SDK Overview

FeatureSignals provides official SDKs for server-side and client-side applications. All SDKs follow a consistent pattern and support OpenFeature for vendor-neutral integration.

Available SDKs

SDK	Language	Type	Package
Go	Go 1.22+	Server	github.com/featuresignals/sdk-go
Node.js	TypeScript/Node 22+	Server	@featuresignals/node
Python	Python 3.9+	Server	featuresignals
Java	Java 17+	Server	com.featuresignals:sdk-java
.NET	.NET 8.0+ / C#	Server	FeatureSignals
Ruby	Ruby 3.1+	Server	featuresignals
React	React 18+	Client	@featuresignals/react
Vue	Vue 3.3+	Client	@featuresignals/vue

SDK Architecture

All SDKs follow the same core design:

┌──────────┐    HTTP/SSE     ┌──────────────┐
│   SDK     │ ──────────────▶│  API Server   │
│           │                │  or Relay     │
│ ┌──────┐  │  initial load  └──────────────┘
│ │Cache │  │
│ └──────┘  │  polling/SSE
│           │  (background)
└──────────┘

1. Initialize with API key and environment key
2. First load: Fetches all flag values via GET /v1/client/{envKey}/flags
3. Background sync: Polls at regular intervals or streams via SSE
4. Local evaluation: Variation methods read from the in-memory cache (no network call)
5. Graceful degradation: Returns fallback values on errors or before ready

Common Patterns

Initialization

All SDKs accept:

Option	Default	Description
sdkKey / sdk_key	(required)	API key for authentication
envKey / env_key	(required)	Environment slug
baseURL / base_url	https://api.featuresignals.com	API server URL
pollingInterval	30 seconds	How often to refresh flags
streaming	false	Use SSE instead of polling

Variation Methods

All SDKs provide typed variation methods:

Method	Returns	SDK Suffix
Boolean	true/false	BoolVariation / boolVariation / bool_variation
String	Text value	StringVariation / stringVariation / string_variation
Number	Numeric value	NumberVariation / numberVariation / number_variation
JSON	Object/map	JSONVariation / jsonVariation / json_variation

Each takes three arguments:

1. Flag key — the flag's unique identifier
2. Evaluation context — user identity and attributes
3. Fallback value — returned if the flag doesn't exist or there's an error

Readiness

SDKs emit a "ready" event after the first successful flag load:

// Node.js
await client.waitForReady();

// Go
<-client.Ready()

// Python
client.wait_for_ready()

// Java
client.waitForReady(5000);

// C#
await client.WaitForReadyAsync();

// Ruby
client.wait_for_ready

Lifecycle

Always close the client when shutting down:

client.close();   // Node.js, Go, Java
client.close()    // Python, Ruby
client.Dispose(); // C#

OpenFeature Support

All server SDKs include an OpenFeature provider for vendor-neutral flag consumption. See the OpenFeature guide for details.

API keys and HTTP behavior

• API key expiration: Environment API keys can be created with an optional expiration time. Expired keys are rejected; rotate keys before they expire.
• SSE authentication: Passing the API key as the api_key query parameter on GET /v1/stream/{envKey} is deprecated and will be removed in a future version. Use the X-API-Key header instead.
• Rate limits: Evaluation and other rate-limited responses include X-RateLimit-Limit, X-RateLimit-Remaining, and X-RateLimit-Reset. SDKs should respect these (for example by backing off or reducing request frequency) to avoid 429 responses.