Rust SDK - GrowthBook Documentation

The official GrowthBook SDK for Rust. This SDK provides a powerful, type-safe way to integrate feature flagging and A/B testing into your Rust applications with automatic feature refreshing, caching, and tracking callbacks.

Requirements

Rust 1.70.0 or higher (as specified in rust-toolchain)
Async runtime: Tokio (recommended) or any async-std compatible runtime

Installation

Add this to your Cargo.toml:

[dependencies]
growthbook-rust = "0.0.4"
tokio = { version = "1", features = ["full"] }
serde_json = "1.0"

Or install via cargo:

cargo add growthbook-rust

Quick Usage

Step 1: Initialize the Client

use growthbook_rust::client::GrowthBookClientBuilder;
use std::time::Duration;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create a GrowthBook client with auto-refresh enabled
    let client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        .ttl(Duration::from_secs(60))           // Cache TTL
        .auto_refresh(true)                     // Enable background updates
        .refresh_interval(Duration::from_secs(30))
        .build()
        .await?;

    Ok(())
}

Step 2: Evaluate Feature Flags

// Simple boolean check
if client.is_on("my-feature", None) {
    println!("Feature is enabled!");
}

// Get typed feature value
let result = client.feature_result("button-color", None);
if let Ok(color) = result.value_as::<String>() {
    println!("Button color: {}", color);
}

// With user attributes
use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};
use std::collections::HashMap;

let mut attrs = Vec::new();
attrs.push(GrowthBookAttribute::new(
    "userId".to_string(),
    GrowthBookAttributeValue::String("user-123".to_string())
));

if client.is_on("premium-feature", Some(attrs)) {
    println!("Premium feature enabled for this user!");
}

Loading Features and Experiments

The Rust SDK provides multiple ways to load and refresh feature definitions from the GrowthBook API.

Built-in Fetching and Auto-Refresh

The recommended approach is to use the builder with auto-refresh enabled:

use growthbook_rust::client::GrowthBookClientBuilder;
use std::time::Duration;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        // Cache settings
        .ttl(Duration::from_secs(60))  // How long to cache features
        // Auto-refresh settings
        .auto_refresh(true)            // Enable background sync
        .refresh_interval(Duration::from_secs(30))  // Refresh every 30 seconds
        .build()
        .await?;

    // Features are now loaded and will refresh automatically
    // in the background every 30 seconds

    Ok(())
}

How Auto-Refresh Works

When auto_refresh is enabled:

Features are fetched immediately during build()
A background task spawns that periodically fetches updates
The cache is updated automatically without blocking your application
The refresh task runs until the client is dropped

Benefits:

Always up-to-date features without manual intervention
Non-blocking updates in the background
Configurable refresh intervals
Automatic retry logic on failures

Manual Refresh

You can manually trigger a feature refresh at any time:

// Force a feature refresh
client.refresh().await;

// Useful for scenarios like:
// - User login/logout events
// - Navigation changes
// - Manual override in admin panels

Starting with Initial Features

If you want to start with a specific set of features (e.g., from a file or cache) and then enable updates:

use serde_json::json;

let initial_features = json!({
    "my-feature": {
        "defaultValue": true
    },
    "button-color": {
        "defaultValue": "blue"
    }
});

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .features_json(initial_features)?  // Start with these features
    .auto_refresh(true)                // Still enable auto-refresh
    .refresh_interval(Duration::from_secs(30))
    .build()
    .await?;

// Client starts with initial features and updates them in the background

Disabling Auto-Refresh

For use cases where you want full control (e.g., testing, edge workers, or custom update logic):

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .auto_refresh(false)  // Disable background sync
    .build()
    .await?;

// Manually control when to refresh
client.refresh().await;

Refresh Callback

Get notified when features are refreshed (useful for logging and debugging):

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .auto_refresh(true)
    .refresh_interval(Duration::from_secs(30))
    .add_on_refresh(Box::new(|| {
        println!("✅ Features refreshed at {}", chrono::Utc::now());
        // Update metrics, logs, or caches
    }))
    .build()
    .await?;

Configuration via Environment Variables

The SDK supports configuration through environment variables:

Variable	Description	Default
`GB_HTTP_CLIENT_TIMEOUT`	HTTP request timeout	10 seconds
`GB_UPDATE_INTERVAL`	Auto-refresh interval	60 seconds
`GB_URL`	GrowthBook API URL	-
`GB_SDK_KEY`	SDK client key	-

// Environment variables will be used if not explicitly set
let client = GrowthBookClientBuilder::new()
    // api_url and client_key will be read from GB_URL and GB_SDK_KEY
    .build()
    .await?;

Encrypted Features

For enhanced security, GrowthBook supports encrypted feature payloads. This prevents sensitive feature configurations and PII data from being exposed in transit or in logs.

Setup

Enable encryption in your GrowthBook SDK Connection settings
Copy the decryption key shown in the GrowthBook dashboard
Pass the key to the SDK during initialization

use growthbook_rust::client::GrowthBookClientBuilder;

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .decryption_key("your-decryption-key-here".to_string())  // Add this
    .build()
    .await?;

// Features are automatically decrypted when loaded

How It Works

Feature payloads from the API are encrypted using AES-256
The SDK automatically decrypts them using your decryption key
Decryption happens transparently - your code doesn’t change
Invalid keys or corrupted data will cause initialization to fail

Security Best Practices

use std::env;

// ✅ DO: Load from environment variables
let decryption_key = env::var("GROWTHBOOK_DECRYPTION_KEY")
    .expect("GROWTHBOOK_DECRYPTION_KEY must be set");

let client = GrowthBookClientBuilder::new()
    .api_url(env::var("GROWTHBOOK_API_URL").unwrap())
    .client_key(env::var("GROWTHBOOK_CLIENT_KEY").unwrap())
    .decryption_key(decryption_key)
    .build()
    .await?;

// ❌ DON'T: Hardcode keys in source code
// let decryption_key = "key-123456789".to_string(); // NEVER DO THIS!

