feat(server): add in memory provider

packages/server/README.md

@@ -186,17 +186,28 @@ A name is a logical identifier which can be used to associate clients with a par
 If a name has no associated provider, the global provider is used.
 
 ```ts
-import { OpenFeature } from "@openfeature/js-sdk";
+import { OpenFeature, InMemoryProvider } from "@openfeature/js-sdk";
+
+const myFlags = {
+  'v2_enabled': {
+    variants: {
+      on: true,
+      off: false
+    },
+    disabled: false,
+    defaultVariant: "on"
+  }
+};
 
 // Registering the default provider
-OpenFeature.setProvider(NewLocalProvider());
+OpenFeature.setProvider(InMemoryProvider(myFlags));
 // Registering a named provider
-OpenFeature.setProvider("clientForCache", new NewCachedProvider());
+OpenFeature.setProvider("otherClient", new InMemoryProvider(someOtherFlags));
 
 // A Client backed by default provider
 const clientWithDefault = OpenFeature.getClient();
 // A Client backed by NewCachedProvider
-const clientForCache = OpenFeature.getClient("clientForCache");
+const clientForCache = OpenFeature.getClient("otherClient");
 ```
 
 ### Eventing

packages/server/src/provider/in-memory-provider/flag-configuration.ts

@@ -0,0 +1,36 @@
+/**
+ * Don't export types from this file publicly.
+ * It might cause confusion since these types are not a part of the general API,
+ * but just for the in-memory provider.
+ */
+import { EvaluationContext, JsonValue } from '@openfeature/shared';
+
+type Variants<T> = Record<string, T>;
+
+/**
+ * A Feature Flag definition, containing it's specification
+ */
+export type Flag = {
+  /**
+   * An object containing all possible flags mappings (variant -> flag value)
+   */
+  variants: Variants<boolean> | Variants<string> | Variants<number> | Variants<JsonValue>;
+  /**
+   * The variant it will resolve to in STATIC evaluation
+   */
+  defaultVariant: string;
+  /**
+   * Determines if flag evaluation is enabled or not for this flag.
+   * If false, falls back to the default value provided to the client
+   */
+  disabled: boolean;
+  /**
+   * Function used in order to evaluate a flag to a specific value given the provided context.
+   * It should return a variant key.
+   * If it does not return a valid variant it falls back to the default value provided to the client
+   * @param EvaluationContext
+   */
+  contextEvaluator?: (ctx: EvaluationContext) => string;
+};
+
+export type FlagConfiguration = Record<string, Flag>;

