Openfeature Sdk Dev

OpenFeature SDK Development

Note: This skill extends patterns from meta-sdk-patterns-eng. See that skill for foundational SDK patterns (architecture, error handling, configuration, testing strategies, packaging).

Guide for implementing OpenFeature SDKs from the specification. This skill covers building SDK implementations, not using existing SDKs (see openfeature-eng for usage).

When to Use This Skill

• Implementing OpenFeature in a new programming language
• Contributing to official OpenFeature SDK repositories
• Building custom providers from scratch
• Extending existing SDKs with new features
• Understanding SDK architecture and compliance requirements

Specification Overview

OpenFeature uses RFC 2119 keywords (MUST, SHOULD, MAY) to define normative requirements. Implementations achieve compliance by satisfying all mandatory clauses.

Stability Levels

Core Architecture

Component Hierarchy

┌─────────────────────────────────────────────────────────────┐
│                     OpenFeature API                         │
│  (Global Singleton - manages providers, hooks, context)     │
├─────────────────────────────────────────────────────────────┤
│                        Client(s)                            │
│  (Domain-scoped flag evaluation interface)                  │
├─────────────────────────────────────────────────────────────┤
│                       Provider(s)                           │
│  (Translates evaluation to flag management system)          │
├─────────────────────────────────────────────────────────────┤
│                         Hooks                               │
│  (Lifecycle middleware: before/after/error/finally)         │
└─────────────────────────────────────────────────────────────┘

SDK Paradigms

Dynamic-Context (Server-Side)

• Context passed per evaluation call
• Stateless evaluation
• Multi-tenant support

Static-Context (Client-Side)

• Context set once, cached
• Requires reconciliation on context change
• Single-user/device focus

Type Definitions

Flag Value Types

// Core value types
type FlagValue = boolean | string | number | Structure;

// Structure: Language-idiomatic structured data (object, dict, map)
interface Structure {
  [key: string]: boolean | string | number | DateTime | Structure;
}

Resolution Details

interface ResolutionDetails<T extends FlagValue> {
  value: T;                    // Required: resolved flag value
  variant?: string;            // Optional: string identifier for value
  reason?: ResolutionReason;   // Optional: semantic cause
  errorCode?: ErrorCode;       // Optional: null/falsy on success
  errorMessage?: string;       // Optional: additional error detail
  flagMetadata?: FlagMetadata; // Optional: arbitrary metadata
}

type ResolutionReason =
  | 'STATIC'           // Statically configured value
  | 'DEFAULT'          // Default value returned
  | 'TARGETING_MATCH'  // Targeting rule matched
  | 'SPLIT'            // Percentage/split allocation
  | 'CACHED'           // Cached value returned
  | 'DISABLED'         // Flag disabled
  | 'UNKNOWN'          // Reason unknown
  | 'STALE'            // Stale cached value
  | 'ERROR'            // Error occurred
  | string;            // Custom reasons allowed

Error Codes

enum ErrorCode {
  PROVIDER_NOT_READY = 'PROVIDER_NOT_READY',
  FLAG_NOT_FOUND = 'FLAG_NOT_FOUND',
  PARSE_ERROR = 'PARSE_ERROR',
  TYPE_MISMATCH = 'TYPE_MISMATCH',
  TARGETING_KEY_MISSING = 'TARGETING_KEY_MISSING',
  INVALID_CONTEXT = 'INVALID_CONTEXT',
  PROVIDER_FATAL = 'PROVIDER_FATAL',
  GENERAL = 'GENERAL'
}

Provider Status

enum ProviderStatus {
  NOT_READY = 'NOT_READY',     // Initial state, not yet initialized
  READY = 'READY',             // Initialization complete
  ERROR = 'ERROR',             // Temporary error, may recover
  STALE = 'STALE',             // Using cached/stale data
  FATAL = 'FATAL',             // Unrecoverable error
  RECONCILING = 'RECONCILING'  // Static-context only: context change in progress
}

API Implementation

Global Singleton (Requirement 1.1.1)

The API SHOULD exist as a global singleton, even with multiple API versions present.