Recommendations:

Use environment variables or secret management systems (AWS Secrets Manager, HashiCorp Vault)
Rotate keys regularly
Use different keys for different environments (dev, staging, production)
Never commit keys to version control

Error Handling

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .decryption_key("wrong-key".to_string())
    .build()
    .await;

match client {
    Ok(client) => {
        println!("Client initialized successfully");
    },
    Err(e) => {
        eprintln!("Failed to initialize client: {}", e);
        // Could be due to:
        // - Wrong decryption key
        // - Network issues
        // - Invalid client key
        // Fall back to safe defaults
    }
}

Attributes

Attributes are used for two main purposes:

Feature targeting - Show different values to different user segments
Experiment bucketing - Ensure consistent variation assignment

Setting Global Attributes

You can set default attributes that apply to all feature evaluations:

use std::collections::HashMap;
use growthbook_rust::model_public::GrowthBookAttributeValue;

// Define global attributes during client creation
let mut global_attrs = HashMap::new();
global_attrs.insert(
    "tenantId".to_string(),
    GrowthBookAttributeValue::String("acme-corp".to_string())
);
global_attrs.insert(
    "plan".to_string(),
    GrowthBookAttributeValue::String("enterprise".to_string())
);

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .attributes(global_attrs)  // Set global attributes
    .build()
    .await?;

Per-Evaluation Attributes

You can override or supplement global attributes on a per-check basis:

use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};

// Create per-evaluation attributes
let mut user_attrs = Vec::new();
user_attrs.push(GrowthBookAttribute::new(
    "userId".to_string(),
    GrowthBookAttributeValue::String("user-456".to_string())
));
user_attrs.push(GrowthBookAttribute::new(
    "country".to_string(),
    GrowthBookAttributeValue::String("US".to_string())
));
user_attrs.push(GrowthBookAttribute::new(
    "isPremium".to_string(),
    GrowthBookAttributeValue::Bool(true)
));

// These attributes are merged with global attributes
if client.is_on("new-dashboard", Some(user_attrs)) {
    // Show new dashboard
}

Attribute Types

The SDK supports all JSON data types as attributes:

use growthbook_rust::model_public::GrowthBookAttributeValue;
use serde_json::json;

let mut attrs = Vec::new();

// String
attrs.push(GrowthBookAttribute::new(
    "email".to_string(),
    GrowthBookAttributeValue::String("user@example.com".to_string())
));

// Number (integer)
attrs.push(GrowthBookAttribute::new(
    "age".to_string(),
    GrowthBookAttributeValue::Number(serde_json::Number::from(25))
));

// Boolean
attrs.push(GrowthBookAttribute::new(
    "isLoggedIn".to_string(),
    GrowthBookAttributeValue::Bool(true)
));

// Array
attrs.push(GrowthBookAttribute::new(
    "tags".to_string(),
    GrowthBookAttributeValue::Array(vec![
        json!("premium"),
        json!("beta-tester")
    ])
));

// Object
attrs.push(GrowthBookAttribute::new(
    "company".to_string(),
    GrowthBookAttributeValue::Object(json!({
        "id": "company-123",
        "name": "Acme Corp"
    }))
));

Common Attribute Patterns

// Web application attributes
let mut web_attrs = Vec::new();
web_attrs.push(GrowthBookAttribute::new("id".to_string(),
    GrowthBookAttributeValue::String(user_id)));
web_attrs.push(GrowthBookAttribute::new("url".to_string(),
    GrowthBookAttributeValue::String(request.uri().to_string())));
web_attrs.push(GrowthBookAttribute::new("userAgent".to_string(),
    GrowthBookAttributeValue::String(user_agent)));
web_attrs.push(GrowthBookAttribute::new("country".to_string(),
    GrowthBookAttributeValue::String(geo_ip_country)));

// API attributes
let mut api_attrs = Vec::new();
api_attrs.push(GrowthBookAttribute::new("apiKey".to_string(),
    GrowthBookAttributeValue::String(api_key)));
api_attrs.push(GrowthBookAttribute::new("requestsToday".to_string(),
    GrowthBookAttributeValue::Number(serde_json::Number::from(request_count))));
api_attrs.push(GrowthBookAttribute::new("tier".to_string(),
    GrowthBookAttributeValue::String("premium".to_string())));

Attribute Merging Behavior

When you provide per-evaluation attributes:

They are merged with global attributes
Per-evaluation attributes take precedence over global ones
This allows you to set common attributes globally and override them as needed

// Global attributes
let mut global = HashMap::new();
global.insert("tenantId".to_string(),
    GrowthBookAttributeValue::String("tenant-1".to_string()));
global.insert("plan".to_string(),
    GrowthBookAttributeValue::String("free".to_string()));

let client = GrowthBookClientBuilder::new()
    // ... other settings ...
    .attributes(global)
    .build()
    .await?;

// Per-evaluation attributes (overrides "plan")
let mut user_attrs = Vec::new();
user_attrs.push(GrowthBookAttribute::new(
    "userId".to_string(),
    GrowthBookAttributeValue::String("user-123".to_string())
));
user_attrs.push(GrowthBookAttribute::new(
    "plan".to_string(),  // This overrides the global "plan"
    GrowthBookAttributeValue::String("premium".to_string())
));

// Final attributes used: { tenantId: "tenant-1", plan: "premium", userId: "user-123" }
let result = client.is_on("premium-feature", Some(user_attrs));

Using Features

The SDK provides multiple methods for evaluating features with different levels of detail.

