fix: Multiple "concurrent" calls to init causing multiple fetches. (#145)

* fix: in-memory lock to only init once at a time per SDK key
* bump version to v3.9.3-alpha.0

Author: typotter

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@eppo/js-client-sdk",
-  "version": "3.9.2-alpha.0",
+  "version": "3.9.3-alpha.0",
   "description": "Eppo SDK for client-side JavaScript applications",
   "main": "dist/index.js",
   "files": [

src/index.spec.ts

@@ -503,6 +503,104 @@ describe('initialization options', () => {
     expect(callCount).toBe(2);
   });
 
+  it('only fetches/does initialization workload once if init is called multiple times concurrently', async () => {
+    let callCount = 0;
+
+    global.fetch = jest.fn(() => {
+      ++callCount;
+      return Promise.resolve({
+        ok: true,
+        status: 200,
+        json: () => Promise.resolve(mockConfigResponse),
+      });
+    }) as jest.Mock;
+
+    const inits: Promise<EppoClient>[] = [];
+    [...Array(10).keys()].forEach(() => {
+      inits.push(
+        init({
+          apiKey,
+          baseUrl,
+          assignmentLogger: mockLogger,
+        }),
+      );
+    });
+
+    // Advance timers mid-init to allow retrying
+    await jest.advanceTimersByTimeAsync(maxRetryDelay);
+
+    // Await for all the initialization calls to resolve
+    const client = await Promise.race(inits);
+    await Promise.all(inits);
+
+    expect(callCount).toBe(1);
+    expect(client.getStringAssignment(flagKey, 'subject', {}, 'default-value')).toBe('control');
+  });
+
+  it('only fetches/does initialization workload once per API key if init is called multiple times concurrently', async () => {
+    let callCount = 0;
+
+    global.fetch = jest.fn(() => {
+      ++callCount;
+      return Promise.resolve({
+        ok: true,
+        status: 200,
+        json: () => Promise.resolve(mockConfigResponse),
+      });
+    }) as jest.Mock;
+
+    const inits: Promise<EppoClient>[] = [];
+    [
+      'KEY_1',
+      'KEY_2',
+      'KEY_1',
+      'KEY_2',
+      'KEY_1',
+      'KEY_2',
+      'KEY_3',
+      'KEY_1',
+      'KEY_2',
+      'KEY_3',
+    ].forEach((varyingAPIKey) => {
+      inits.push(
+        init({
+          apiKey: varyingAPIKey,
+          baseUrl,
+          forceReinitialize: true,
+          assignmentLogger: mockLogger,
+        }),
+      );
+    });
+
+    // Advance timers mid-init to allow retrying
+    await jest.advanceTimersByTimeAsync(maxRetryDelay);
+
+    // Await for all the initialization calls to resolve
+    const client = await Promise.race(inits);
+    await Promise.all(inits);
+
+    expect(callCount).toBe(3);
+    callCount = 0;
+    expect(client.getStringAssignment(flagKey, 'subject', {}, 'default-value')).toBe('control');
+
+    const reInits: Promise<EppoClient>[] = [];
+    ['KEY_1', 'KEY_2', 'KEY_3', 'KEY_4'].forEach((varyingAPIKey) => {
+      reInits.push(
+        init({
+          apiKey: varyingAPIKey,
+          forceReinitialize: true,
+          baseUrl,
+          assignmentLogger: mockLogger,
+        }),
+      );
+    });
+
+    await Promise.all(reInits);
+
+    expect(callCount).toBe(4);
+    expect(client.getStringAssignment(flagKey, 'subject', {}, 'default-value')).toBe('control');
+  });
+
   it('do not reinitialize if already initialized', async () => {
     let callCount = 0;
 

src/index.ts

@@ -315,6 +315,13 @@ export function offlineInit(config: IClientConfigSync): EppoClient {
   return EppoJSClient.instance;
 }
 
+type SDKKey = string;
+
+/**
+ * Tracks initialization by API key. After an initialization completes, the value is removed from the map.
+ */
+const initializationPromises: Map<SDKKey, Promise<EppoClient>> = new Map();
+
 /**
  * Initializes the Eppo client with configuration parameters.
  * This method should be called once on application startup.
@@ -323,6 +330,24 @@ export function offlineInit(config: IClientConfigSync): EppoClient {
  */
 export async function init(config: IClientConfig): Promise<EppoClient> {
   validation.validateNotBlank(config.apiKey, 'API key required');
+
+  // If there is already an init in progress for this apiKey, return that.
+  let initPromise = initializationPromises.get(config.apiKey);
+  if (initPromise) {
+    return initPromise;
+  }
+
+  initPromise = explicitInit(config);
+
+  initializationPromises.set(config.apiKey, initPromise);
+
+  const client = await initPromise;
+  initializationPromises.delete(config.apiKey);
+  return client;
+}
+
+async function explicitInit(config: IClientConfig): Promise<EppoClient> {
+  validation.validateNotBlank(config.apiKey, 'API key required');
   let initializationError: Error | undefined;
   const instance = EppoJSClient.instance;
   const {