packages/server/src/provider/in-memory-provider/in-memory-provider.ts

@@ -0,0 +1,139 @@
+import {
+  EvaluationContext,
+  FlagNotFoundError,
+  FlagValueType,
+  GeneralError,
+  JsonValue,
+  Logger,
+  OpenFeatureError,
+  OpenFeatureEventEmitter,
+  ProviderEvents,
+  ResolutionDetails,
+  StandardResolutionReasons,
+  TypeMismatchError
+} from '@openfeature/shared';
+import { Provider } from '../provider';
+import { Flag, FlagConfiguration } from './flag-configuration';
+import { VariantFoundError } from './variant-not-found-error';
+
+/**
+ * A simple OpenFeature provider intended for demos and as a test stub.
+ */
+export class InMemoryProvider implements Provider {
+  public readonly events = new OpenFeatureEventEmitter();
+  public readonly runsOn = 'server';
+  readonly metadata = {
+    name: 'in-memory',
+  } as const;
+  private _flagConfiguration: FlagConfiguration;
+
+  constructor(flagConfiguration: FlagConfiguration = {}) {
+    this._flagConfiguration = { ...flagConfiguration };
+  }
+
+  /**
+   * Overwrites the configured flags.
+   * @param { FlagConfiguration } flagConfiguration new flag configuration
+   */
+  putConfiguration(flagConfiguration: FlagConfiguration) {
+    const flagsChanged = Object.entries(flagConfiguration)
+      .filter(([key, value]) => this._flagConfiguration[key] !== value)
+      .map(([key]) => key);
+
+    this._flagConfiguration = { ...flagConfiguration };
+    this.events.emit(ProviderEvents.ConfigurationChanged, { flagsChanged });
+  }
+
+  resolveBooleanEvaluation(
+    flagKey: string,
+    defaultValue: boolean,
+    context?: EvaluationContext,
+    logger?: Logger,
+  ): Promise<ResolutionDetails<boolean>> {
+    return this.resolveFlagWithReason<boolean>(flagKey, defaultValue, context, logger);
+  }
+
+  resolveNumberEvaluation(
+    flagKey: string,
+    defaultValue: number,
+    context?: EvaluationContext,
+    logger?: Logger,
+  ): Promise<ResolutionDetails<number>> {
+    return this.resolveFlagWithReason<number>(flagKey, defaultValue, context, logger);
+  }
+
+  async resolveStringEvaluation(
+    flagKey: string,
+    defaultValue: string,
+    context?: EvaluationContext,
+    logger?: Logger,
+  ): Promise<ResolutionDetails<string>> {
+    return this.resolveFlagWithReason<string>(flagKey, defaultValue, context, logger);
+  }
+
+  async resolveObjectEvaluation<T extends JsonValue>(
+    flagKey: string,
+    defaultValue: T,
+    context?: EvaluationContext,
+    logger?: Logger,
+  ): Promise<ResolutionDetails<T>> {
+    return this.resolveFlagWithReason<T>(flagKey, defaultValue, context, logger);
+  }
+
+  private async resolveFlagWithReason<T extends JsonValue | FlagValueType>(
+    flagKey: string,
+    defaultValue: T,
+    ctx?: EvaluationContext,
+    logger?: Logger,
+  ): Promise<ResolutionDetails<T>> {
+    try {
+      const resolutionResult = this.lookupFlagValue(flagKey, defaultValue, ctx, logger);
+
+      if (typeof resolutionResult?.value != typeof defaultValue) {
+        throw new TypeMismatchError();
+      }
+
+      return resolutionResult;
+    } catch (error: unknown) {
+      if (!(error instanceof OpenFeatureError)) {
+        throw new GeneralError((error as Error)?.message || 'unknown error'); 
+      }
+      throw error;
+    }
+  }
+
+  private lookupFlagValue<T extends JsonValue | FlagValueType>(
+    flagKey: string,
+    defaultValue: T,
+    ctx?: EvaluationContext,
+    logger?: Logger,
+  ): ResolutionDetails<T> {
+    if (!(flagKey in this._flagConfiguration)) {
+      const message = `no flag found with key ${flagKey}`;
+      logger?.debug(message);
+      throw new FlagNotFoundError(message);
+    }
+    const flagSpec: Flag = this._flagConfiguration[flagKey];
+
+    if (flagSpec.disabled) {
+      return { value: defaultValue, reason: StandardResolutionReasons.DISABLED };
+    }
+
+    const isContextEval = ctx && flagSpec?.contextEvaluator;
+    const variant = isContextEval ? flagSpec.contextEvaluator?.(ctx) : flagSpec.defaultVariant;
+
+    const value = variant && flagSpec?.variants[variant];
+
+    if (value === undefined) {
+      const message = `no value associated with variant ${variant}`;
+      logger?.error(message);
+      throw new VariantFoundError(message);
+    }
+
+    return {
+      value: value as T,
+      ...(variant && { variant }),
+      reason: isContextEval ? StandardResolutionReasons.TARGETING_MATCH : StandardResolutionReasons.STATIC,
+    };
+  }
+}

packages/server/src/provider/in-memory-provider/index.ts

@@ -0,0 +1 @@
+export * from './in-memory-provider';

packages/server/src/provider/in-memory-provider/variant-not-found-error.ts

@@ -0,0 +1,15 @@
+import { ErrorCode, OpenFeatureError } from '@openfeature/shared';
+
+/**
+ * A custom error for the in-memory provider.
+ * Indicates the resolved or default variant doesn't exist.
+ */
+export class VariantFoundError extends OpenFeatureError {
+  code: ErrorCode;
+  constructor(message?: string) {
+    super(message);
+    Object.setPrototypeOf(this, VariantFoundError.prototype);
+    this.name = 'VariantFoundError';
+    this.code = ErrorCode.GENERAL;
+  }
+}

packages/server/src/provider/index.ts

@@ -1,2 +1,3 @@
 export * from './provider';
 export * from './no-op-provider';
+export * from './in-memory-provider';