Basic Feature Checks

`is_on()` - Simple Boolean Check

Check if a feature is enabled (evaluates to a truthy value):

// Without attributes
if client.is_on("new-navigation", None) {
    println!("Show new navigation");
}

// With attributes
let mut attrs = Vec::new();
attrs.push(GrowthBookAttribute::new(
    "userId".to_string(),
    GrowthBookAttributeValue::String("user-123".to_string())
));

if client.is_on("beta-feature", Some(attrs)) {
    println!("User is in beta program");
}

`is_off()` - Inverse Boolean Check

Check if a feature is disabled (evaluates to a falsy value):

if client.is_off("maintenance-mode", None) {
    // Allow normal operations
    process_request();
}

Getting Feature Values

`feature_result()` - Get Detailed Feature Information

Get the full feature result with metadata:

let result = client.feature_result("button-color", None);

// Access the raw value
println!("Value: {:?}", result.value);

// Check if enabled
if result.on {
    println!("Feature is on");
}

// Get typed value with error handling
match result.value_as::<String>() {
    Ok(color) => println!("Button color: {}", color),
    Err(e) => println!("Error getting value: {}", e),
}

// Understand why this value was assigned
println!("Source: {:?}", result.source);  // e.g., "experiment", "force", "defaultValue"

Type-Safe Feature Values

The value_as::<T>() method provides type-safe access to feature values:

// String values
let color_result = client.feature_result("button-color", None);
let color: String = color_result.value_as::<String>()
    .unwrap_or("blue".to_string());

// Integer values
let max_items_result = client.feature_result("max-items", None);
let max_items: i32 = max_items_result.value_as::<i32>()
    .unwrap_or(10);

// Boolean values
let enabled_result = client.feature_result("new-feature", None);
let enabled: bool = enabled_result.value_as::<bool>()
    .unwrap_or(false);

// Complex types (JSON)
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize)]
struct ThemeConfig {
    primary_color: String,
    secondary_color: String,
    font_size: i32,
}

let theme_result = client.feature_result("theme-config", None);
match theme_result.value_as::<ThemeConfig>() {
    Ok(theme) => {
        println!("Primary: {}, Font: {}", theme.primary_color, theme.font_size);
    },
    Err(_) => {
        // Use default theme
        let default_theme = ThemeConfig {
            primary_color: "blue".to_string(),
            secondary_color: "gray".to_string(),
            font_size: 16,
        };
    }
}

Feature Result Properties

The FeatureResult struct contains detailed information about the feature evaluation:

let result = client.feature_result("my-feature", None);

// The actual feature value (Option<serde_json::Value>)
println!("Value: {:?}", result.value);

// Boolean helpers
if result.on {
    // Feature value is truthy
}
if result.off {
    // Feature value is falsy
}

// Why was this value assigned?
match result.source {
    FeatureResultSource::DefaultValue => println!("Using default value"),
    FeatureResultSource::Force => println!("Forced value from targeting rule"),
    FeatureResultSource::Experiment => println!("Value from A/B test"),
    FeatureResultSource::UnknownFeature => println!("Feature not found"),
}

// Experiment information (if from an A/B test)
if let Some(exp) = &result.experiment {
    println!("Experiment key: {}", exp.key);
}

if let Some(exp_result) = &result.experiment_result {
    println!("Variation ID: {}", exp_result.variation_id);
    println!("In experiment: {}", exp_result.in_experiment);
}

Handling Missing Features

Features that don’t exist return None as their value:

let result = client.feature_result("non-existent-feature", None);

if result.value.is_none() {
    println!("Feature not found, using default behavior");
    // Fallback logic
}

// Or use value_as with a default
let value = result.value_as::<String>()
    .unwrap_or("default-value".to_string());

Feature Flags Usage - Best Practices

// ✅ Good: Use descriptive feature keys
if client.is_on("enable-dark-mode", None) {
    // ...
}

// ✅ Good: Provide fallback values
let timeout = client.feature_result("api-timeout", None)
    .value_as::<i32>()
    .unwrap_or(30);

// ✅ Good: Handle errors gracefully
match client.feature_result("config", None).value_as::<Config>() {
    Ok(config) => use_config(config),
    Err(_) => use_default_config(),
}

// ❌ Bad: Using magic values without fallbacks
let timeout = client.feature_result("timeout", None)
    .value_as::<i32>()
    .unwrap();  // Panics if feature doesn't exist!

// ❌ Bad: Not handling type mismatches
let value = client.feature_result("my-feature", None).value;
// Assuming type without checking

Tracking Callbacks

Tracking callbacks allow you to integrate GrowthBook with your analytics systems (Segment, Mixpanel, Amplitude, etc.) to track when users are exposed to experiments.

Experiment Viewed Callback

This callback fires when a user is assigned a variation in an A/B test:

use growthbook_rust::client::GrowthBookClientBuilder;

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .on_experiment_viewed(Box::new(|experiment_result| {
        // Track in your analytics system
        println!("🧪 Experiment Viewed:");
        println!("  Experiment: {}", experiment_result.key);
        println!("  Variation: {}", experiment_result.variation_id);
        println!("  Value: {:?}", experiment_result.value);
        println!("  In Experiment: {}", experiment_result.in_experiment);
        println!("  Hash Used: {}", experiment_result.hash_used);

        // Example: Send to Segment
        // analytics.track("Experiment Viewed", json!({
        //     "experiment_id": experiment_result.key,
        //     "variation_id": experiment_result.variation_id,
        //     "variation_value": experiment_result.value,
        // }));
    }))
    .build()
    .await?;

When is the Callback Triggered?