// TypeScript example structure
class OpenFeatureAPI {
  private static instance: OpenFeatureAPI;
  private providers: Map<string, FeatureProvider> = new Map();
  private hooks: Hook[] = [];
  private globalContext: EvaluationContext = {};

  static getInstance(): OpenFeatureAPI {
    if (!OpenFeatureAPI.instance) {
      OpenFeatureAPI.instance = new OpenFeatureAPI();
    }
    return OpenFeatureAPI.instance;
  }
}

Provider Management

interface OpenFeatureAPI {
  // Requirement 1.1.2: Set default provider
  setProvider(provider: FeatureProvider): void;

  // Requirement 1.1.2: Set provider and await initialization
  setProviderAndWait(provider: FeatureProvider): Promise<void>;

  // Requirement 1.1.3: Bind provider to domain
  setProvider(domain: string, provider: FeatureProvider): void;

  // Requirement 1.1.5: Get provider metadata
  getProviderMetadata(domain?: string): ProviderMetadata;

  // Requirement 1.6.1: Shutdown all providers
  shutdown(): Promise<void>;
}

Provider Lifecycle Rules:

• MUST invoke initialize on newly registered provider before resolution
• MUST call shutdown on previously registered provider when replaced
• Requirement 1.6.2: Shutdown MUST reset all API state

Hook Management

interface OpenFeatureAPI {
  // Requirement 1.1.4: Add hooks (append to collection)
  addHooks(...hooks: Hook[]): void;

  // Clear hooks
  clearHooks(): void;
}

Client Creation

interface OpenFeatureAPI {
  // Requirement 1.1.6: Create client with optional domain
  getClient(domain?: string): Client;
}

Requirement 1.1.7: Client creation MUST NOT throw or abnormally terminate.

Client Implementation

Client Interface

interface Client {
  // Requirement 1.2.2: Immutable domain field
  readonly domain?: string;

  // Client-level hooks
  addHooks(...hooks: Hook[]): void;

  // Client-level context (dynamic paradigm)
  setContext(context: EvaluationContext): void;
}

Flag Evaluation Methods

Dynamic-Context Paradigm (Requirement 1.3.1.1)

interface Client {
  getBooleanValue(
    flagKey: string,
    defaultValue: boolean,
    context?: EvaluationContext,
    options?: EvaluationOptions
  ): boolean;

  getStringValue(
    flagKey: string,
    defaultValue: string,
    context?: EvaluationContext,
    options?: EvaluationOptions
  ): string;

  getNumberValue(
    flagKey: string,
    defaultValue: number,
    context?: EvaluationContext,
    options?: EvaluationOptions
  ): number;

  getObjectValue<T extends Structure>(
    flagKey: string,
    defaultValue: T,
    context?: EvaluationContext,
    options?: EvaluationOptions
  ): T;
}

Static-Context Paradigm (Requirement 1.3.2.1)

Context parameter omitted; set at API/client level.

interface StaticContextClient {
  getBooleanValue(
    flagKey: string,
    defaultValue: boolean,
    options?: EvaluationOptions
  ): boolean;
  // ... same pattern for other types
}

Requirement 1.3.3.1: Languages with separate integer/float types SHOULD provide distinct methods.

Detailed Evaluation Methods

Return EvaluationDetails with supplementary metadata.

interface EvaluationDetails<T extends FlagValue> extends ResolutionDetails<T> {
  flagKey: string;  // Requirement 1.4.5: Must contain passed flag key
}

interface Client {
  getBooleanDetails(
    flagKey: string,
    defaultValue: boolean,
    context?: EvaluationContext,
    options?: EvaluationOptions
  ): EvaluationDetails<boolean>;

  // ... same pattern for string, number, object
}

Type Safety (Requirement 1.3.4)

If provider returns wrong type, return default value:

function evaluateFlag<T>(flagKey: string, defaultValue: T): T {
  const result = provider.resolve(flagKey, defaultValue);

  // Type mismatch → return default
  if (typeof result.value !== typeof defaultValue) {
    return defaultValue;
  }

  return result.value as T;
}

Error Handling (Requirement 1.4.10)

CRITICAL: Methods MUST NOT throw exceptions. Always return default value on error.

function getBooleanValue(flagKey: string, defaultValue: boolean): boolean {
  try {
    const details = provider.resolveBooleanValue(flagKey, defaultValue, context);
    return details.value;
  } catch (error) {
    // Log via hooks, not direct logging (Requirement 1.4.11)
    return defaultValue;
  }
}

