Adds LocalStorageAssignmentCache with persistent implementation between sessions to lower assignment logging; enabled by default (FF-839) (#43)

* Adds LocalStorageAssignmentCache with persistent implementation between sessions to lower assignment logging; enabled by default (FF-839)

* use commons 2.0.0

* augment test

Author: leoromanovsky

package.json

@@ -57,7 +57,7 @@
     "xhr-mock": "^2.5.1"
   },
   "dependencies": {
-    "@eppo/js-client-sdk-common": "^2.0.0",
+    "@eppo/js-client-sdk-common": "2.0.0",
     "axios": "^1.6.0",
     "md5": "^2.3.0"
   }

src/index.spec.ts

@@ -15,6 +15,7 @@ import {
 } from '../test/testHelpers';
 
 import { EppoLocalStorage } from './local-storage';
+import { LocalStorageAssignmentCache } from './local-storage-assignment-cache';
 
 import { EppoJSClient, IAssignmentLogger, IEppoClient, init } from './index';
 
@@ -256,6 +257,53 @@ describe('EppoJSClient E2E test', () => {
     });
   });
 
+  describe('LocalStorageAssignmentCache', () => {
+    it('typical behavior', () => {
+      const cache = new LocalStorageAssignmentCache();
+      expect(
+        cache.hasLoggedAssignment({
+          subjectKey: 'subject-1',
+          flagKey: 'flag-1',
+          allocationKey: 'allocation-1',
+          variationValue: EppoValue.String('control'),
+        }),
+      ).toEqual(false);
+
+      cache.setLastLoggedAssignment({
+        subjectKey: 'subject-1',
+        flagKey: 'flag-1',
+        allocationKey: 'allocation-1',
+        variationValue: EppoValue.String('control'),
+      });
+
+      expect(
+        cache.hasLoggedAssignment({
+          subjectKey: 'subject-1',
+          flagKey: 'flag-1',
+          allocationKey: 'allocation-1',
+          variationValue: EppoValue.String('control'),
+        }),
+      ).toEqual(true); // this key has been logged
+
+      // change variation
+      cache.setLastLoggedAssignment({
+        subjectKey: 'subject-1',
+        flagKey: 'flag-1',
+        allocationKey: 'allocation-1',
+        variationValue: EppoValue.String('variant'),
+      });
+
+      expect(
+        cache.hasLoggedAssignment({
+          subjectKey: 'subject-1',
+          flagKey: 'flag-1',
+          allocationKey: 'allocation-1',
+          variationValue: EppoValue.String('control'),
+        }),
+      ).toEqual(false); // this key has not been logged
+    });
+  });
+
   function getAssignmentsWithSubjectAttributes(
     subjectsWithAttributes: {
       subjectKey: string;

src/index.ts

@@ -11,6 +11,7 @@ import {
 import axios from 'axios';
 
 import { EppoLocalStorage } from './local-storage';
+import { LocalStorageAssignmentCache } from './local-storage-assignment-cache';
 import { sdkName, sdkVersion } from './sdk-data';
 
 /**
@@ -150,9 +151,9 @@ export async function init(config: IClientConfig): Promise<IEppoClient> {
     });
     EppoJSClient.instance.setLogger(config.assignmentLogger);
 
-    // default behavior is to use a non-expiring cache.
+    // default behavior is to use a LocalStorage-based assignment cache.
     // this can be overridden after initialization.
-    EppoJSClient.instance.useNonExpiringInMemoryAssignmentCache();
+    EppoJSClient.instance.useCustomAssignmentCache(new LocalStorageAssignmentCache());
 
     const configurationRequestor = new ExperimentConfigurationRequestor(localStorage, httpClient);
     await configurationRequestor.fetchAndStoreConfigurations();