The on_experiment_viewed callback is called when:

A feature evaluation runs an experiment
The user is included in the experiment (passes targeting rules)
The user is randomly assigned a variation (not forced)

It is NOT called when:

A feature uses a forced value (no experiment)
The user is excluded from the experiment due to targeting
The feature doesn’t exist

Feature Usage Callback

Track every feature evaluation, regardless of whether it’s part of an experiment:

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .on_feature_usage(Box::new(|feature_key, result| {
        // Track feature usage
        println!("📊 Feature Used:");
        println!("  Key: {}", feature_key);
        println!("  Value: {:?}", result.value);
        println!("  On: {}", result.on);
        println!("  Source: {:?}", result.source);

        // Example: Send to monitoring system
        // monitoring.record_metric("feature.used", 1, vec![
        //     format!("feature:{}", feature_key),
        //     format!("enabled:{}", result.on),
        // ]);
    }))
    .build()
    .await?;

Use Cases for Feature Usage Tracking:

Monitor which features are being evaluated
Debug feature flag behavior
Track adoption of new features
Send metrics to monitoring systems (DataDog, New Relic)

Using Both Callbacks Together

You can use both callbacks for comprehensive tracking:

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    // Track A/B test exposures for analytics
    .on_experiment_viewed(Box::new(|exp_result| {
        // Critical for experiment analysis
        analytics::track_experiment_viewed(
            exp_result.key.clone(),
            exp_result.variation_id,
            exp_result.value.clone(),
        );
    }))
    // Track all feature usage for monitoring
    .on_feature_usage(Box::new(|key, result| {
        // For debugging and monitoring
        monitoring::record_feature_usage(key, result.on);
    }))
    .build()
    .await?;

Integration Examples

Segment Integration

// Assuming you have a Segment client
use segment::{HttpClient, Message};

let segment_client = HttpClient::default();

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .on_experiment_viewed(Box::new(move |exp_result| {
        let message = Message::track("user-123", "Experiment Viewed")
            .properties(json!({
                "experiment_id": exp_result.key,
                "variation_id": exp_result.variation_id,
                "variation_value": exp_result.value,
            }));

        segment_client.send(message);
    }))
    .build()
    .await?;

Custom Analytics System

use tokio::spawn;

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .on_experiment_viewed(Box::new(|exp_result| {
        // Send async without blocking
        spawn(async move {
            let event = json!({
                "event": "experiment_viewed",
                "experiment_id": exp_result.key,
                "variation_id": exp_result.variation_id,
                "timestamp": chrono::Utc::now().to_rfc3339(),
            });

            // Send to your analytics endpoint
            if let Err(e) = send_analytics_event(event).await {
                eprintln!("Failed to send analytics: {}", e);
            }
        });
    }))
    .build()
    .await?;

Context and Caching

Context

The SDK uses a context object internally to manage state. You typically don’t interact with it directly, but it’s useful to understand how it works:

// The builder pattern creates and manages context for you
let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .attributes(global_attributes)  // Context attributes
    .build()
    .await?;

// Per-evaluation attributes merge with context attributes
let result = client.is_on("my-feature", Some(user_specific_attributes));

Caching

The SDK implements intelligent caching to minimize network requests:

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .ttl(Duration::from_secs(60))  // Cache features for 60 seconds
    .build()
    .await?;

How Caching Works:

Features are cached in memory after the first fetch
Cache is automatically refreshed based on TTL
Manual refresh with client.refresh().await bypasses cache
Cache is shared across all evaluations
TTL defaults to 60 seconds if not specified

Cache Behavior:

// First call: fetches from API
let result1 = client.is_on("my-feature", None);

// Second call within TTL: uses cache (fast)
let result2 = client.is_on("my-feature", None);

// After TTL expires: fetches from API again
tokio::time::sleep(Duration::from_secs(61)).await;
let result3 = client.is_on("my-feature", None);

Debugging and Logging

Enable Debug Output

The Rust SDK uses standard Rust logging. Enable it using env_logger or tracing:

// Add to Cargo.toml
// [dependencies]
// env_logger = "0.11"
// log = "0.4"

use log::{info, debug};

fn main() {
    // Initialize logger
    env_logger::init();

    // Or with custom format
    env_logger::Builder::from_default_env()
        .filter_level(log::LevelFilter::Debug)
        .init();

    // Now SDK operations will log details
    let client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        .build()
        .await?;
}

Set the log level via environment variable:

RUST_LOG=debug cargo run
# or
RUST_LOG=growthbook_rust=debug cargo run

Common Issues and Solutions

Issue: Features not loading

// Check initialization
let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .build()
    .await;

match client {
    Ok(c) => println!("✅ Client initialized"),
    Err(e) => eprintln!("❌ Initialization failed: {}", e),
}

// Check if features are loaded
let result = client.feature_result("test-feature", None);
if result.value.is_none() {
    eprintln!("Feature not found - features may not be loaded");
}

Issue: Wrong feature values

// Debug feature evaluation
let result = client.feature_result("my-feature", None);

println!("Feature: my-feature");
println!("  Value: {:?}", result.value);
println!("  Source: {:?}", result.source);
println!("  On: {}", result.on);

// Check attributes being used
let mut attrs = Vec::new();
attrs.push(GrowthBookAttribute::new(
    "userId".to_string(),
    GrowthBookAttributeValue::String("test-123".to_string())
));

let result_with_attrs = client.feature_result("my-feature", Some(attrs));
println!("With attributes: {:?}", result_with_attrs.value);

Issue: Auto-refresh not working

use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};

let refresh_count = Arc::new(AtomicUsize::new(0));
let counter = refresh_count.clone();

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .auto_refresh(true)
    .refresh_interval(Duration::from_secs(10))
    .add_on_refresh(Box::new(move || {
        let count = counter.fetch_add(1, Ordering::SeqCst);
        println!("Refresh #{} at {:?}", count + 1, std::time::SystemTime::now());
    }))
    .build()
    .await?;

// Wait and watch for refreshes
tokio::time::sleep(Duration::from_secs(35)).await;
println!("Total refreshes: {}", refresh_count.load(Ordering::SeqCst));

Testing and QA

Testing with Forced Values

// In your test environment, use features_json to control values
use serde_json::json;

let test_features = json!({
    "feature-under-test": {
        "defaultValue": true
    },
    "config-value": {
        "defaultValue": "test-mode"
    }
});

let client = GrowthBookClientBuilder::new()
    .api_url("http://localhost:8080".to_string())
    .client_key("test-key".to_string())
    .features_json(test_features)?
    .auto_refresh(false)  // Disable refresh in tests
    .build()
    .await?;