Evaluation Options

interface EvaluationOptions {
  // Requirement 1.5.1: Additional hooks for this evaluation
  hooks?: Hook[];

  // Hook hints passed to all hooks
  hookHints?: HookHints;
}

Provider Implementation

Provider Interface

interface FeatureProvider {
  // Requirement 2.1.1: Metadata with name field
  readonly metadata: ProviderMetadata;

  // Requirement 2.4.1: Optional initialization
  initialize?(context: EvaluationContext): Promise<void>;

  // Requirement 2.5.1: Optional shutdown
  shutdown?(): Promise<void>;

  // Requirement 2.6.1: Optional context change handler (static paradigm)
  onContextChange?(
    oldContext: EvaluationContext,
    newContext: EvaluationContext
  ): Promise<void>;

  // Requirement 2.3.1: Optional provider hooks
  hooks?: Hook[];

  // Requirement 2.2.1: Resolution methods
  resolveBooleanValue(
    flagKey: string,
    defaultValue: boolean,
    context: EvaluationContext
  ): ResolutionDetails<boolean>;

  resolveStringValue(
    flagKey: string,
    defaultValue: string,
    context: EvaluationContext
  ): ResolutionDetails<string>;

  resolveNumberValue(
    flagKey: string,
    defaultValue: number,
    context: EvaluationContext
  ): ResolutionDetails<number>;

  resolveObjectValue<T extends Structure>(
    flagKey: string,
    defaultValue: T,
    context: EvaluationContext
  ): ResolutionDetails<T>;
}

interface ProviderMetadata {
  name: string;
}

Provider Lifecycle States

                    ┌──────────────────┐
                    │    NOT_READY     │
                    │  (Initial State) │
                    └────────┬─────────┘
                             │ initialize()
                    ┌────────┴─────────┐
          success   │                  │ failure
        ┌───────────┤                  ├───────────┐
        ▼           │                  │           ▼
┌───────────┐       │                  │    ┌───────────┐
│   READY   │◄──────┘                  └────│   ERROR   │
└─────┬─────┘                               └─────┬─────┘
      │                                           │
      │ config change / recovery                  │ recovery
      │ ┌─────────┐                               │
      ├─┤  STALE  │◄──────────────────────────────┘
      │ └─────────┘
      │
      │ shutdown()
      ▼
┌───────────┐
│ NOT_READY │
└───────────┘

┌───────────┐
│   FATAL   │  (Unrecoverable - no transitions out)
└───────────┘

Resolution Details Requirements

function resolveValue<T>(flagKey: string, defaultValue: T): ResolutionDetails<T> {
  // Requirement 2.2.3: Value field MUST contain resolved value
  // Requirement 2.2.4: Variant SHOULD be set if available
  // Requirement 2.2.5: Reason SHOULD indicate semantic cause
  // Requirement 2.2.6: Error code MUST be null/falsy on success
  // Requirement 2.2.9: Flag metadata SHOULD be populated

  return {
    value: resolvedValue,
    variant: 'variant-a',
    reason: 'TARGETING_MATCH',
    errorCode: undefined,
    flagMetadata: { version: '1.0.0' }
  };
}

Error Handling in Providers

function resolveValue(flagKey: string): ResolutionDetails {
  try {
    // Normal resolution
    return { value: resolved, reason: 'TARGETING_MATCH' };
  } catch (error) {
    // Requirement 2.2.7: Indicate errors with error codes
    // Requirement 2.3.3: May include error message
    return {
      value: defaultValue,
      reason: 'ERROR',
      errorCode: 'GENERAL',
      errorMessage: error.message
    };
  }
}

Provider Shutdown (Requirement 2.5.2-2.5.3)

class MyProvider implements FeatureProvider {
  private initialized = false;

  async shutdown(): Promise<void> {
    if (!this.initialized) return; // Idempotent

    // Clean up resources
    await this.connection?.close();
    this.initialized = false;

    // Requirement 2.5.2: Revert to uninitialized state
  }
}

Evaluation Context

Context Structure

interface EvaluationContext {
  // Requirement 3.1.1: Optional targeting key
  targetingKey?: string;

  // Requirement 3.1.2: Custom fields
  [key: string]: boolean | string | number | DateTime | Structure | undefined;
}

Context Merging (Requirement 3.2.3)

Precedence order (highest wins):

