diff --git a/Backend/config/database.js b/Backend/config/database.js
new file mode 100644
index 0000000..a852edb
--- /dev/null
+++ b/Backend/config/database.js
@@ -0,0 +1,25 @@
+const boolFromEnv = (value, fallback = false) => {
+  if (value === undefined || value === '') return fallback
+  return ['1', 'true', 'yes', 'on'].includes(value.toLowerCase())
+}
+
+const numberFromEnv = (value, fallback) => {
+  if (value === undefined || value === '') return fallback
+  const parsed = Number(value)
+  return Number.isFinite(parsed) ? parsed : fallback
+}
+
+const config = {
+  uri: process.env.MONGO_URI || 'mongodb://127.0.0.1:27017',
+  dbName: process.env.MONGO_DB || 'chatapp',
+  options: {
+    user: process.env.MONGO_USER || undefined,
+    pass: process.env.MONGO_PASSWORD || undefined,
+    socketTimeoutMS: numberFromEnv(process.env.MONGO_SOCKET_TIMEOUT, 45000),
+    connectTimeoutMS: numberFromEnv(process.env.MONGO_CONNECT_TIMEOUT, 30000),
+    replicaSet: process.env.MONGO_REPLICA_SET || undefined,
+    tls: boolFromEnv(process.env.MONGO_TLS, false),
+  },
+}
+
+module.exports = config
diff --git a/Backend/config/index.js b/Backend/config/index.js
new file mode 100644
index 0000000..1c45829
--- /dev/null
+++ b/Backend/config/index.js
@@ -0,0 +1,29 @@
+const fs = require('fs')
+const path = require('path')
+
+loadLocalEnv()
+
+module.exports = {
+  port: process.env.PORT || 4000,
+  clientOrigin: process.env.CLIENT_ORIGIN || '*',
+}
+
+function loadLocalEnv() {
+  const envPath = path.join(__dirname, '..', '.env')
+  if (!fs.existsSync(envPath)) {
+    return
+  }
+
+  const content = fs.readFileSync(envPath, 'utf-8')
+  for (const line of content.split(/\r?\n/)) {
+    if (!line || line.trim().startsWith('#')) continue
+
+    const [key, ...rest] = line.split('=')
+    if (!key) continue
+
+    const value = rest.join('=').trim()
+    if (key && value && process.env[key] === undefined) {
+      process.env[key] = value
+    }
+  }
+}
diff --git a/Backend/lib/datastore.js b/Backend/lib/datastore.js
new file mode 100644
index 0000000..31f4f55
--- /dev/null
+++ b/Backend/lib/datastore.js
@@ -0,0 +1,34 @@
+const path = require('path')
+const JsonStore = require('./stores/jsonStore')
+
+async function createDataStore() {
+  const requestedStore = process.env.DATA_STORE
+  const forceJson = requestedStore === 'json'
+  const strictMongo = requestedStore === 'mongo'
+
+  if (forceJson) {
+    const filePath = path.join(__dirname, '..', 'data', 'db.json')
+    console.log(`Using JSON fallback store at ${filePath}`)
+    return new JsonStore(filePath)
+  }
+
+  try {
+    const createMongoStore = require('./stores/mongoStore')
+    return await createMongoStore()
+  } catch (error) {
+    if (strictMongo) {
+      throw error
+    }
+
+    console.warn(
+      'MongoDB store unavailable, falling back to JSON store.',
+      error.message,
+    )
+  }
+
+  const filePath = path.join(__dirname, '..', 'data', 'db.json')
+  console.log(`Using JSON fallback store at ${filePath}`)
+  return new JsonStore(filePath)
+}
+
+module.exports = createDataStore
diff --git a/Backend/lib/stores/jsonStore.js b/Backend/lib/stores/jsonStore.js
new file mode 100644
index 0000000..f21cf83
--- /dev/null
+++ b/Backend/lib/stores/jsonStore.js
@@ -0,0 +1,150 @@
+const fs = require('fs')
+const path = require('path')
+
+class JsonStore {
+  constructor(filePath) {
+    this.filePath = filePath
+    this.state = { users: [], messages: [] }
+    this._ensureStore()
+  }
+
+  _ensureStore() {
+    const dir = path.dirname(this.filePath)
+    fs.mkdirSync(dir, { recursive: true })
+    if (!fs.existsSync(this.filePath)) {
+      this._persist()
+      return
+    }
+    try {
+      const contents = fs.readFileSync(this.filePath, 'utf-8')
+      this.state = JSON.parse(contents)
+    } catch (error) {
+      console.warn('JSON store damaged, recreating', error)
+      this._persist()
+    }
+  }
+
+  _persist() {
+    fs.writeFileSync(this.filePath, JSON.stringify(this.state, null, 2), 'utf-8')
+  }
+
+  async upsertUser({ id, username, room }) {
+    const trimmed = username?.trim()
+    if (!trimmed) {