// Now you can test with predictable feature values
assert!(client.is_on("feature-under-test", None));

Unit Testing Helpers

#[cfg(test)]
mod tests {
    use super::*;

    async fn create_test_client() -> GrowthBookClient {
        let features = json!({
            "test-feature": {
                "defaultValue": true
            }
        });

        GrowthBookClientBuilder::new()
            .api_url("http://test".to_string())
            .client_key("test".to_string())
            .features_json(features).unwrap()
            .auto_refresh(false)
            .build()
            .await
            .unwrap()
    }

    #[tokio::test]
    async fn test_feature_enabled() {
        let client = create_test_client().await;
        assert!(client.is_on("test-feature", None));
    }

    #[tokio::test]
    async fn test_with_attributes() {
        let client = create_test_client().await;

        let mut attrs = Vec::new();
        attrs.push(GrowthBookAttribute::new(
            "userId".to_string(),
            GrowthBookAttributeValue::String("test-user".to_string())
        ));

        let result = client.is_on("test-feature", Some(attrs));
        assert!(result);
    }
}

Integration Examples

This section provides real-world integration examples with popular Rust frameworks.

Actix Web Integration

A complete example of integrating GrowthBook with Actix Web:

use actix_web::{web, App, HttpRequest, HttpResponse, HttpServer};
use growthbook_rust::client::{GrowthBookClient, GrowthBookClientBuilder};
use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};
use std::sync::Arc;
use std::time::Duration;

// Application state
struct AppState {
    gb_client: Arc<GrowthBookClient>,
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    env_logger::init();

    // Initialize GrowthBook client (singleton)
    let gb_client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        .auto_refresh(true)
        .refresh_interval(Duration::from_secs(30))
        .on_experiment_viewed(Box::new(|exp_result| {
            log::info!("Experiment viewed: {} -> {}",
                exp_result.key, exp_result.variation_id);
        }))
        .build()
        .await
        .expect("Failed to initialize GrowthBook");

    let gb_client = Arc::new(gb_client);

    // Start HTTP server
    HttpServer::new(move || {
        App::new()
            .app_data(web::Data::new(AppState {
                gb_client: gb_client.clone(),
            }))
            .route("/", web::get().to(index))
            .route("/api/features", web::get().to(get_features))
    })
    .bind(("127.0.0.1", 8080))?
    .run()
    .await
}

// Handler with per-request attributes
async fn index(
    data: web::Data<AppState>,
    req: HttpRequest,
) -> HttpResponse {
    // Extract user attributes from request
    let user_id = req.headers()
        .get("X-User-ID")
        .and_then(|v| v.to_str().ok())
        .unwrap_or("anonymous");

    let mut attrs = Vec::new();
    attrs.push(GrowthBookAttribute::new(
        "userId".to_string(),
        GrowthBookAttributeValue::String(user_id.to_string())
    ));
    attrs.push(GrowthBookAttribute::new(
        "url".to_string(),
        GrowthBookAttributeValue::String(req.uri().to_string())
    ));

    // Check features with user-specific attributes
    let show_new_ui = data.gb_client.is_on("new-ui", Some(attrs.clone()));
    let theme_result = data.gb_client.feature_result("theme", Some(attrs));
    let theme = theme_result.value_as::<String>()
        .unwrap_or("light".to_string());

    HttpResponse::Ok().json(serde_json::json!({
        "new_ui": show_new_ui,
        "theme": theme,
        "user_id": user_id,
    }))
}

// Handler to inspect all features (useful for debugging)
async fn get_features(data: web::Data<AppState>) -> HttpResponse {
    // Return feature flags for debugging
    HttpResponse::Ok().json(serde_json::json!({
        "status": "ok",
        "message": "Features loaded"
    }))
}

Axum Integration

Modern async web framework integration:

use axum::{
    extract::{Extension, Path},
    http::{Request, StatusCode},
    response::{IntoResponse, Json},
    routing::{get, post},
    Router,
};
use growthbook_rust::client::{GrowthBookClient, GrowthBookClientBuilder};
use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};
use serde::{Deserialize, Serialize};
use std::net::SocketAddr;
use std::sync::Arc;
use std::time::Duration;

#[derive(Clone)]
struct AppState {
    gb_client: Arc<GrowthBookClient>,
}

#[tokio::main]
async fn main() {
    // Initialize tracing
    tracing_subscriber::fmt::init();

    // Create GrowthBook client
    let gb_client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        .auto_refresh(true)
        .refresh_interval(Duration::from_secs(30))
        .build()
        .await
        .expect("Failed to create GrowthBook client");

    let state = AppState {
        gb_client: Arc::new(gb_client),
    };

    // Build router
    let app = Router::new()
        .route("/", get(root))
        .route("/user/:id", get(user_features))
        .route("/experiment", post(run_experiment))
        .layer(Extension(state));

    // Run server
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    tracing::info!("Listening on {}", addr);
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

async fn root(Extension(state): Extension<AppState>) -> impl IntoResponse {
    let is_maintenance = state.gb_client.is_on("maintenance-mode", None);

    if is_maintenance {
        return (
            StatusCode::SERVICE_UNAVAILABLE,
            "Maintenance mode enabled",
        ).into_response();
    }

    (StatusCode::OK, "Service operational").into_response()
}

#[derive(Debug, Serialize)]
struct UserFeatures {
    user_id: String,
    premium_enabled: bool,
    theme: String,
    max_uploads: i32,
}

async fn user_features(
    Path(user_id): Path<String>,
    Extension(state): Extension<AppState>,
) -> Json<UserFeatures> {
    // Build user attributes
    let mut attrs = Vec::new();
    attrs.push(GrowthBookAttribute::new(
        "userId".to_string(),
        GrowthBookAttributeValue::String(user_id.clone())
    ));