1. Before hooks (highest)
2. Invocation context
3. Client context
4. Transaction context
5. API (global) context (lowest)

function mergeContext(...contexts: EvaluationContext[]): EvaluationContext {
  // Later contexts override earlier ones
  return contexts.reduce((merged, ctx) => ({
    ...merged,
    ...ctx
  }), {});
}

// Usage in evaluation
const finalContext = mergeContext(
  api.getContext(),           // Lowest precedence
  transactionContext,
  client.getContext(),
  invocationContext,
  beforeHookContext           // Highest precedence
);

Hooks Implementation

Hook Interface

interface Hook {
  // At least one stage required
  before?(context: HookContext, hints: HookHints): EvaluationContext | void;
  after?(context: HookContext, details: EvaluationDetails, hints: HookHints): void;
  error?(context: HookContext, error: Error, hints: HookHints): void;
  finally?(context: HookContext, hints: HookHints): void;
}

interface HookContext {
  readonly flagKey: string;
  readonly flagValueType: FlagValueType;
  readonly defaultValue: FlagValue;
  readonly evaluationContext: EvaluationContext;
  readonly clientMetadata: ClientMetadata;
  readonly providerMetadata: ProviderMetadata;

  // Mutable: allows inter-stage communication
  hookData: Record<string, unknown>;
}

interface HookHints {
  readonly [key: string]: boolean | string | number | DateTime | Structure;
}

Hook Execution Order

API Hooks ──┬── before() ────────────────────────────────────┐
            │                                                 │
Client Hooks┼── before() ───────────────────────────────────┐│
            │                                                ││
Invocation ─┼── before() ──────────────────────────────────┐││
Hooks       │                                               │││
            │                                               │││
Provider ───┼── before() ─────────────────────────────────┐││││
Hooks       │                                              │││││
            │          ┌─────────────────┐                 │││││
            │          │  FLAG RESOLVE   │                 │││││
            │          └─────────────────┘                 │││││
            │                                              │││││
Provider ───┼── after() ──────────────────────────────────┘││││
Hooks       │                                               ││││
            │                                               ││││
Invocation ─┼── after() ───────────────────────────────────┘│││
Hooks       │                                                │││
            │                                                │││
Client Hooks┼── after() ────────────────────────────────────┘││
            │                                                 ││
API Hooks ──┴── after() ─────────────────────────────────────┘│
                                                               │
            finally() executes in same reverse order ◄─────────┘

Hook Error Handling

async function executeHooks(hooks: Hook[], stage: string): Promise<void> {
  for (const hook of hooks) {
    try {
      await hook[stage]?.();
    } catch (error) {
      // Hooks MUST NOT propagate exceptions
      // Error hooks still execute
      // Finally hooks always execute
      if (stage !== 'error' && stage !== 'finally') {
        await executeErrorHooks(hooks, error);
      }
    }
  }
}

Before Hook Context Modification

// Dynamic-context paradigm only
function beforeHook(context: HookContext): EvaluationContext {
  // Can return modified context (merged with highest precedence)
  return {
    ...context.evaluationContext,
    additionalKey: 'added-by-hook'
  };
}

Events Implementation

Event Types

enum ProviderEvent {
  PROVIDER_READY = 'PROVIDER_READY',
  PROVIDER_ERROR = 'PROVIDER_ERROR',
  PROVIDER_FATAL = 'PROVIDER_FATAL',
  PROVIDER_STALE = 'PROVIDER_STALE',
  PROVIDER_CONFIGURATION_CHANGED = 'PROVIDER_CONFIGURATION_CHANGED',
  PROVIDER_RECONCILING = 'PROVIDER_RECONCILING',        // Static-context only
  PROVIDER_CONTEXT_CHANGED = 'PROVIDER_CONTEXT_CHANGED' // Static-context only
}

interface EventDetails {
  providerName: string;
  errorCode?: ErrorCode;
  errorMessage?: string;
  flagsChanged?: string[];
  metadata?: Record<string, unknown>;
}

type EventHandler = (details: EventDetails) => void;

Event Handler Registration

interface EventEmitter {
  // Requirement 5.2.1: API-level handlers
  addHandler(event: ProviderEvent, handler: EventHandler): void;

  // Requirement 5.2.2: Client-level handlers
  addHandler(event: ProviderEvent, handler: EventHandler): void;

