targeting, you can provide the input data using the evaluation context.

// Create a global evaluation context and set it into the API.
// Note that this is optional. By default it uses an empty one.
let mut api = OpenFeature::singleton_mut().await;
api.set_evaluation_context(global_evaluation_context).await;

// Set client level evaluation context.
// It will overwrite the global one for the existing keys.
let mut client = api.create_client();
client.set_evaluation_context(client_evaluation_context);

// Pass evaluation context in evaluation functions.
// This one will overwrite the globla evaluation context and 
// the client level one.
client.get_int_value("flag", Some(&evaluation_context), None);

Hooks

Hooks allow for custom logic to be added at well-defined points of the flag evaluation life-cycle. Look here for a complete list of available hooks. If the hook you're looking for hasn't been created yet, see the develop a hook section to learn how to build it yourself.

Once you've added a hook as a dependency, it can be registered at the global, client, or flag invocation level.

let mut api = OpenFeature::singleton_mut().await;

// Set a global hook.
api.set_hook(MyHook::default()).await;

// Create a client and set a client level hook.
let client = api.create_client();
client.set_hook(MyHook::default());

// Get a flag value with a hook.
let eval = EvaluationOptions::default().with_hook(MyHook::default());
client.get_int_value("key", None, Some(&eval)).await;

Example of a hook implementation you can find in examples/hooks.rs.

To run the example, execute the following command:

cargo run --example hooks

Logging

Note that in accordance with the OpenFeature specification, the SDK doesn't generally log messages during flag evaluation.

Logging hook

The Rust SDK provides a logging hook that can be used to log messages during flag evaluation. This hook is not enabled by default and must be explicitly set.

let mut api = OpenFeature::singleton_mut().await;

let client = api.create_client().with_logging_hook(false);

...

// Note: You can include evaluation context to log output.
let client = api.create_client().with_logging_hook(true);

Both text and structured logging are supported. To enable structured logging, enable feature structured-logging in your Cargo.toml:

open-feature = { version = "0.2.4", features = ["structured-logging"] }

Example of a logging hook usage you can find in examples/logging.rs.

To run the example, execute the following command:

cargo run --example logging

Output:

[2025-01-10T18:53:11Z DEBUG open_feature::hooks::logging] Before stage: domain=, provider_name=Dummy Provider, flag_key=my_feature, default_value=Some(Bool(false)), evaluation_context=EvaluationContext { targeting_key: None, custom_fields: {} }
[2025-01-10T18:53:11Z DEBUG open_feature::hooks::logging] After stage: domain=, provider_name=Dummy Provider, flag_key=my_feature, default_value=Some(Bool(false)), reason=None, variant=None, value=Bool(true), evaluation_context=EvaluationContext { targeting_key: None, custom_fields: {} }

or with structured logging:

cargo run --example logging --features structured-logging

Output:

{"default_value":"Some(Bool(false))","domain":"","evaluation_context":"EvaluationContext { targeting_key: None, custom_fields: {} }","flag_key":"my_feature","level":"DEBUG","message":"Before stage","provider_name":"No-op Provider","target":"open_feature","timestamp":1736537120828}
{"default_value":"Some(Bool(false))","domain":"","error_message":"Some(\"No-op provider is never ready\")","evaluation_context":"EvaluationContext { targeting_key: None, custom_fields: {} }","file":"src/hooks/logging.rs","flag_key":"my_feature","level":"ERROR","line":162,"message":"Error stage","module":"open_feature::hooks::logging::structured","provider_name":"No-op Provider","target":"open_feature","timestamp":1736537120828}

Named clients

Clients can be given a name. A name is a logical identifier that can be used to associate clients with a particular provider. If a name has no associated provider, the global provider is used.

// Create a named provider and bind it.
api.set_named_provider("named", NoOpProvider::default()).await;

// This named client will use the feature provider bound to this name.
let client = api.create_named_client("named");

assert_eq!(client.get_int_value("key", None, None).await.unwrap(), 42);

Eventing

Events are not yet available in the Rust SDK.

Shutdown

The OpenFeature API provides a close function to perform a cleanup of all registered providers. This should only be called when your application is in the process of shutting down.

// This will clean all the registered providers and invoke their `shutdown()` function.
let api = OpenFeature::singleton_mut().await;
api.shutdown();

Extending

Develop a provider

To develop a provider, you need to create a new project and include the OpenFeature SDK as a dependency. This can be a new repository or included in the existing contrib repository available under the OpenFeature organization. You’ll then need to write the provider by implementing the FeatureProvider interface exported by the OpenFeature SDK.

Check the source of NoOpProvider for an example.

Built a new provider? Let us know so we can add it to the docs!

Develop a hook

To develop a hook, you need to create a new project and include the OpenFeature SDK as a dependency. This can be a new repository or included in the existing contrib repository available under the OpenFeature organization. Implement your own hook by conforming to the Hook interface. To satisfy the interface, all methods (before/after/finally/error) need to be defined.

use open_feature::{
    EvaluationContext, EvaluationDetails, EvaluationError,
    Hook, HookContext, HookHints, Value,
};

struct MyHook;

#[async_trait::async_trait]
impl Hook for MyHook {
    async fn before<'a>(
        &self,
        context: &HookContext<'a>,
        hints: Option<&'a HookHints>,
    ) -> Result<Option<EvaluationContext>, EvaluationError> {
        todo!()
    }

    async fn after<'a>(
        &self,
        context: &HookContext<'a>,
        details: &EvaluationDetails<Value>,
        hints: Option<&'a HookHints>,
    ) -> Result<(), EvaluationError> {
        todo!()
    }

    async fn error<'a>(
        &self,
        context: &HookContext<'a>,
        error: &EvaluationError,
        hints: Option<&'a HookHints>,
    ) {
        todo!()
    }

    async fn finally<'a>(
        &self,
        context: &HookContext<'a>,
        detaild: &EvaluationDetails<Value>,
        hints: Option<&'a HookHints>,
    ) {
        todo!()
    }
}

Built a new hook? Let us know so we can add it to the docs!

⭐️ Support the project

• Give this repo a ⭐️!
• Follow us on social media:
  • Twitter: @openfeature
  • LinkedIn: OpenFeature
• Join us on Slack
• For more, check out our community page

🤝 Contributing

Interested in contributing? Great, we'd love your help! To get started, take a look at the CONTRIBUTING guide.

Thanks to everyone who has already contributed

Made with contrib.rocks.