Online presence system in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the main component that tracks user status.

HLD

class PresenceTracker {
  constructor() {
    this.users = new Map();
  }

  updateStatus(userId, status) {
    this.users.set(userId, [1]);
  }
}

Drag options to blanks, or click blank then click option'

Athis

Bstatus

CuserId

Dusers

Attempts:

3 left

💡 Hint

Common Mistakes

Using userId instead of status as the value.

Trying to assign this or users instead of status.

The updateStatus method should store the status for the given userId in the users map.

2fill in blank

medium

Complete the code to check if a user is currently online.

HLD

isUserOnline(userId) {
  return this.users.get(userId) === [1];
}

Drag options to blanks, or click blank then click option'

A'away'

B'offline'

C'online'

D'busy'

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'offline' instead of 'online'.

Using a status that means away or busy.

The method should return true if the user's status is 'online'.

3fill in blank

hard

Fix the error in the method that removes a user from the presence map.

HLD

removeUser(userId) {
  this.users.[1](userId);
}

Drag options to blanks, or click blank then click option'

Adelete

Bpop

Cdiscard

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove or pop which are not Map methods.

Using discard which is from other languages.

The correct method to remove a key from a JavaScript Map is delete.

4fill in blank

hard

Fill both blanks to create a method that returns all users currently online.

HLD

getOnlineUsers() {
  return [...this.users.entries()].filter(([[1], [2]]) => [2] === 'online').map(([userId]) => userId);
}

Drag options to blanks, or click blank then click option'

AuserId

Bstatus

Cuser

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping userId and status names.

Using incorrect variable names that don't match the pair.

The entries() method returns [userId, status] pairs. We filter by status === 'online' and map to userId.

5fill in blank

hard

Fill all three blanks to implement a method that returns a count of users by status.

HLD

countByStatus() {
  const counts = {};
  for (const [1] of this.users.values()) {
    counts[[2]] = (counts[[3]] || 0) + 1;
  }
  return counts;
}

Drag options to blanks, or click blank then click option'

Astatus

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'user' instead of 'status' as the loop variable.

Using different variable names for the key and lookup.

We iterate over each status value, then increment the count for that status in the counts object.

Practice

1. What is the primary purpose of an online presence system in a chat application?

easy

A. To track if users are currently online or offline

B. To store chat message history permanently

C. To encrypt messages between users

D. To manage user account passwords

Solution

1. Step 1: Understand the role of presence system

   An online presence system tracks user activity to know if they are online or offline.

2. Step 2: Differentiate from other chat features

   Message storage, encryption, and password management are separate features not handled by presence systems.

3. Final Answer:

   To track if users are currently online or offline -> Option A

4. Quick Check:

   Presence system = user online status [OK]

Hint: Presence means tracking user online/offline status [OK]

Common Mistakes:

• Confusing presence with message storage
• Thinking presence handles security features
• Mixing presence with account management

2. Which event is NOT typically used in an online presence system to track user status?

easy

A. message_send

B. connect

C. disconnect

D. heartbeat

Solution

1. Step 1: Identify presence tracking events

   Presence systems use connect, heartbeat, and disconnect to track user activity.

2. Step 2: Recognize unrelated events

   message_send relates to sending chat messages, not presence tracking.

3. Final Answer:

   message_send -> Option A

4. Quick Check:

   Presence events exclude message sending [OK]

Hint: Presence tracks connection, not message sending [OK]

Common Mistakes:

• Assuming message events track presence
• Confusing heartbeat with message_send
• Ignoring disconnect event importance

3. Given this simplified presence update code snippet, what will be the user's status after 10 seconds?

user_last_seen = 0
current_time = 10
heartbeat_interval = 5
if current_time - user_last_seen <= heartbeat_interval:
    status = 'online'
else:
    status = 'offline'

medium

A. Error due to comparison

B. "online"

C. "offline"

D. "unknown"

Solution

1. Step 1: Calculate time difference

   current_time - user_last_seen = 10 - 0 = 10 seconds.

2. Step 2: Compare with heartbeat interval

   10 <= 5 is false, so status is set to 'offline'.

3. Final Answer:

   "offline" -> Option C

4. Quick Check:

   Time diff > heartbeat means offline [OK]

Hint: Compare last seen time difference with heartbeat [OK]

Common Mistakes:

• Mixing up less than and greater than
• Forgetting to subtract last seen time
• Assuming status is always online

4. Identify the bug in this presence update logic:

def update_status(last_seen, current_time, heartbeat=10):
    if current_time - last_seen > heartbeat:
        return 'online'
    else:
        return 'offline'

medium

A. Function should return a boolean, not string

B. The comparison sign should be reversed

C. Heartbeat value should be negative

D. No bug, logic is correct

Solution

1. Step 1: Analyze the condition meaning

   If time difference is greater than heartbeat, user should be offline, not online.

2. Step 2: Correct the comparison

   The condition should return 'offline' when difference > heartbeat, so the comparison sign must be reversed.

3. Final Answer:

   The comparison sign should be reversed -> Option B

4. Quick Check:

   Time diff > heartbeat means offline [OK]

Hint: Online if time diff ≤ heartbeat, else offline [OK]

Common Mistakes:

• Returning online when user is inactive
• Misunderstanding heartbeat meaning
• Ignoring return type correctness

5. You need to design an online presence system for a messaging app with millions of users. Which approach best ensures scalability and real-time accuracy?

hard

A. Store presence data in user profile tables updated once per day

B. Store user status in a centralized database and query it on every client request

C. Send presence updates only when users log in or log out, ignoring heartbeats

D. Use distributed in-memory cache with TTL and heartbeat updates from clients

Solution

1. Step 1: Evaluate centralized database approach

   Querying a centralized DB for every request causes high latency and bottlenecks at scale.

2. Step 2: Consider distributed cache with TTL and heartbeats

   This approach keeps presence data fresh, reduces DB load, and supports real-time updates efficiently.

3. Step 3: Analyze other options

   Ignoring heartbeats or updating once per day leads to stale presence info, unsuitable for real-time apps.

4. Final Answer:

   Use distributed in-memory cache with TTL and heartbeat updates from clients -> Option D

5. Quick Check:

   Distributed cache + heartbeat = scalable real-time presence [OK]

Hint: Use cache with TTL and heartbeats for scalable presence [OK]

Common Mistakes:

• Relying on centralized DB for real-time presence
• Ignoring heartbeat updates causing stale data
• Updating presence too infrequently