  // Requirement 5.2.7: Handler removal
  removeHandler(event: ProviderEvent, handler: EventHandler): void;
}

Event Emission Rules

class ProviderWrapper {
  private handlers: Map<ProviderEvent, EventHandler[]> = new Map();

  async initialize(): Promise<void> {
    try {
      await this.provider.initialize?.(this.context);
      // Requirement 5.3.1: Emit READY on success
      this.emit(ProviderEvent.PROVIDER_READY);
    } catch (error) {
      // Requirement 5.3.2: Emit ERROR on failure
      this.emit(ProviderEvent.PROVIDER_ERROR, {
        errorCode: 'GENERAL',
        errorMessage: error.message
      });
    }
  }

  private emit(event: ProviderEvent, details?: Partial<EventDetails>): void {
    const handlers = this.handlers.get(event) ?? [];
    for (const handler of handlers) {
      try {
        handler({ providerName: this.provider.metadata.name, ...details });
      } catch {
        // Requirement 5.2.5: Handler errors don't prevent other handlers
      }
    }
  }
}

Deferred Handler Execution (Requirement 5.3.3)

function addHandler(event: ProviderEvent, handler: EventHandler): void {
  this.handlers.get(event)?.push(handler);

  // If provider already in target state, execute immediately
  if (event === ProviderEvent.PROVIDER_READY && this.status === 'READY') {
    handler({ providerName: this.provider.metadata.name });
  }
}

Provider Status Management

Status Tracking (Requirement 1.7.1)

class ProviderManager {
  private status: ProviderStatus = ProviderStatus.NOT_READY;

  getStatus(): ProviderStatus {
    return this.status;
  }

  async initialize(): Promise<void> {
    try {
      await this.provider.initialize?.(this.context);
      // Requirement 1.7.3: READY on normal initialization
      this.status = ProviderStatus.READY;
    } catch (error) {
      if (error.code === 'PROVIDER_FATAL') {
        // Requirement 1.7.5: FATAL on fatal error
        this.status = ProviderStatus.FATAL;
      } else {
        // Requirement 1.7.4: ERROR on abnormal termination
        this.status = ProviderStatus.ERROR;
      }
    }
  }
}

Evaluation During Non-Ready States

function evaluate(flagKey: string, defaultValue: boolean): EvaluationDetails<boolean> {
  // Requirement 1.7.6: Return error code if NOT_READY
  if (this.status === ProviderStatus.NOT_READY) {
    return {
      value: defaultValue,
      flagKey,
      reason: 'ERROR',
      errorCode: ErrorCode.PROVIDER_NOT_READY
    };
  }

  // Requirement 1.7.7: Return error code if FATAL
  if (this.status === ProviderStatus.FATAL) {
    return {
      value: defaultValue,
      flagKey,
      reason: 'ERROR',
      errorCode: ErrorCode.PROVIDER_FATAL
    };
  }

  // Normal evaluation
  return this.provider.resolveBooleanValue(flagKey, defaultValue, this.context);
}

Testing Requirements

Gherkin Test Suites

The specification includes Gherkin test suites for compliance testing:

Feature: Flag Evaluation

Scenario: Boolean flag evaluation returns expected value
  Given a provider is registered
  When a boolean flag "my-flag" is evaluated with default "false"
  Then the returned value should be "true"
  And the variant should be "on"
  And the reason should be "TARGETING_MATCH"

Scenario: Evaluation returns default on provider error
  Given a provider is registered that throws errors
  When a boolean flag "error-flag" is evaluated with default "false"
  Then the returned value should be "false"
  And the error code should be "GENERAL"

Compliance Testing Structure

// Test categories to implement
describe('Flag Evaluation API', () => {
  describe('Singleton behavior', () => {
    it('should return same instance', () => {
      expect(OpenFeature.getInstance()).toBe(OpenFeature.getInstance());
    });
  });

  describe('Provider management', () => {
    it('should call initialize on provider registration');
    it('should call shutdown on previous provider when replaced');
  });

  describe('Flag evaluation', () => {
    it('should return correct type for boolean flags');
    it('should return default value on type mismatch');
    it('should never throw exceptions');
  });

  describe('Hooks', () => {
    it('should execute in correct order');
    it('should not propagate hook errors');
  });

  describe('Events', () => {
    it('should emit PROVIDER_READY on successful initialization');
    it('should execute deferred handlers immediately');
  });
});