    // Evaluate features for this user
    let premium_enabled = state.gb_client.is_on("premium-features", Some(attrs.clone()));

    let theme_result = state.gb_client.feature_result("theme", Some(attrs.clone()));
    let theme = theme_result.value_as::<String>()
        .unwrap_or("default".to_string());

    let uploads_result = state.gb_client.feature_result("max-uploads", Some(attrs));
    let max_uploads = uploads_result.value_as::<i32>()
        .unwrap_or(5);

    Json(UserFeatures {
        user_id,
        premium_enabled,
        theme,
        max_uploads,
    })
}

#[derive(Debug, Deserialize)]
struct ExperimentRequest {
    user_id: String,
    feature_key: String,
}

async fn run_experiment(
    Extension(state): Extension<AppState>,
    Json(payload): Json<ExperimentRequest>,
) -> impl IntoResponse {
    let mut attrs = Vec::new();
    attrs.push(GrowthBookAttribute::new(
        "userId".to_string(),
        GrowthBookAttributeValue::String(payload.user_id.clone())
    ));

    let result = state.gb_client.feature_result(&payload.feature_key, Some(attrs));

    Json(serde_json::json!({
        "feature_key": payload.feature_key,
        "user_id": payload.user_id,
        "value": result.value,
        "on": result.on,
        "source": format!("{:?}", result.source),
    }))
}

Rocket Integration

#[macro_use] extern crate rocket;

use rocket::{State, http::Status};
use rocket::serde::json::Json;
use growthbook_rust::client::{GrowthBookClient, GrowthBookClientBuilder};
use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};
use std::sync::Arc;
use std::time::Duration;

struct GBState {
    client: Arc<GrowthBookClient>,
}

#[get("/")]
async fn index(state: &State<GBState>) -> Result<String, Status> {
    if state.client.is_on("maintenance-mode", None) {
        return Err(Status::ServiceUnavailable);
    }
    Ok("Hello, World!".to_string())
}

#[get("/feature/<user_id>")]
async fn check_feature(
    user_id: String,
    state: &State<GBState>,
) -> Json<serde_json::Value> {
    let mut attrs = Vec::new();
    attrs.push(GrowthBookAttribute::new(
        "userId".to_string(),
        GrowthBookAttributeValue::String(user_id.clone())
    ));

    let enabled = state.client.is_on("beta-feature", Some(attrs));

    Json(serde_json::json!({
        "user_id": user_id,
        "beta_enabled": enabled,
    }))
}

#[launch]
async fn rocket() -> _ {
    let gb_client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        .auto_refresh(true)
        .build()
        .await
        .expect("Failed to init GrowthBook");

    rocket::build()
        .manage(GBState {
            client: Arc::new(gb_client),
        })
        .mount("/", routes![index, check_feature])
}

CLI Application Example

Using GrowthBook in a command-line application:

use growthbook_rust::client::GrowthBookClientBuilder;
use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};
use clap::Parser;
use std::time::Duration;

#[derive(Parser, Debug)]
#[clap(author, version, about, long_about = None)]
struct Args {
    /// User ID
    #[clap(short, long)]
    user_id: String,

    /// Command to run
    #[clap(short, long)]
    command: String,
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    env_logger::init();

    let args = Args::parse();

    // Initialize GrowthBook
    let client = GrowthBookClientBuilder::new()
        .api_url("https://cdn.growthbook.io".to_string())
        .client_key("sdk-abc123".to_string())
        .auto_refresh(false)  // No auto-refresh for CLI
        .build()
        .await?;

    // Create user attributes
    let mut attrs = Vec::new();
    attrs.push(GrowthBookAttribute::new(
        "userId".to_string(),
        GrowthBookAttributeValue::String(args.user_id.clone())
    ));
    attrs.push(GrowthBookAttribute::new(
        "cli".to_string(),
        GrowthBookAttributeValue::Bool(true)
    ));

    // Check feature flags
    let can_use_beta_commands = client.is_on("cli-beta-commands", Some(attrs.clone()));

    match args.command.as_str() {
        "beta-feature" if can_use_beta_commands => {
            println!("✅ Beta feature enabled for user {}", args.user_id);
            run_beta_feature();
        }
        "beta-feature" => {
            println!("❌ Beta feature not available for user {}", args.user_id);
        }
        "standard" => {
            println!("Running standard command");
            run_standard_command();
        }
        _ => {
            println!("Unknown command: {}", args.command);
        }
    }

    Ok(())
}

fn run_beta_feature() {
    println!("Executing beta feature...");
    // Beta feature logic
}

fn run_standard_command() {
    println!("Executing standard command...");
    // Standard logic
}

Background Worker / Job Processor

Using GrowthBook in async background workers:

use growthbook_rust::client::{GrowthBookClient, GrowthBookClientBuilder};
use growthbook_rust::model_public::{GrowthBookAttribute, GrowthBookAttributeValue};
use std::sync::Arc;
use std::time::Duration;
use tokio::time::sleep;

struct JobProcessor {
    gb_client: Arc<GrowthBookClient>,
}

impl JobProcessor {
    async fn new() -> Result<Self, Box<dyn std::error::Error>> {
        let gb_client = GrowthBookClientBuilder::new()
            .api_url("https://cdn.growthbook.io".to_string())
            .client_key("sdk-abc123".to_string())
            .auto_refresh(true)
            .refresh_interval(Duration::from_secs(60))
            .build()
            .await?;

        Ok(Self {
            gb_client: Arc::new(gb_client),
        })
    }

    async fn process_job(&self, job: Job) {
        // Create attributes for this job
        let mut attrs = Vec::new();
        attrs.push(GrowthBookAttribute::new(
            "userId".to_string(),
            GrowthBookAttributeValue::String(job.user_id.clone())
        ));
        attrs.push(GrowthBookAttribute::new(
            "jobType".to_string(),
            GrowthBookAttributeValue::String(job.job_type.clone())
        ));

