small error handling for database.service.ts and refactoring

src/db/services/db/database.service.ts

@@ -2,10 +2,45 @@ import "dotenv/config";
 
 import mongoose from "mongoose";
 
-export async function connectToDatabase() {
-    await mongoose.connect(process.env.DB_CONN_STRING!, {
-        dbName: process.env.DB_NAME!,
-    });
+let isConnected = false;
+
+export async function connectToDatabase() {
+    if (isConnected) {
+        console.log("Already connected to MongoDB.");
+        return;
+    }
+    const dbConnString = process.env.DB_CONN_STRING;
+    const dbName = process.env.DB_NAME;
+
+    if (!dbConnString) {
+        throw new Error("Missing environment variable: DB_CONN_STRING is required for database connection.");
+    }
+    if (!dbName) {
+        throw new Error("Missing environment variable: DB_NAME is required for database connection.");
+    }
+    try {
+        await mongoose.connect(dbConnString, {
+            dbName: dbName,
+            serverSelectionTimeoutMS: 5000,
+            socketTimeoutMS: 45000,
+            family: 4
+        });
+        isConnected = true;
+        console.log("Connected to MongoDB with Mongoose");
+
+        mongoose.connection.on('disconnected', () => {
+            // TODO: add reconnecting
+            console.warn('Mongoose disconnected from DB. Attempting to reconnect...');
+            isConnected = false;
+        });
+        mongoose.connection.on('error', (err) => {
+            console.error('Mongoose connection error:', err);
+            isConnected = false;
+        });
+
+    } catch (error) {
+        console.error("Failed to connect to MongoDB:", error);
+        process.exit(1);
+    }
 
-    console.log("Connected to MongoDB with Mongoose");
 }