SDK Implementation Checklist

Phase 1: Core Types

• Define FlagValue union type
• Define ResolutionDetails structure
• Define EvaluationDetails structure
• Implement ErrorCode enum
• Implement ProviderStatus enum
• Implement ResolutionReason type
• Define EvaluationContext interface
• Define FlagMetadata type

Phase 2: Provider Interface

• Define FeatureProvider interface
• Define ProviderMetadata interface
• Implement NoOpProvider (for testing/defaults)
• Support initialize lifecycle method
• Support shutdown lifecycle method
• Support onContextChange (static paradigm)

Phase 3: API Implementation

• Implement global singleton
• Implement provider registration
• Implement domain-scoped providers
• Implement hook registration
• Implement context management
• Implement shutdown

Phase 4: Client Implementation

• Implement client creation (non-throwing)
• Implement value methods (boolean, string, number, object)
• Implement details methods
• Implement client-level hooks
• Implement client-level context
• Ensure type safety

Phase 5: Hooks

• Define Hook interface
• Define HookContext interface
• Define HookHints interface
• Implement hook execution order
• Implement error isolation
• Implement finally stage guarantee
• Support context modification (dynamic)

Phase 6: Events

• Define ProviderEvent enum
• Define EventDetails interface
• Implement event emitter
• Implement handler registration
• Implement deferred execution
• Ensure handler error isolation

Phase 7: Testing

• Unit tests for all components
• Integration tests with providers
• Gherkin compliance tests
• Edge case coverage
• Concurrency testing (if applicable)

Server SDK Implementation

Server SDKs use the dynamic-context paradigm where evaluation context is passed per request. This is the most common pattern for backend services.

Key Characteristics

Architecture

┌─────────────────────────────────────────────────────────────┐
│                    Server Application                        │
├─────────────────────────────────────────────────────────────┤
│  Request 1          Request 2          Request 3            │
│  ┌─────────┐        ┌─────────┐        ┌─────────┐          │
│  │Context A│        │Context B│        │Context C│          │
│  └────┬────┘        └────┬────┘        └────┬────┘          │
│       │                  │                  │                │
│       └──────────────────┼──────────────────┘                │
│                          ▼                                   │
│                 ┌─────────────────┐                          │
│                 │ OpenFeature API │ (Singleton)              │
│                 └────────┬────────┘                          │
│                          ▼                                   │
│                 ┌─────────────────┐                          │
│                 │    Provider     │ (Shared, thread-safe)    │
│                 └─────────────────┘                          │
└─────────────────────────────────────────────────────────────┘

Server SDK Requirements

// Dynamic-context evaluation signature
interface ServerClient {
  getBooleanValue(
    flagKey: string,
    defaultValue: boolean,
    context: EvaluationContext,  // Required per-call
    options?: EvaluationOptions
  ): boolean;
}

// Context passed with each request
app.get('/api/feature', (req, res) => {
  const context: EvaluationContext = {
    targetingKey: req.user.id,
    email: req.user.email,
    plan: req.user.subscription,
    // Request-specific context
  };

  const enabled = client.getBooleanValue('new-feature', false, context);
  res.json({ enabled });
});

Server-Specific Considerations

Thread Safety

// Provider must be thread-safe
class ThreadSafeProvider implements FeatureProvider {
  private cache: Map<string, FlagValue> = new Map();
  private mutex = new Mutex();

  async resolveBooleanValue(
    flagKey: string,
    defaultValue: boolean,
    context: EvaluationContext
  ): Promise<ResolutionDetails<boolean>> {
    // Use mutex for cache access in multi-threaded environments
    return this.mutex.runExclusive(async () => {
      // Resolution logic
    });
  }
}

Connection Pooling

// Go - Reuse HTTP connections
type Provider struct {
    client *http.Client
}

func NewProvider() *Provider {
    return &Provider{
        client: &http.Client{
            Transport: &http.Transport{
                MaxIdleConns:        100,
                MaxIdleConnsPerHost: 100,
                IdleConnTimeout:     90 * time.Second,
            },
        },
    }
}

Request-Scoped Context Propagation

# Python - Using contextvars for request context
from contextvars import ContextVar

request_context: ContextVar[EvaluationContext] = ContextVar('request_context')

