Allow async initialization of data sources

CHANGELOG.md

@@ -6,6 +6,7 @@ The version headers in this history reflect the versions of Apollo Server itself
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the the appropriate changes within that release will be moved into the new section.
 
+- `apollo-server-core`: Allow asynchronous initialization of datasources: the `initialize` method on datasources may now return a Promise, which will be settled before any resolvers are called. [#3639](https://github.com/apollographql/apollo-server/pull/3639)
 - `apollo-server-core`: Update apollo-tooling dependencies, resolve TS build error (missing types for node-fetch) [#3662](https://github.com/apollographql/apollo-server/pull/3662)
 
 ### v2.9.15

packages/apollo-datasource/src/index.ts

@@ -6,5 +6,5 @@ export interface DataSourceConfig<TContext> {
 }
 
 export abstract class DataSource<TContext = any> {
-  initialize?(config: DataSourceConfig<TContext>): void;
+  initialize?(config: DataSourceConfig<TContext>): void | Promise<void>;
 }

packages/apollo-server-core/src/__tests__/dataSources.test.ts

@@ -0,0 +1,105 @@
+import { ApolloServerBase } from '../ApolloServer';
+import gql from 'graphql-tag';
+
+const typeDefs = gql`
+  type Query {
+    hello: String
+  }
+`;
+
+describe('ApolloServerBase dataSources', () => {
+  it('initializes synchronous datasources from a datasource creator function', async () => {
+    const initialize = jest.fn();
+
+    const server = new ApolloServerBase({
+      typeDefs,
+      resolvers: {
+        Query: {
+          hello() {
+            return 'world';
+          }
+        }
+      },
+      dataSources: () => ({ x: { initialize }, y: { initialize } })
+    });
+
+    await server.executeOperation({ query: "query { hello }"});
+
+    expect(initialize).toHaveBeenCalledTimes(2);
+  });
+
+  it('initializes all async and sync datasources before calling resolvers', async () => {
+    const INITIALIZE = "datasource initializer call";
+    const METHOD_CALL = "datasource method call";
+
+    const expectedCallOrder = [
+      INITIALIZE,
+      INITIALIZE,
+      INITIALIZE,
+      METHOD_CALL
+    ];
+
+    const actualCallOrder: string[] = [];
+
+    const server = new ApolloServerBase({
+      typeDefs,
+      resolvers: {
+        Query: {
+          hello(_, __, context) {
+            context.dataSources.x.getData();
+            return "world";
+          }
+        },
+      },
+      dataSources: () => ({
+        x: {
+          initialize() {
+            return Promise.resolve().then(
+              () => { actualCallOrder.push(INITIALIZE); }
+            );
+          },
+          getData() { actualCallOrder.push(METHOD_CALL); }
+        },
+        y: {
+          initialize() {
+            return new Promise(res => {
+              setTimeout(() => {
+                actualCallOrder.push(INITIALIZE);
+                res();
+              }, 0);
+            });
+          },
+        },
+        z: {
+          initialize() { actualCallOrder.push(INITIALIZE); }
+        }
+      })
+    });
+
+    await server.executeOperation({ query: "query { hello }"});
+
+    expect(actualCallOrder).toEqual(expectedCallOrder);
+  });
+
+  it('makes datasources available on resolver contexts', async () => {
+    const message = 'hi from dataSource';
+    const getData = jest.fn(() => message);
+
+    const server = new ApolloServerBase({
+      typeDefs,
+      resolvers: {
+        Query: {
+          hello(_, __, context) {
+            return context.dataSources.x.getData();
+          }
+        },
+      },
+      dataSources: () => ({ x: { initialize() {}, getData } })
+    });
+
+    const res = await server.executeOperation({ query: "query { hello }"});
+
+    expect(getData).toHaveBeenCalled();
+    expect(res.data?.hello).toBe(message);
+  });
+});

packages/apollo-server-core/src/requestPipeline.ts

@@ -114,7 +114,7 @@ export async function processGraphQLRequest<TContext>(
 
   const dispatcher = initializeRequestListenerDispatcher();
 
-  initializeDataSources();
+  await initializeDataSources();
 
   const metrics = requestContext.metrics || Object.create(null);
   if (!requestContext.metrics) {
@@ -611,21 +611,26 @@ export async function processGraphQLRequest<TContext>(
     return new GraphQLExtensionStack(extensions);
   }
 
-  function initializeDataSources() {
+  async function initializeDataSources() {
     if (config.dataSources) {
       const context = requestContext.context;
 
       const dataSources = config.dataSources();
 
+      const initializers: any[] = [];
       for (const dataSource of Object.values(dataSources)) {
         if (dataSource.initialize) {
-          dataSource.initialize({
-            context,
-            cache: requestContext.cache,
-          });
+          initializers.push(
+            dataSource.initialize({
+              context,
+              cache: requestContext.cache,
+            })
+          );
         }
       }
 
+      await Promise.all(initializers);
+
       if ('dataSources' in context) {
         throw new Error(
           'Please use the dataSources config option instead of putting dataSources on the context yourself.',