allow overriding the DB implementation

docs/api/Server.md

@@ -26,8 +26,12 @@ const app = Server({
   // The return value of Game().
   game: game,
 
-  // The number of players.
-  numPlayers: 2
+  // Optional, if you want to hook it up to a
+  // custom storage backend not supported by
+  // the framework. DbImpl must implement the
+  // same interface shown in db.js:
+  // https://github.com/google/boardgame.io/blob/master/src/server/db.js
+  db: new DbImpl(),
 });
 
 app.listen(8000);

src/server/index.js

@@ -12,13 +12,16 @@ const Redux = require('redux');
 import { InMemory } from './db';
 import { createGameReducer } from '../core/reducer';
 
-function Server({ games }) {
+function Server({ games, db }) {
   const app = new Koa();
   const io = new IO();
   app.context.io = io;
   io.attach(app);
 
-  const db = new InMemory();
+  if (db === undefined) {
+    db = new InMemory();
+  }
+
   const clientInfo = new Map();
   const roomInfo = new Map();
 

src/server/index.test.js

@@ -155,3 +155,18 @@ test('playerView', () => {
     }
   });
 });
+
+test('custom db implementation', () => {
+  let getId = null;
+  class Custom {
+    get(id) { getId = id }
+    set() {}
+  }
+
+  const game = Game({});
+  const server = Server({ games: [game], db: new Custom() });
+  const io = server.context.io;
+
+  io.socket.receive('sync', 'gameID');
+  expect(getId).toBe('gameID');
+});