diff --git a/apps/back/package.json b/apps/back/package.json
index ef92764..bd58dae 100644
--- a/apps/back/package.json
+++ b/apps/back/package.json
@@ -2,11 +2,15 @@
   "name": "@roomruler/back",
   "version": "0.0.0",
   "scripts": {
-    "build": "tsc --build",
+    "build": "rollup --config rollup.config.js",
     "start": "node dist/index.js"
   },
   "devDependencies": {
+    "@rollup/plugin-node-resolve": "^13.0.4",
+    "@rollup/plugin-typescript": "^8.2.5",
     "@types/ws": "^7.4.7",
+    "rollup": "^2.56.3",
+    "tslib": "^2.3.1",
     "typescript": "^4.4.2"
   },
   "dependencies": {
diff --git a/apps/back/rollup.config.js b/apps/back/rollup.config.js
new file mode 100644
index 0000000..f43f516
--- /dev/null
+++ b/apps/back/rollup.config.js
@@ -0,0 +1,11 @@
+const typescript = require("@rollup/plugin-typescript");
+const { nodeResolve } = require("@rollup/plugin-node-resolve");
+
+export default {
+  input: "src/index.ts",
+  output: {
+    dir: "dist",
+    format: "cjs",
+  },
+  plugins: [typescript(), nodeResolve({ resolveOnly: [/^@roomruler\/.*$/] })],
+};
diff --git a/apps/back/src/index.ts b/apps/back/src/index.ts
index 6e8bc51..8c2a14d 100644
--- a/apps/back/src/index.ts
+++ b/apps/back/src/index.ts
@@ -1,7 +1,12 @@
 import "reflect-metadata";
 import { Server, OPEN } from "ws";
 import { connect, getRoomList, updateFree } from "./db";
-import { isMessage, isUpdateMessage } from "@roomruler/messages";
+import {
+  composeMessage,
+  isMessage,
+  isUpdateMessage,
+  ListMessage,
+} from "@roomruler/messages";
 
 const main = async () => {
   const connection = await connect();
@@ -15,7 +20,11 @@ const main = async () => {
 
   wss.on("connection", async (wsc, req) => {
     console.log("New user connected from " + req.socket.remoteAddress);
-    wsc.send(JSON.stringify(await getRoomList(connection)));
+    wsc.send(
+      JSON.stringify(
+        composeMessage<ListMessage>("list", await getRoomList(connection))
+      )
+    );
 
     wsc.on("message", async (data) => {
       console.log("Got message from " + req.socket.remoteAddress);
diff --git a/apps/back/tsconfig.json b/apps/back/tsconfig.json
index ffce736..93205f0 100644
--- a/apps/back/tsconfig.json
+++ b/apps/back/tsconfig.json
@@ -5,5 +5,6 @@
     "module": "commonjs",
     "outDir": "dist"
   },
+  "include": ["src"],
   "references": [{ "path": "../../packages/messages/tsconfig.json" }]
 }
diff --git a/packages/messages/package.json b/packages/messages/package.json
index 5acb0d1..35f4d64 100644
--- a/packages/messages/package.json
+++ b/packages/messages/package.json
@@ -3,6 +3,7 @@
   "main": "dist/src/index.js",
   "version": "0.0.0",
   "types": "dist/src/index.d.ts",
+  "type": "module",
   "scripts": {
     "build": "tsc --build"
   },
diff --git a/packages/messages/src/index.ts b/packages/messages/src/index.ts
index 3c76f1c..0f4027f 100644
--- a/packages/messages/src/index.ts
+++ b/packages/messages/src/index.ts
@@ -11,6 +11,21 @@ export type UpdateMessage = Message & {
   };
 };
 
+export type Room = {
+  id: number;
+  title: string;
+  free: boolean;
+  x: number;
+  y: number;
+  width: number;
+  height: number;
+};
+
+export type ListMessage = Message & {
+  type: "list";
+  args: Room[];
+};
+
 const isObjLike = (obj: unknown): obj is object =>
   Boolean(obj) && typeof obj === "object";
 
@@ -31,3 +46,40 @@ export const isUpdateMessage = (message: Message): message is UpdateMessage =>
   typeof message.args.id === "number" &&
   hasProperty(message.args, "value") &&
   typeof message.args.value === "boolean";
+
+export const isRoom = (obj: unknown): obj is Room => {
+  if (!(typeof obj === "object") || !obj) return false;
+
+  const ns = ["id", "x", "y", "width", "height"] as const;
+  for (const key of ns) {
+    if (!hasProperty(obj, key) || typeof obj[key] !== "number") return false;
+  }
+
+  if (
+    !hasProperty(obj, "title") ||
+    typeof obj.title !== "string" ||
+    !hasProperty(obj, "free") ||
+    typeof obj.free !== "boolean"
+  )
+    return false;
+
+  return true;
+};
+
+export const isArrayOf = <T extends unknown>(
+  obj: T,
+  itemCheck: (arg: T) => boolean
+) => {
+  if (Array.isArray(obj)) {
+    for (const el of obj) if (!itemCheck(el)) return false;
+    return true;
+  } else return false;
+};
+
+export const composeMessage = <T extends Message>(
+  type: T["type"],
+  args: T["args"]
+): Message => ({
+  type,
+  args,
+});
diff --git a/packages/messages/tsconfig.json b/packages/messages/tsconfig.json
index 74d847c..3d87a68 100644
--- a/packages/messages/tsconfig.json
+++ b/packages/messages/tsconfig.json
@@ -1,10 +1,10 @@
 {
   "extends": "../../tsconfig.json",
   "compilerOptions": {
-    "module": "commonjs",
+    "module": "es6",
     "target": "es6",
     "outDir": "dist",
     "composite": true
   },
-  "include": ["src"]
+  "include": ["src"],
 }