+      throw new Error('Username is required')
+    }
+    const now = new Date().toISOString()
+    const index = this.state.users.findIndex((user) => user.id === id)
+    if (index >= 0) {
+      const updated = {
+        ...this.state.users[index],
+        username: trimmed,
+        room,
+        isOnline: true,
+        lastActiveAt: now,
+      }
+      this.state.users[index] = updated
+      this._persist()
+      return updated
+    }
+    const user = {
+      id,
+      username: trimmed,
+      room,
+      isOnline: true,
+      joinedAt: now,
+      lastActiveAt: now,
+    }
+    this.state.users.push(user)
+    this._persist()
+    return user
+  }
+
+  async setUserOnline(userId, room) {
+    const user = this.state.users.find((entry) => entry.id === userId)
+    if (!user) {
+      return null
+    }
+    Object.assign(user, {
+      isOnline: true,
+      room,
+      lastActiveAt: new Date().toISOString(),
+    })
+    this._persist()
+    return user
+  }
+
+  async setUserOffline(userId) {
+    const user = this.state.users.find((entry) => entry.id === userId)
+    if (!user) {
+      return null
+    }
+    Object.assign(user, {
+      isOnline: false,
+      lastActiveAt: new Date().toISOString(),
+    })
+    this._persist()
+    return user
+  }
+
+  async updateUsername(userId, username) {
+    const user = this.state.users.find((entry) => entry.id === userId)
+    if (!user) return null
+    user.username = username.trim()
+    user.lastActiveAt = new Date().toISOString()
+    this._persist()
+    return user
+  }
+
+  async removeUser(userId) {
+    const index = this.state.users.findIndex((entry) => entry.id === userId)
+    if (index === -1) return null
+    const [removed] = this.state.users.splice(index, 1)
+    this._persist()
+    return removed
+  }
+
+  async getUser(userId) {
+    return this.state.users.find((entry) => entry.id === userId) || null
+  }
+
+  async getParticipants(room) {
+    return this.state.users
+      .filter((entry) => entry.room === room)
+      .map(({ id, username, isOnline, joinedAt, lastActiveAt }) => ({
+        id,
+        username,
+        isOnline,
+        joinedAt,
+        lastActiveAt,
+      }))
+      .sort((a, b) => {
+        if (a.isOnline === b.isOnline) {
+          return new Date(b.lastActiveAt) - new Date(a.lastActiveAt)
+        }
+        return a.isOnline ? -1 : 1
+      })
+  }
+
+  async addMessage({ id, room, userId, username, text }) {
+    const message = {
+      id,
+      room,
+      userId,
+      username,
+      text,
+      createdAt: new Date().toISOString(),
+    }
+    this.state.messages.push(message)
+    this._persist()
+    return message
+  }
+
+  async getMessages(room, limit = 100) {
+    return this.state.messages
+      .filter((message) => message.room === room)
+      .slice(-limit)
+  }
+}
+
+module.exports = JsonStore
diff --git a/Backend/lib/stores/mongoStore.js b/Backend/lib/stores/mongoStore.js
new file mode 100644
index 0000000..8584df5
--- /dev/null
+++ b/Backend/lib/stores/mongoStore.js
@@ -0,0 +1,217 @@
+const config = require('../../config/database')
+
+
+let MongoClient
+try {
+  ;({ MongoClient } = require('mongodb'))
+} catch (error) {
+  MongoClient = null
+}
+
+
+const collectionNames = {
+  users: 'users',
+  messages: 'messages',
+}
+
+function normalizeUser(doc) {
+  if (!doc) return null
+  const {
+    _id,
+    username,
+    room,
+    isOnline,
+    joinedAt,
+    lastActiveAt,
+  } = doc
+  return {
+    id: _id,
+    username,
+    room,
+    isOnline,
+    joinedAt,
+    lastActiveAt,
+  }
+}
+
+function normalizeMessage(doc) {
+  if (!doc) return null
+  const {
+    _id,
+    room,
+    userId,
+    username,
+    text,
+    createdAt,
+  } = doc
+  return {
+    id: _id,
+    room,
+    userId,
+    username,
+    text,
+    createdAt,
+  }
+}
+
+function extractDocument(result) {
+  if (!result) return null
+  return Object.prototype.hasOwnProperty.call(result, 'value')
+    ? result.value
+    : result
+}
+
+async function createMongoStore() {
+  // Guard: fail fast with an actionable message if the driver is missing.