@app.middleware("http")
async def add_context(request: Request, call_next):
    context = EvaluationContext(
        targeting_key=request.user.id,
        attributes={"path": request.url.path}
    )
    token = request_context.set(context)
    try:
        response = await call_next(request)
    finally:
        request_context.reset(token)
    return response

Server SDK Checklist

• Thread-safe provider implementation
• Connection pooling for external calls
• Request context propagation
• Graceful shutdown with in-flight request handling
• Metrics/telemetry per evaluation
• Bulk evaluation support (optional)
• Caching strategy for high-throughput

Client SDK Implementation

Client SDKs use the static-context paradigm where context is set once and cached. This pattern is used for mobile apps, browser applications, and edge devices.

Key Characteristics

Architecture

┌─────────────────────────────────────────────────────────────┐
│                    Client Application                        │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│  ┌──────────────────────────────────────────────────────┐   │
│  │                  EvaluationContext                    │   │
│  │  (Set once: user ID, device, app version, etc.)      │   │
│  └──────────────────────┬───────────────────────────────┘   │
│                         │                                    │
│                         ▼                                    │
│  ┌──────────────────────────────────────────────────────┐   │
│  │               OpenFeature API                         │   │
│  │  ┌────────────────────────────────────────────────┐  │   │
│  │  │              Flag Cache                         │  │   │
│  │  │  (Evaluated flags stored locally)              │  │   │
│  │  └────────────────────────────────────────────────┘  │   │
│  └──────────────────────┬───────────────────────────────┘   │
│                         │                                    │
│                         ▼                                    │
│  ┌──────────────────────────────────────────────────────┐   │
│  │                    Provider                           │   │
│  │  (Syncs with server, manages local cache)            │   │
│  └──────────────────────────────────────────────────────┘   │
│                                                              │
└─────────────────────────────────────────────────────────────┘

Client SDK Requirements

// Static-context evaluation signature (no context parameter)
interface ClientSdkClient {
  getBooleanValue(
    flagKey: string,
    defaultValue: boolean,
    options?: EvaluationOptions
  ): boolean;  // Synchronous - uses cached values
}

// Context set at initialization or on user change
OpenFeature.setContext({
  targetingKey: user.id,
  email: user.email,
  deviceType: 'mobile',
  appVersion: '2.1.0',
});

// Evaluations use cached context
const enabled = client.getBooleanValue('new-feature', false);

Client-Specific Events

// Additional events for client SDKs
enum ClientProviderEvent {
  // Standard events
  PROVIDER_READY = 'PROVIDER_READY',
  PROVIDER_ERROR = 'PROVIDER_ERROR',
  PROVIDER_STALE = 'PROVIDER_STALE',

  // Client-specific events
  PROVIDER_RECONCILING = 'PROVIDER_RECONCILING',      // Context change in progress
  PROVIDER_CONTEXT_CHANGED = 'PROVIDER_CONTEXT_CHANGED', // Context change complete
}

// Listen for context reconciliation
client.addHandler(ProviderEvent.PROVIDER_CONTEXT_CHANGED, () => {
  // Re-render UI with new flag values
  refreshUI();
});

Context Reconciliation

When context changes, client SDKs must reconcile cached values:

interface ClientProvider extends FeatureProvider {
  // Called when context changes
  onContextChange(
    oldContext: EvaluationContext,
    newContext: EvaluationContext
  ): Promise<void>;
}

class MyClientProvider implements ClientProvider {
  private cache: Map<string, ResolutionDetails<FlagValue>> = new Map();

  async onContextChange(
    oldContext: EvaluationContext,
    newContext: EvaluationContext
  ): Promise<void> {
    // Emit RECONCILING event
    this.emit(ProviderEvent.PROVIDER_RECONCILING);

    try {
      // Fetch new flag values for new context
      const newFlags = await this.fetchFlags(newContext);
      this.cache = new Map(newFlags);

      // Emit CONTEXT_CHANGED event
      this.emit(ProviderEvent.PROVIDER_CONTEXT_CHANGED);
    } catch (error) {
      this.emit(ProviderEvent.PROVIDER_ERROR, { error });
    }
  }
}

Platform-Specific Patterns

Mobile (iOS/Android)