        // Check if new processing algorithm is enabled
        let use_new_algorithm = self.gb_client.is_on(
            "new-job-processing",
            Some(attrs.clone())
        );

        if use_new_algorithm {
            log::info!("Using new processing algorithm for job {}", job.id);
            self.process_with_new_algorithm(&job).await;
        } else {
            log::info!("Using standard processing for job {}", job.id);
            self.process_with_standard_algorithm(&job).await;
        }

        // Check rate limits from feature flags
        let rate_limit_result = self.gb_client.feature_result(
            "job-rate-limit",
            Some(attrs)
        );

        if let Ok(rate_limit) = rate_limit_result.value_as::<i32>() {
            log::info!("Rate limit for this job: {}", rate_limit);
            // Apply rate limiting
        }
    }

    async fn process_with_new_algorithm(&self, job: &Job) {
        // New algorithm logic
        log::info!("Processing job {} with new algorithm", job.id);
    }

    async fn process_with_standard_algorithm(&self, job: &Job) {
        // Standard logic
        log::info!("Processing job {} with standard algorithm", job.id);
    }
}

#[derive(Debug)]
struct Job {
    id: String,
    user_id: String,
    job_type: String,
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    env_logger::init();

    let processor = JobProcessor::new().await?;

    // Simulate job queue
    loop {
        // Fetch job from queue (Redis, RabbitMQ, etc.)
        let job = Job {
            id: uuid::Uuid::new_v4().to_string(),
            user_id: "user-123".to_string(),
            job_type: "data-processing".to_string(),
        };

        processor.process_job(job).await;

        sleep(Duration::from_secs(5)).await;
    }
}

Advanced Usage

Multiple SDK Instances

You can create multiple GrowthBook clients for different environments or projects:

use std::collections::HashMap;

struct MultiTenantGrowthBook {
    clients: HashMap<String, Arc<GrowthBookClient>>,
}

impl MultiTenantGrowthBook {
    async fn new(tenants: Vec<(&str, &str, &str)>) -> Result<Self, Box<dyn std::error::Error>> {
        let mut clients = HashMap::new();

        for (tenant_id, api_url, client_key) in tenants {
            let client = GrowthBookClientBuilder::new()
                .api_url(api_url.to_string())
                .client_key(client_key.to_string())
                .auto_refresh(true)
                .build()
                .await?;

            clients.insert(tenant_id.to_string(), Arc::new(client));
        }

        Ok(Self { clients })
    }

    fn get_client(&self, tenant_id: &str) -> Option<&Arc<GrowthBookClient>> {
        self.clients.get(tenant_id)
    }
}

// Usage
let multi_tenant = MultiTenantGrowthBook::new(vec![
    ("tenant-a", "https://cdn.growthbook.io", "sdk-key-a"),
    ("tenant-b", "https://cdn.growthbook.io", "sdk-key-b"),
]).await?;

if let Some(client) = multi_tenant.get_client("tenant-a") {
    let enabled = client.is_on("feature", None);
}

TypeScript / Rust Interop

If you’re building a hybrid application with TypeScript frontend and Rust backend, you can use the same SDK concepts across both: Rust Backend:

// backend/src/main.rs
let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .build()
    .await?;

let features_for_frontend = serde_json::json!({
    "dark_mode": client.is_on("dark-mode", Some(user_attrs)),
    "theme": client.feature_result("theme", Some(user_attrs)).value,
});

// Send to frontend
HttpResponse::Ok().json(features_for_frontend)

TypeScript Frontend:

// frontend/src/features.ts
interface Features {
  dark_mode: boolean;
  theme: string;
}

const features: Features = await fetch('/api/features').then(r => r.json());

if (features.dark_mode) {
  enableDarkMode();
}

Performance Considerations

Memory Usage

Each client instance maintains an in-memory cache of features
Auto-refresh spawns a background task
Features are deserialized from JSON on each fetch

Optimization Tips:

// ✅ Good: Single client instance, reused across requests
lazy_static! {
    static ref GB_CLIENT: Arc<GrowthBookClient> = {
        // Initialize once at startup
        tokio::runtime::Runtime::new()
            .unwrap()
            .block_on(async {
                Arc::new(
                    GrowthBookClientBuilder::new()
                        .api_url("https://cdn.growthbook.io".to_string())
                        .client_key("sdk-abc123".to_string())
                        .build()
                        .await
                        .expect("Failed to create GrowthBook client")
                )
            })
    };
}

// ❌ Bad: Creating new client on each request
async fn handler() {
    let client = GrowthBookClientBuilder::new() // DON'T DO THIS
        .build()
        .await
        .unwrap();
}

Network Performance

Features are cached based on TTL
Consider longer refresh intervals for stable features

// For frequently changing features
let client = GrowthBookClientBuilder::new()
    .refresh_interval(Duration::from_secs(30))  // 30 seconds
    .build()
    .await?;

// For stable features
let client = GrowthBookClientBuilder::new()
    .refresh_interval(Duration::from_secs(300))  // 5 minutes
    .build()
    .await?;

Troubleshooting

Common Error Messages

”Failed to fetch features"

// Possible causes:
// 1. Wrong API URL or client key
// 2. Network connectivity issues
// 3. API is down

// Solution: Check configuration and network
let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())  // Verify this
    .client_key("sdk-abc123".to_string())              // Verify this
    .build()
    .await
    .map_err(|e| {
        eprintln!("Client creation failed: {}", e);
        e
    })?;

"Decryption failed"

// Cause: Wrong decryption key
// Solution: Verify the key from GrowthBook dashboard
let correct_key = env::var("GROWTHBOOK_DECRYPTION_KEY")?;
let client = GrowthBookClientBuilder::new()
    .decryption_key(correct_key)
    .build()
    .await?;

"Feature not found”

let result = client.feature_result("my-feature", None);
if result.value.is_none() {
    // Feature doesn't exist in your GrowthBook project
    // Check the feature key spelling
    eprintln!("Feature 'my-feature' not found");
}

