diff --git a/back/src/constants.ts b/back/src/constants.ts
new file mode 100644
index 0000000..f207bf5
--- /dev/null
+++ b/back/src/constants.ts
@@ -0,0 +1 @@
+export const idMsgTypes = ["freed", "occupied"] as const;
diff --git a/back/src/index.ts b/back/src/index.ts
index 3da887c..1bed51b 100644
--- a/back/src/index.ts
+++ b/back/src/index.ts
@@ -1,6 +1,7 @@
 import "reflect-metadata";
 import { Server } from "ws";
 import { connect, getRoomList } from "./db";
+import { isIdMessage, isMessage } from "./types";
 
 (async () => {
   const connection = await connect();
@@ -11,5 +12,13 @@ import { connect, getRoomList } from "./db";
 
   wss.on("connection", async (ws) => {
     ws.send(JSON.stringify(await getRoomList(connection)));
+
+    ws.on("message", (data) => {
+      const message: unknown = JSON.parse(data.toString());
+      if (!isMessage(message)) throw new Error("Message corrupted");
+
+      if (isIdMessage(message))
+        updateFree(message.args.id, message.type === "freed");
+    });
   });
 })();
diff --git a/back/src/types.ts b/back/src/types.ts
new file mode 100644
index 0000000..78acf14
--- /dev/null
+++ b/back/src/types.ts
@@ -0,0 +1,32 @@
+import { idMsgTypes } from "./constants";
+
+export type IdMsgTypes = typeof idMsgTypes[number];
+
+export type Message = {
+  type: string;
+  args: unknown;
+};
+
+export type IdMessage = Message & {
+  type: IdMsgTypes;
+  args: {
+    id: number;
+  };
+};
+
+const hasProperty = <T extends {}, U extends PropertyKey>(
+  obj: T,
+  prop: U
+): obj is T & Record<U, unknown> => prop in obj;
+
+export const isMessage = (obj: unknown): obj is Message =>
+  typeof obj === "object" &&
+  hasProperty(obj, "type") &&
+  typeof obj.type === "string" &&
+  hasProperty(obj, "args");
+
+export const isIdMessage = (message: Message): message is IdMessage =>
+  idMsgTypes.reduce((prev, curr) => prev || curr === message.type, false) &&
+  typeof message.args === "object" &&
+  hasProperty(message.args, "id") &&
+  typeof message.args.id === "number";