// Swift - iOS lifecycle handling
class OpenFeatureManager {
    func applicationDidBecomeActive() {
        // Refresh flags when app becomes active
        provider.refresh()
    }

    func applicationDidEnterBackground() {
        // Persist cache before backgrounding
        provider.persistCache()
    }
}

// Kotlin - Android lifecycle
class FeatureFlagViewModel : ViewModel() {
    private val client = OpenFeature.getClient()

    init {
        // Observe provider events
        OpenFeature.addHandler(ProviderEvent.PROVIDER_READY) {
            refreshFlags()
        }
    }

    override fun onCleared() {
        // Cleanup when ViewModel is destroyed
        OpenFeature.shutdown()
    }
}

Browser/Web

// Handle page visibility changes
document.addEventListener('visibilitychange', () => {
  if (document.visibilityState === 'visible') {
    // Refresh flags when tab becomes visible
    provider.refresh();
  }
});

// Handle online/offline
window.addEventListener('online', () => {
  provider.reconnect();
});

window.addEventListener('offline', () => {
  // Use cached values, emit STALE if needed
  provider.setStatus(ProviderStatus.STALE);
});

Edge/Embedded

// Rust - Resource-constrained environments
pub struct EdgeProvider {
    cache: HashMap<String, FlagValue>,
    max_cache_size: usize,
    storage: Box<dyn PersistentStorage>,
}

impl EdgeProvider {
    pub fn new(max_cache_size: usize) -> Self {
        Self {
            cache: HashMap::with_capacity(max_cache_size),
            max_cache_size,
            storage: Box::new(FileStorage::new("/data/flags")),
        }
    }

    // Load from persistent storage on init
    pub async fn initialize(&mut self) -> Result<(), Error> {
        self.cache = self.storage.load().await?;
        Ok(())
    }
}

Offline Support

interface OfflineCapableProvider extends ClientProvider {
  // Check if operating offline
  isOffline(): boolean;

  // Get cached value (works offline)
  getCachedValue<T>(flagKey: string): T | undefined;

  // Queue context changes for when online
  queueContextChange(context: EvaluationContext): void;
}

class OfflineProvider implements OfflineCapableProvider {
  private pendingContext: EvaluationContext | null = null;

  async onContextChange(old: EvaluationContext, new_: EvaluationContext) {
    if (this.isOffline()) {
      this.pendingContext = new_;
      return; // Will reconcile when back online
    }
    await this.reconcile(new_);
  }

  async onOnline() {
    if (this.pendingContext) {
      await this.reconcile(this.pendingContext);
      this.pendingContext = null;
    }
  }
}

Client SDK Checklist

• Static-context evaluation methods (no context param)
• Context reconciliation (onContextChange)
• RECONCILING and CONTEXT_CHANGED events
• Local flag cache
• Persistent storage for offline support
• Application lifecycle handling
• Network state awareness (online/offline)
• Background refresh strategies
• Memory-efficient caching (size limits)
• Synchronous evaluation from cache

Language-Specific Considerations

Static vs Dynamic Typing

Statically Typed (Go, Rust, Java, TypeScript)

• Use generics for type-safe evaluation
• Compile-time type checking preferred
• Requirement 2.2.8.1: Support generic resolution details

Dynamically Typed (Python, Ruby, JavaScript)

• Runtime type checking in evaluation
• Duck typing for providers
• Clear documentation of expected types

Concurrency Models

Single-threaded (JavaScript/Browser)

• Async/await for provider operations
• Event loop considerations
• Requirement 1.4.12: Provide async mechanisms

Multi-threaded (Go, Rust, Java)

• Thread-safe singleton implementation
• Concurrent provider access
• Lock-free where possible

Error Handling Idioms

Exceptions (Java, Python, C#)

• Catch and suppress in evaluation
• Return defaults on any exception

Result Types (Rust, Go)

• Map errors to default values
• Error details in resolution structure

Memory Management

Garbage Collected (Java, Go, Python)

• Standard cleanup in shutdown

Manual/RAII (Rust, C++)

• Implement Drop/destructor
• Consider Rc/Arc for shared providers

References

• OpenFeature Specification
• Flag Evaluation API
• Provider Interface
• Hooks
• Evaluation Context
• Events
• Types and Data Structures
• Gherkin Test Suites
• Official SDK Repositories