Enable Verbose Logging

# Maximum verbosity
RUST_LOG=trace cargo run

# GrowthBook-specific logs
RUST_LOG=growthbook_rust=debug cargo run

# Filter by module
RUST_LOG=growthbook_rust::client=debug cargo run

Health Check Endpoint

use axum::{Extension, Json};

async fn health_check(
    Extension(state): Extension<AppState>,
) -> Json<serde_json::Value> {
    // Check if GrowthBook is accessible
    let test_result = state.gb_client.feature_result("health-check", None);

    Json(serde_json::json!({
        "status": "ok",
        "growthbook": {
            "initialized": true,
            "features_loaded": !test_result.value.is_none(),
        }
    }))
}

Migration from Other SDKs

From Node.js/JavaScript SDK

JavaScript:

const gb = new GrowthBook({
  apiHost: "https://cdn.growthbook.io",
  clientKey: "sdk-abc123",
  attributes: { userId: "123" }
});

await gb.init();
const enabled = gb.isOn("my-feature");

Rust:

let mut attrs = HashMap::new();
attrs.insert("userId".to_string(),
    GrowthBookAttributeValue::String("123".to_string()));

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .attributes(attrs)
    .build()
    .await?;

let enabled = client.is_on("my-feature", None);

From Python SDK

Python:

gb = GrowthBook(
    api_host="https://cdn.growthbook.io",
    client_key="sdk-abc123",
    attributes={"userId": "123"}
)
gb.load_features()
enabled = gb.is_on("my-feature")

Rust:

let mut attrs = HashMap::new();
attrs.insert("userId".to_string(),
    GrowthBookAttributeValue::String("123".to_string()));

let client = GrowthBookClientBuilder::new()
    .api_url("https://cdn.growthbook.io".to_string())
    .client_key("sdk-abc123".to_string())
    .attributes(attrs)
    .build()
    .await?;

let enabled = client.is_on("my-feature", None);

​Requirements

​Installation

​Quick Usage

​Step 1: Initialize the Client

​Step 2: Evaluate Feature Flags

​Loading Features and Experiments

​Built-in Fetching and Auto-Refresh

​How Auto-Refresh Works

​Manual Refresh

​Starting with Initial Features

​Disabling Auto-Refresh

​Refresh Callback

​Configuration via Environment Variables

​Encrypted Features

​Setup

​How It Works

​Security Best Practices

​Error Handling

​Attributes

​Setting Global Attributes

​Per-Evaluation Attributes

​Attribute Types

​Common Attribute Patterns

​Attribute Merging Behavior

​Using Features

​Basic Feature Checks

​is_on() - Simple Boolean Check

​is_off() - Inverse Boolean Check

​Getting Feature Values

​feature_result() - Get Detailed Feature Information

​Type-Safe Feature Values

​Feature Result Properties

​Handling Missing Features

​Feature Flags Usage - Best Practices

​Tracking Callbacks

​Experiment Viewed Callback

​When is the Callback Triggered?

​Feature Usage Callback

​Using Both Callbacks Together

​Integration Examples

​Segment Integration

​Custom Analytics System

​Context and Caching

​Context

​Caching

​Debugging and Logging

​Enable Debug Output

​Common Issues and Solutions

​Issue: Features not loading

​Issue: Wrong feature values

​Issue: Auto-refresh not working

​Testing and QA

​Testing with Forced Values

​Unit Testing Helpers

​Integration Examples

​Actix Web Integration

​Axum Integration

​Rocket Integration

​CLI Application Example

​Background Worker / Job Processor

​Advanced Usage

​Multiple SDK Instances

​TypeScript / Rust Interop

​Performance Considerations

​Memory Usage

​Network Performance

​Troubleshooting

​Common Error Messages

​”Failed to fetch features"

​"Decryption failed"

​"Feature not found”

​Enable Verbose Logging

​Health Check Endpoint

​Migration from Other SDKs

​From Node.js/JavaScript SDK

​From Python SDK

​Further Reading

​Supported Features

Requirements

Installation

Quick Usage

Step 1: Initialize the Client

Step 2: Evaluate Feature Flags

Loading Features and Experiments

Built-in Fetching and Auto-Refresh

How Auto-Refresh Works

Manual Refresh

Starting with Initial Features

Disabling Auto-Refresh

Refresh Callback

Configuration via Environment Variables

Encrypted Features

Setup

How It Works

Security Best Practices

Error Handling

Attributes

Setting Global Attributes

Per-Evaluation Attributes

Attribute Types

Common Attribute Patterns

Attribute Merging Behavior

Using Features

Basic Feature Checks

`is_on()` - Simple Boolean Check

`is_off()` - Inverse Boolean Check

Getting Feature Values

`feature_result()` - Get Detailed Feature Information

Type-Safe Feature Values

Feature Result Properties

Handling Missing Features

Feature Flags Usage - Best Practices

Tracking Callbacks

Experiment Viewed Callback

When is the Callback Triggered?

Feature Usage Callback

Using Both Callbacks Together

Integration Examples

Segment Integration

Custom Analytics System

Context and Caching

Context

Caching

Debugging and Logging

Enable Debug Output

Common Issues and Solutions

Issue: Features not loading

Issue: Wrong feature values

Issue: Auto-refresh not working

Testing and QA

Testing with Forced Values

Unit Testing Helpers

Integration Examples

Actix Web Integration

Axum Integration

Rocket Integration

CLI Application Example

Background Worker / Job Processor

Advanced Usage

Multiple SDK Instances

TypeScript / Rust Interop

Performance Considerations

Memory Usage

Network Performance

Troubleshooting

Common Error Messages

”Failed to fetch features"

"Decryption failed"

"Feature not found”

Enable Verbose Logging

Health Check Endpoint

Migration from Other SDKs

From Node.js/JavaScript SDK

From Python SDK

Further Reading

Supported Features