+  if (!MongoClient) {
+    const message =
+      'MongoDB driver is not installed. Run `npm install mongodb` in Backend/.'
+    const error = new Error(message)
+    error.code = 'MODULE_NOT_INSTALLED'
+    throw error
+  }
+
+  const client = new MongoClient(config.uri, {
+    ...config.options,
+    appName: 'chat-app',
+  })
+
+  await client.connect()
+  const db = client.db(config.dbName)
+
+  const users = db.collection(collectionNames.users)
+  const messages = db.collection(collectionNames.messages)
+
+  await Promise.all([
+    users.createIndex({ room: 1, isOnline: -1, lastActiveAt: -1 }),
+    messages.createIndex({ room: 1, createdAt: 1 }),
+  ])
+
+  console.log(
+    `Connected to MongoDB at ${
+      config.uri
+    }/${config.dbName} (collections: ${Object.values(collectionNames).join(', ')})`,
+  )
+
+  return {
+    async upsertUser({ id, username, room }) {
+      const now = new Date()
+      const result = await users.findOneAndUpdate(
+        { _id: id },
+        {
+          $set: {
+            username: username.trim(),
+            room,
+            isOnline: true,
+            lastActiveAt: now,
+            updatedAt: now,
+          },
+          $setOnInsert: {
+            joinedAt: now,
+          },
+        },
+        {
+          upsert: true,
+          returnDocument: 'after',
+        },
+      )
+      const document = extractDocument(result)
+      if (document) {
+        return normalizeUser(document)
+      }
+
+
+      const inserted = await users.findOne({ _id: id })
+      return normalizeUser(inserted)
+    },
+
+    async setUserOnline(userId, room) {
+      const result = await users.findOneAndUpdate(
+        { _id: userId },
+        {
+          $set: {
+            isOnline: true,
+            room,
+            lastActiveAt: new Date(),
+          },
+        },
+        { returnDocument: 'after' },
+      )
+      return normalizeUser(extractDocument(result))
+    },
+
+    async setUserOffline(userId) {
+      const result = await users.findOneAndUpdate(
+        { _id: userId },
+        {
+          $set: {
+            isOnline: false,
+            lastActiveAt: new Date(),
+          },
+        },
+        { returnDocument: 'after' },
+      )
+      return normalizeUser(extractDocument(result))
+    },
+
+    async updateUsername(userId, username) {
+      const result = await users.findOneAndUpdate(
+        { _id: userId },
+        {
+          $set: {
+            username: username.trim(),
+            lastActiveAt: new Date(),
+          },
+        },
+        { returnDocument: 'after' },
+      )
+      return normalizeUser(extractDocument(result))
+    },
+
+    async removeUser(userId) {
+      const result = await users.findOneAndDelete({ _id: userId })
+      return normalizeUser(extractDocument(result))
+    },
+
+    async getUser(userId) {
+      const doc = await users.findOne({ _id: userId })
+      return normalizeUser(doc)
+    },
+
+    async getParticipants(room) {
+      const docs = await users
+        .find({ room })
+        .sort({ isOnline: -1, lastActiveAt: -1 })
+        .toArray()
+      return docs.map(normalizeUser)
+    },
+
+    async addMessage({ id, room, userId, username, text }) {
+      const message = {
+        _id: id,
+        room,
+        userId,
+        username,
+        text,
+        createdAt: new Date(),
+      }
+      await messages.insertOne(message)
+      return normalizeMessage(message)
+    },
+
+    async getMessages(room, limit = 100) {
+      const docs = await messages
+        .find({ room })
+        .sort({ createdAt: 1 })
+        .limit(limit)
+        .toArray()
+      return docs.map(normalizeMessage)
+    },
+
+    async disconnect() {
+      await client.close()
+    },
+  }
+}
+
+module.exports = createMongoStore