Pavel/add async (#3)

* Add prefetchConfig function

* Make running of vercel function async

* Adjust vercion of sdk

* Adjust example

* Fix for test

* Adjust README

* Add check for config expiration to avoid running Vercel Function too much

* Add section about using cron job to readme

* Adjust readme

Author: pavelflux

README.md

@@ -23,7 +23,8 @@ npm install @eppo/vercel-edge-sdk
 
 ## Quick start
 
-This SDK is inteded to be used in [Vercel Edge Middleware](https://vercel.com/docs/functions/edge-middleware)
+This SDK is inteded to be used in [Vercel Edge Middleware](https://vercel.com/docs/functions/edge-middleware) and in [Vercel Function](https://vercel.com/docs/functions/quickstart) for hydration and keeping stored config up-to-date.
+
 [Vercel Edge Config Store ](https://vercel.com/docs/storage/edge-config/using-edge-config) is required for storing Eppo configs.
 
 #### Example of usage in middleware.ts file
@@ -67,6 +68,8 @@ export async function middleware(request: NextRequest) {
         edgeConfig: process.env.EDGE_CONFIG,
         edgeConfigStoreId: process.env.EDGE_CONFIG_STORE_ID,
         vercelApiToken: process.env.EDGE_CONFIG_TOKEN,
+        vercelFunctionUrl: process.env.VERCEL_FUNCTION_URL // e.g. https://domain/api/eppo-prefetch
+        edgeConfigExpirationSeconds: 1000,
       }
     });
 
