Notification to all parties in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Design a scalable notification system for multiple parties

You need to design a system that sends notifications to all parties involved in a transaction. The system must handle millions of notifications per day and ensure delivery even if some parties are temporarily offline.

Which architectural component is essential to ensure reliable delivery and scalability?

A A message queue to buffer notifications and retry delivery

B A single synchronous API call to notify all parties at once

C A direct database update without any intermediate layer

D A batch job that sends notifications once a day

💡 Hint

Think about how to handle temporary failures and high volume.

A message queue helps buffer notifications and retry sending if a party is offline, ensuring reliability and scalability.

❓ scaling

intermediate

Estimating capacity for notification delivery

Your notification system must support 10 million users, each receiving an average of 5 notifications per day. Estimate the number of notifications your system must handle per second.

A Approximately 50 notifications per second

B Approximately 5000 notifications per second

C Approximately 600 notifications per second

D Approximately 10000 notifications per second

💡 Hint

Calculate total notifications per day and divide by seconds in a day.

10 million users * 5 notifications = 50 million notifications per day. 50 million / 86400 seconds ≈ 578 notifications per second, rounded to 600.

❓ tradeoff

advanced

Choosing between push and pull notification models

For notifying all parties, you can either push notifications immediately or let parties pull notifications when they check. What is a key tradeoff of using a push model?

A Push is simpler to implement than pull and requires no client changes

B Push reduces server load but increases latency for users

C Push eliminates the need for retries and guarantees delivery

D Push provides real-time updates but requires more server resources to maintain connections

💡 Hint

Consider resource usage and immediacy of updates.

Push notifications deliver updates instantly but need servers to keep connections open, increasing resource use.

❓ component

advanced

Identify the component responsible for handling notification retries

In a notification system, which component should handle retrying failed notification deliveries to ensure eventual consistency?

A The message queue or broker with dead-letter queue support

B The client application requesting notifications

C The frontend UI displaying notifications

D The database storing user information

💡 Hint

Think about where failed messages can be stored and retried automatically.

Message queues with dead-letter queues can hold failed messages and retry delivery, ensuring no notifications are lost.

🧠 Conceptual

expert

Ensuring notification ordering across multiple parties

You must guarantee that all parties receive notifications in the exact order events occurred, even under high load and failures. Which design approach best supports this requirement?

A Send notifications in parallel without ordering guarantees to improve speed

B Use partitioned message queues with ordering keys and idempotent consumers

C Store notifications in a relational database and poll clients for updates

D Use a simple broadcast without tracking delivery order

💡 Hint

Consider how to keep order while scaling and handling retries.

Partitioned queues with ordering keys ensure messages for the same key are processed in order. Idempotent consumers prevent duplicates.

Practice

1.

What is the main purpose of a notification system that sends messages to all parties?

easy

A. To quickly share important messages with everyone involved

B. To store large amounts of data securely

C. To perform complex calculations on user data

D. To create user profiles and preferences

Solution

1. Step 1: Understand the role of notifications

   Notifications are designed to deliver messages to users or parties quickly and efficiently.

2. Step 2: Identify the main goal

   The main goal is to share important information with all involved parties without delay.

3. Final Answer:

   To quickly share important messages with everyone involved -> Option A

4. Quick Check:

   Notification purpose = quick message sharing [OK]

Hint: Notifications = fast message delivery to all involved [OK]

Common Mistakes:

• Confusing notifications with data storage
• Thinking notifications perform data processing
• Assuming notifications create user profiles

2.

Which of the following is the correct way to represent a notification service that sends messages to all parties in pseudocode?

function notifyAll(parties, message) {
  for (let i = 0; i < parties.length; i++) {
    parties[i].send(message);
  }
}

easy

A. Call send(message) once without looping

B. Send message only to the first party

C. Loop through parties and call send(message) on each

D. Loop through parties but do not send any message

Solution

1. Step 1: Analyze the pseudocode loop

   The code loops through each party in the parties list using a for loop.

2. Step 2: Check the send method call

   Inside the loop, it calls send(message) on each party, ensuring all get notified.

3. Final Answer:

   Loop through parties and call send(message) on each -> Option C

4. Quick Check:

   Loop + send call = notify all [OK]

Hint: Loop through all parties to send message [OK]

Common Mistakes:

• Sending message only once
• Not looping through all parties
• Calling send outside the loop

3.

Consider this code snippet for notifying parties:

parties = ["Alice", "Bob", "Charlie"]
function notifyAll(parties, message) {
  let notified = []
  for (const person of parties) {
    notified.push(person + ": " + message)
  }
  return notified
}

console.log(notifyAll(parties, "Meeting at 5 PM"))

What will be the output?

medium

A. ["Alice: Meeting at 5 PM", "Bob: Meeting at 5 PM", "Charlie: Meeting at 5 PM"]

B. ["Meeting at 5 PM", "Meeting at 5 PM", "Meeting at 5 PM"]

C. ["Alice", "Bob", "Charlie"]

D. Error: notifyAll is not defined

Solution

1. Step 1: Understand the loop behavior

   The function loops over each person in parties and creates a string combining their name and the message.

2. Step 2: Check the returned list

   The notified list contains strings like "Alice: Meeting at 5 PM" for each party.

3. Final Answer:

   ["Alice: Meeting at 5 PM", "Bob: Meeting at 5 PM", "Charlie: Meeting at 5 PM"] -> Option A

4. Quick Check:

   Loop + string concat = list of personalized messages [OK]

Hint: Each party gets message with their name prefixed [OK]

Common Mistakes:

• Returning only messages without names
• Returning original party list
• Assuming function is undefined

4.

Identify the bug in this notification function and select the fix:

function notifyAll(parties, message) {
  for (let i = 0; i < parties.length; i++) {
    parties.send(message)
  }
}

medium

A. Remove the loop and call parties.send(message) once

B. Add a return statement inside the loop

C. Change i < parties.length to i <= parties.length

D. Change parties.send(message) to parties[i].send(message)

Solution

1. Step 1: Identify incorrect method call

   The code calls send on the entire parties array instead of individual party objects.

2. Step 2: Fix by indexing the array

   Use parties[i].send(message) to call send on each party in the loop.

3. Final Answer:

   Change parties.send(message) to parties[i].send(message) -> Option D

4. Quick Check:

   Call send on each party object [OK]

Hint: Call send on parties[i], not parties array [OK]

Common Mistakes:

• Calling send on the whole array
• Using wrong loop condition
• Adding unnecessary return inside loop

5.

You are designing a notification system to alert all parties involved in a project. Which design choice best ensures scalability and reliability?

• A. Use a single server to send notifications sequentially to all parties.
• B. Send notifications only to a random subset of parties to reduce load.
• C. Store all notifications in a database and send them manually when needed.
• D. Use a message queue to distribute notification tasks to multiple worker servers.

hard

A. Single server sending sequentially

B. Message queue with multiple workers

C. Store notifications and send manually

D. Send to random subset to reduce load

Solution

1. Step 1: Evaluate single server approach

   Sending sequentially from one server limits scalability and can cause delays or failures.

2. Step 2: Consider message queue with workers

   Using a message queue allows distributing notification tasks to multiple workers, improving scalability and reliability.

3. Step 3: Assess other options

   Storing notifications for manual sending is slow; sending to random subset misses parties.

4. Final Answer:

   Message queue with multiple workers -> Option B

5. Quick Check:

   Queue + workers = scalable, reliable notifications [OK]

Hint: Use queues and workers for scalable notifications [OK]

Common Mistakes:

• Relying on single server for all notifications
• Sending notifications manually
• Skipping parties to reduce load