rs-feature-flags

In-memory feature flag evaluation with rollout, environment, targeting, and A/B variant support for Rust

Installation

[dependencies]
philiprehberger-feature-flags = "0.2.4"

To enable JSON deserialization via serde:

[dependencies]
philiprehberger-feature-flags = { version = "0.2.4", features = ["serde"] }

Usage

use philiprehberger_feature_flags::{FeatureFlags, FlagConfig, Context};

let mut flags = FeatureFlags::new();

flags.set("dark-mode", FlagConfig::new(true));
flags.set(
    "beta-feature",
    FlagConfig::new(true)
        .with_rollout(50)
        .with_environments(vec!["production".into()]),
);

// Simple check
assert!(flags.is_enabled("dark-mode"));

// Check with context
let ctx = Context::new()
    .with_user_id("user-123")
    .with_environment("production");

let enabled = flags.is_enabled_for("beta-feature", &ctx);
println!("beta-feature enabled: {enabled}");

Targeting

Grant access to specific users or roles, bypassing percentage rollout:

use philiprehberger_feature_flags::{FeatureFlags, FlagConfig, Context};

let mut flags = FeatureFlags::new();
flags.set(
    "internal-tool",
    FlagConfig::new(true)
        .with_rollout(0) // 0% general rollout
        .with_allowed_users(vec!["alice".into(), "bob".into()])
        .with_allowed_roles(vec!["admin".into()]),
);

// Alice gets access via allowed_users
let ctx = Context::new().with_user_id("alice");
assert!(flags.is_enabled_for("internal-tool", &ctx));

// Any admin gets access via allowed_roles
let ctx = Context::new().with_user_id("charlie").with_role("admin");
assert!(flags.is_enabled_for("internal-tool", &ctx));

You can also require context attributes to match:

use std::collections::HashMap;
use philiprehberger_feature_flags::{FeatureFlags, FlagConfig, Context};

let mut attrs = HashMap::new();
attrs.insert("plan".to_owned(), "enterprise".to_owned());

let mut flags = FeatureFlags::new();
flags.set(
    "enterprise-only",
    FlagConfig::new(true).with_required_attributes(attrs),
);

let ctx = Context::new().with_attribute("plan", "enterprise");
assert!(flags.is_enabled_for("enterprise-only", &ctx));

Variants

Deterministic A/B test variant assignment based on user ID hashing:

use philiprehberger_feature_flags::FeatureFlags;

let flags = FeatureFlags::new();
let variant = flags.get_variant("experiment", "user-42", &["control", "variant-a", "variant-b"]);
// Always returns the same variant for the same flag + user pair
println!("assigned: {}", variant.unwrap());

API

Item	Description
FlagConfig	Configuration for a single flag: enabled, rollout, environments, targeting, variants, attributes
FlagConfig::new(enabled)	Create a new flag config
.with_rollout(pct)	Set rollout percentage (0-100)
.with_environments(envs)	Restrict flag to specific environments
.with_allowed_users(users)	Set users that bypass rollout
.with_allowed_roles(roles)	Set roles that bypass rollout (matched against role attribute)
.with_variants(variants)	Set variant names for A/B testing
.with_required_attributes(attrs)	Set required context attributes
Context	Evaluation context with user ID, environment, and custom attributes
Context::new()	Create an empty context
.with_user_id(id)	Set the user ID for rollout evaluation
.with_environment(env)	Set the environment for filtering
.with_attribute(key, value)	Add a custom attribute
.with_role(role)	Set the role attribute (shorthand)
FeatureFlags	In-memory flag store
FeatureFlags::new()	Create an empty store
.set(name, config)	Add or update a flag
.set_config(name, config)	Add or update a flag (accepts &str)
.remove(name)	Remove a flag
.is_enabled(name)	Check if a flag is enabled (no context)
.is_enabled_for(name, ctx)	Evaluate a flag with full context
.evaluate_with_config(name, ctx)	Evaluate a stored flag with context
.get_variant(flag, user_id, variants)	Get a deterministic A/B variant
.all_flags()	Get a sorted list of all flag names
FeatureFlags::from_json(json)	Parse flags from JSON (requires serde feature)

Development

cargo test
cargo clippy -- -D warnings

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License

MIT