src/local-storage-assignment-cache.ts

@@ -0,0 +1,51 @@
+import { AssignmentCache } from '@eppo/js-client-sdk-common';
+
+import { hasWindowLocalStorage } from './local-storage';
+
+class LocalStorageAssignmentShim {
+  LOCAL_STORAGE_KEY = 'EPPO_LOCAL_STORAGE_ASSIGNMENT_CACHE';
+
+  public has(key: string): boolean {
+    if (!hasWindowLocalStorage()) {
+      return false;
+    }
+
+    return this.getCache().has(key);
+  }
+
+  public get(key: string): string {
+    if (!hasWindowLocalStorage()) {
+      return null;
+    }
+
+    return this.getCache().get(key);
+  }
+
+  public set(key: string, value: string) {
+    if (!hasWindowLocalStorage()) {
+      return;
+    }
+
+    const cache = this.getCache();
+    cache.set(key, value);
+    this.setCache(cache);
+  }
+
+  private getCache(): Map<string, string> {
+    const cache = window.localStorage.getItem(this.LOCAL_STORAGE_KEY);
+    return cache ? new Map(JSON.parse(cache)) : new Map();
+  }
+
+  private setCache(cache: Map<string, string>) {
+    window.localStorage.setItem(
+      this.LOCAL_STORAGE_KEY,
+      JSON.stringify(Array.from(cache.entries())),
+    );
+  }
+}
+
+export class LocalStorageAssignmentCache extends AssignmentCache<LocalStorageAssignmentShim> {
+  constructor() {
+    super(new LocalStorageAssignmentShim());
+  }
+}

src/local-storage.ts

@@ -1,6 +1,6 @@
 export class EppoLocalStorage {
   public get<T>(key: string): T {
-    if (this.hasWindowLocalStorage()) {
+    if (hasWindowLocalStorage()) {
       const serializedEntry = window.localStorage.getItem(key);
       if (serializedEntry) {
         return JSON.parse(serializedEntry);
@@ -9,21 +9,21 @@ export class EppoLocalStorage {
     return null;
   }
 
-  // Checks whether local storage is enabled in the browser (the user might have disabled it).
-  private hasWindowLocalStorage(): boolean {
-    try {
-      return typeof window !== 'undefined' && !!window.localStorage;
-    } catch {
-      // Chrome throws an error if local storage is disabled and you try to access it
-      return false;
-    }
-  }
-
   public setEntries<T>(entries: Record<string, T>) {
-    if (this.hasWindowLocalStorage()) {
+    if (hasWindowLocalStorage()) {
       Object.entries(entries).forEach(([key, val]) => {
         window.localStorage.setItem(key, JSON.stringify(val));
       });
     }
   }
 }
+
+// Checks whether local storage is enabled in the browser (the user might have disabled it).
+export function hasWindowLocalStorage(): boolean {
+  try {
+    return typeof window !== 'undefined' && !!window.localStorage;
+  } catch {
+    // Chrome throws an error if local storage is disabled and you try to access it
+    return false;
+  }
+}

yarn.lock

@@ -373,7 +373,7 @@
   resolved "https://registry.yarnpkg.com/@discoveryjs/json-ext/-/json-ext-0.5.7.tgz#1d572bfbbe14b7704e0ba0f39b74815b84870d70"
   integrity sha512-dBVuXR082gk3jsFp7Rd/JI4kytwGHecnCoTtXFb7DB6CNHp4rg5k1bhg0nWdLGLnOV71lmDzGQaLMy8iPLY0pw==
 
-"@eppo/js-client-sdk-common@^2.0.0":
+"@eppo/[email protected]":
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/@eppo/js-client-sdk-common/-/js-client-sdk-common-2.0.0.tgz#562006a128fb33653ca8c5e17df7680f7d91d0b4"
   integrity sha512-yHi4BFb6N7jISPK4y++N9OwjJBQAONSLqqCoCUHAyFSlNmUm+aNc/a+j/HHPsnXYZ1GUBWAZJOIQDNne4svfPg==