@@ -95,6 +98,86 @@ export const config = {
 
 ```
 
+This script will not fetch configs from Eppo, only from Vercel Config Store.
+
+To fetch configs from Eppo and store them in Vercel Config Store, you need to create a Vercel Function.
+
+Example:
+
+`pages/api/eppo-prefetch.ts`
+
+```ts
+export const runtime = 'nodejs';
+
+import { IAssignmentLogger, prefetchConfig } from '@eppo/vercel-edge-sdk';
+import { NextApiRequest, NextApiResponse } from 'next';
+
+const assignmentLogger: IAssignmentLogger = {
+  logAssignment(assignment) {
+    console.log('assignement', assignment)
+  },
+};
+
+export default async function handler(req: NextApiRequest, res: NextApiResponse) {
+
+  try {
+    if (!process.env.EDGE_CONFIG) {
+      throw new Error('Define EDGE_CONFIG env variable');
+    }
+
+    if (!process.env.EDGE_CONFIG_STORE_ID) {
+      throw new Error('Define EDGE_CONFIG_STORE_ID env variable')
+    }
+
+    if (!process.env.EDGE_CONFIG_TOKEN) {
+      throw new Error('Define EDGE_CONFIG_TOKEN env variable')
+    }
+
+    if (!process.env.INTERNAL_FEATURE_FLAG_API_KEY) {
+      throw new Error('Define INTERNAL_FEATURE_FLAG_API_KEY env variable')
+    }
+
+    prefetchConfig({
+      apiKey: process.env.INTERNAL_FEATURE_FLAG_API_KEY,
+        assignmentLogger,
+        vercelParams: {
+          edgeConfig: process.env.EDGE_CONFIG,
+          edgeConfigStoreId: process.env.EDGE_CONFIG_STORE_ID,
+          vercelApiToken: process.env.VERCEL_API_TOKEN,
+        },
+    });
+
+    res.status(200).json({ message: 'Prefetch success' });
+  } catch(e) {
+    res.status(500).json({ message: 'Prefetch error'});
+  }
+}
+```
+
+Your middleware, each time running, will start this cloud function (by doing an async request to the url specidifed in `vercelParams.vercelFunctionUrl`), and it will fetch and store configs.
+
+The flow is next:
+- if config stored in Vercel Config Store is not outdated, middleware will give return up-to-date assignment;
+- if config stored in Vercel Config Store is outdated, middleware will still give an assignment requested, just outdated, and send a request to start Vercel Function to prefetch up-to-date config; Next run of the middleware will give an updated result;
+
+
+### Vercel Cron Job
+
+You can hydrate data using [Vercel Cron Job](https://vercel.com/guides/how-to-setup-cron-jobs-on-vercel).
+For this, in your middleware, do not provide a URL to Vercel Function.
+Create a Vercel Function and as in the example above, and create a cron job like:
+
+```ts
+{
+  "crons": [
+    {
+      "path": "/api/eppo-prefetch",
+      "schedule": "0 5 * * *"
+    }
+  ]
+}
+```
+
 ## Assignment functions
 
 Every Eppo flag has a return type that is set once on creation in the dashboard. Once a flag is created, assignments in code should be made using the corresponding typed function: 

example.ts

@@ -1,6 +1,6 @@
 import { IAssignmentLogger } from '@eppo/js-client-sdk-common';
 
-import { init } from './src/index';
+import { init, prefetchConfig } from './src/index';
 
 const assignmentLogger: IAssignmentLogger = {
   logAssignment(assignment) {
@@ -9,13 +9,24 @@ const assignmentLogger: IAssignmentLogger = {
 };
 
 async function main() {
+  await prefetchConfig({
+    apiKey: '...',
+    assignmentLogger,
+    vercelParams: {
+      edgeConfig: 'https://edge-config.vercel.com/...',
+      edgeConfigStoreId: '...',
+      vercelApiToken: '..',
+    },
+  });
+
   const eppoClient = await init({
-    apiKey: '<your-eppo-sdk-key>',
+    apiKey: '...',
     assignmentLogger,
     vercelParams: {
       edgeConfig: 'https://edge-config.vercel.com/...',
       edgeConfigStoreId: '...',
       vercelApiToken: '...',
+      vercelFunctionUrl: 'http://localhost:3001/api/eppo-prefetch',
     },
   });
 

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@eppo/vercel-edge-sdk",
-  "version": "3.0.0",
+  "version": "3.0.1",
   "description": "Eppo SDK for use in Vercel Edge Middleware",
   "main": "dist/index.js",
   "files": [

src/index.spec.ts

@@ -275,6 +275,7 @@ describe('EppoJSClient E2E test', () => {
       'test variation assignment splits',
       async ({ flag, variationType, defaultValue, subjects }: IAssignmentTestCase) => {
         const client = getInstance();
+        await client.fetchFlagConfigurations();
 
         const typeAssignmentFunctions = {
           [VariationType.BOOLEAN]: client.getBoolAssignment.bind(client),

src/index.ts

@@ -43,6 +43,7 @@ export interface IClientConfig {
     edgeConfigStoreId: string;
     vercelApiToken: string;
     edgeConfigExpirationSeconds?: number;
+    vercelFunctionUrl?: string;
   };
 
   /**
@@ -153,7 +154,13 @@ export class EppoJSClient extends EppoClient {
  * @public
  */
 export async function init(config: IClientConfig): Promise<IEppoClient> {
-  validation.validateNotBlank(config.apiKey, 'API key required');
+  await initClient(config);
+  EppoJSClient.initialized = true;
+  return EppoJSClient.instance;
+}
+
+async function initClient(config: IClientConfig) {
+  validateConfig(config);
   try {
     // If any existing instances; ensure they are not polling
     if (EppoJSClient.instance) {
@@ -190,7 +197,12 @@ export async function init(config: IClientConfig): Promise<IEppoClient> {
     EppoJSClient.instance.setLogger(config.assignmentLogger);
     EppoJSClient.instance.setConfigurationRequestParameters(requestConfiguration);
 
-    await EppoJSClient.instance.fetchFlagConfigurations();
+    if (config.vercelParams.vercelFunctionUrl) {
+      const isConfigExpired = await configurationStore.isExpired();
+      if (isConfigExpired) {
+        fetch(config.vercelParams.vercelFunctionUrl);
+      }
+    }
   } catch (error) {
     console.warn(
       'Eppo SDK encountered an error initializing, assignment calls will return the default value and not be logged',
@@ -199,8 +211,22 @@ export async function init(config: IClientConfig): Promise<IEppoClient> {
       throw error;
     }
   }
-  EppoJSClient.initialized = true;
-  return EppoJSClient.instance;
+}
+
+export async function prefetchConfig(config: IClientConfig) {
+  await initClient(config);
+
+  await EppoJSClient.instance.fetchFlagConfigurations();
+}
+
+function validateConfig(config: IClientConfig) {
+  validation.validateNotBlank(config.apiKey, 'API key required');
+  validation.validateNotBlank(config.vercelParams.edgeConfig, 'EDGE_CONFIG is required');
+  validation.validateNotBlank(config.vercelParams.vercelApiToken, 'Vercel api token is required');
+  validation.validateNotBlank(
+    config.vercelParams.edgeConfigStoreId,
+    'Edge Config Store Id is required',
+  );
 }
 
 /**