extract @flags-sdk/utils

packages/adapter-launchdarkly/src/index.ts

@@ -3,9 +3,9 @@ import {
   type LDClient,
   type LDContext,
 } from '@launchdarkly/vercel-server-sdk';
-import { createClient, type EdgeConfigClient } from '@vercel/edge-config';
-import { AsyncLocalStorage } from 'async_hooks';
+import { createClient } from '@vercel/edge-config';
 import type { Adapter } from 'flags';
+import { createCachedEdgeConfigClient as cacheEdgeConfigClient } from 'flags/utils';
 
 export { getProviderData } from './provider';
 export type { LDContext };
@@ -46,31 +46,10 @@ export function createLaunchDarklyAdapter({
   edgeConfigConnectionString: string;
 }): AdapterResponse {
   const edgeConfigClient = createClient(edgeConfigConnectionString);
-
-  const store = new AsyncLocalStorage<WeakKey>();
-  const cache = new WeakMap<WeakKey, Promise<unknown>>();
-
-  const patchedEdgeConfigClient: EdgeConfigClient = {
-    ...edgeConfigClient,
-    get: async <T>(key: string) => {
-      const h = store.getStore();
-      if (h) {
-        const cached = cache.get(h);
-        if (cached) {
-          return cached as Promise<T>;
-        }
-      }
-
-      const promise = edgeConfigClient.get<T>(key);
-      if (h) cache.set(h, promise);
-
-      return promise;
-    },
-  };
+  const { run, client } = cacheEdgeConfigClient(edgeConfigClient);
 
   let initPromise: Promise<unknown> | null = null;
-
-  const ldClient = init(clientSideId, patchedEdgeConfigClient);
+  const ldClient = init(clientSideId, client);
 
   function origin(key: string) {
     return `https://app.launchdarkly.com/projects/${projectSlug}/flags/${key}/`;
@@ -87,7 +66,7 @@ export function createLaunchDarklyAdapter({
           await initPromise;
         }
 
-        return store.run(
+        return run(
           headers,
           () =>
             ldClient.variation(

packages/flags/package.json

@@ -26,6 +26,10 @@
       "import": "./dist/index.js",
       "require": "./dist/index.cjs"
     },
+    "./utils": {
+      "import": "./dist/utils.js",
+      "require": "./dist/utils.cjs"
+    },
     "./next": {
       "import": "./dist/next.js",
       "require": "./dist/next.cjs"
@@ -50,6 +54,10 @@
         "dist/*.d.ts",
         "dist/*.d.cts"
       ],
+      "utils": [
+        "dist/utils.d.ts",
+        "dist/utils.d.cts"
+      ],
       "next": [
         "dist/next.d.ts",
         "dist/next.d.cts"
@@ -79,6 +87,7 @@
   },
   "dependencies": {
     "@edge-runtime/cookies": "^5.0.2",
+    "@vercel/edge-config": "^1.4.0",
     "jose": "^5.2.1"
   },
   "devDependencies": {

packages/flags/src/utils.ts

@@ -0,0 +1,59 @@
+/** This file is an entry point for flags/utils, so its exports are public */
+import type { EdgeConfigClient } from '@vercel/edge-config';
+import { AsyncLocalStorage } from 'async_hooks';
+
+/**
+ * Returns a patched version of the EdgeConfigClient that cache reads
+ * for the duration of a request.
+ *
+ * Uses the headers as a cache key.
+ */
+export function createCachedEdgeConfigClient(
+  edgeConfigClient: EdgeConfigClient,
+): {
+  /**
+   * A patched version of the Edge Config client, which only
+   * reads Edge Config once per request and caches the result
+   * for the duration of the request.
+   */
+  client: EdgeConfigClient;
+  run: <R>(headers: HeadersInit, fn: () => R) => R;
+} {
+  const store = new AsyncLocalStorage<WeakKey>();
+  const cache = new WeakMap<WeakKey, Promise<unknown>>();
+
+  const patchedEdgeConfigClient: EdgeConfigClient = {
+    ...edgeConfigClient,
+    get: async <T>(key: string) => {
+      const h = store.getStore();
+      if (h) {
+        const cached = cache.get(h);
+        if (cached) {
+          return cached as Promise<T>;
+        }
+      }
+
+      const promise = edgeConfigClient.get<T>(key);
+      if (h) cache.set(h, promise);
+
+      return promise;
+    },
+  };
+
+  return {
+    client: patchedEdgeConfigClient,
+    // The "run" function puts the headers into the AsyncLocalStorage, which
+    // allows the patchedEdgeConfigClient to read the headers without otherwise
+    // having access to them.
+    //
+    // The patchedEdgeConfigClient then uses the headers as a cache key to
+    // deduplicate Edge Config reads for the duration of a request.
+    //
+    // Performance wise it would actually be fine to read Edge Config on every
+    // flag evaluation, but this would also cause a lot of unnecessary reads,
+    // and as Edge Config is charged per read, this would cause unnecessary charges.
+    //
+    // So this approach helps with performance and ensures a fair usage of Edge Config.
+    run: store.run.bind(store),
+  };
+}

packages/flags/tsup.config.js

@@ -14,6 +14,7 @@ const defaultConfig = {
 export default defineConfig({
   entry: {
     index: 'src/index.ts',
+    utils: 'src/utils.ts',
     next: 'src/next/index.ts',
     sveltekit: 'src/sveltekit/index.ts',
     react: 'src/react/index.tsx',