Message delivery guarantees in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the message delivery guarantee that ensures a message is delivered at least once.

HLD

delivery_guarantee = "[1]"

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

Aat-most-once

Bexactly-once

Cat-least-once

Dbest-effort

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'at-most-once' which may lose messages.

Confusing 'exactly-once' with 'at-least-once'.

The at-least-once guarantee ensures that messages are delivered one or more times, preventing loss but possibly causing duplicates.

2fill in blank

medium

Complete the code to select the message delivery guarantee that ensures a message is delivered no more than once.

HLD

delivery_guarantee = "[1]"

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

Abest-effort

Bat-least-once

Cexactly-once

Dat-most-once

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'at-least-once' which allows duplicates.

Confusing 'exactly-once' with 'at-most-once'.

The at-most-once guarantee ensures messages are delivered zero or one time, which may result in message loss but no duplicates.

3fill in blank

hard

Fix the error in the code to select the message delivery guarantee that ensures a message is delivered exactly once.

HLD

delivery_guarantee = "[1]"

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

Aexactly-once

Bbest-effort

Cat-most-once

Dat-least-once

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'at-least-once' which may cause duplicates.

Choosing 'at-most-once' which may lose messages.

The exactly-once guarantee ensures that each message is delivered one and only one time, avoiding duplicates and loss.

4fill in blank

hard

Fill both blanks to complete the message delivery flow that ensures exactly-once delivery using acknowledgments and deduplication.

HLD

if message_received and not [1]:
    process_message()
    send_[2]()

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

Amessage_processed

Bmessage_ack

Cmessage_duplicate

Dmessage_retry

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'message_duplicate' instead of 'message_processed' for the check.

Sending 'message_retry' instead of acknowledgment.

To ensure exactly-once delivery, the system checks if the message was already processed (message_processed) to avoid duplicates, then sends an acknowledgment (message_ack) after processing.

5fill in blank

hard

Fill all three blanks to complete the code snippet that implements at-least-once delivery with retries and acknowledgment.

HLD

while not [1]:
    send_message()
    [2] = wait_for_[3]()

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

Aack_received

Cack

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'message' with 'ack' in the wait function.

Using inconsistent variable names for acknowledgment.

The loop continues until an acknowledgment (ack_received) is true. The system waits for an ack message after sending.

Practice

1. Which message delivery guarantee ensures that a message is never delivered more than once, but some messages might be lost?

easy

A. Exactly once

B. At most once

C. At least once

D. None of the above

Solution

1. Step 1: Understand 'At most once' guarantee

   This guarantee means messages can be lost but never duplicated.

2. Step 2: Compare with other guarantees

   'At least once' may duplicate messages, 'Exactly once' avoids loss and duplication but is complex.

3. Final Answer:

   At most once -> Option B

4. Quick Check:

   At most once = no duplicates, possible loss [OK]

Hint: No duplicates but possible loss means 'At most once' [OK]

Common Mistakes:

• Confusing 'At most once' with 'At least once'
• Thinking 'Exactly once' is simple
• Assuming 'At most once' never loses messages

2. Which of the following is the correct description of the 'Exactly once' message delivery guarantee?

easy

A. Messages are delivered once without loss or duplication

B. Messages may be lost but never duplicated

C. Messages may be duplicated but never lost

D. Messages are delivered at least once, possibly duplicated

Solution

1. Step 1: Define 'Exactly once' guarantee

   This guarantee ensures each message is delivered once, no loss, no duplication.

2. Step 2: Eliminate other options

   'Messages may be lost but never duplicated' is 'At most once', 'Messages may be duplicated but never lost' and 'delivered at least once, possibly duplicated' describe 'At least once'.

3. Final Answer:

   Messages are delivered once without loss or duplication -> Option A

4. Quick Check:

   Exactly once = no loss, no duplicates [OK]

Hint: Exactly once means no loss and no duplicates [OK]

Common Mistakes:

• Mixing 'Exactly once' with 'At least once'
• Believing 'Exactly once' allows duplicates
• Confusing 'At most once' with 'Exactly once'

3. Consider a message queue system that uses 'At least once' delivery. If a message is sent and the receiver crashes before acknowledging, what is the likely outcome?

medium

A. The message is lost and never delivered

B. The message is delivered once without duplication

C. The message is delivered at most once

D. The message may be delivered multiple times

Solution

1. Step 1: Understand 'At least once' behavior on receiver crash

   If receiver crashes before ack, sender retries, causing possible duplicates.

2. Step 2: Analyze options

   The message is not lost but sender retries on no ack, so may be delivered multiple times to ensure at least once.

3. Final Answer:

   The message may be delivered multiple times -> Option D

4. Quick Check:

   At least once = possible duplicates [OK]

Hint: Receiver crash before ack causes duplicates in 'At least once' [OK]

Common Mistakes:

• Assuming message is lost on crash
• Confusing 'At least once' with 'At most once'
• Ignoring possibility of duplicates

4. A system claims to provide 'Exactly once' delivery but sometimes messages are duplicated. What is the most likely cause?

medium

A. The system uses 'At least once' delivery without deduplication

B. The system uses 'At most once' delivery internally

C. The system does not handle acknowledgments properly

D. The system drops messages on network failure

Solution

1. Step 1: Identify cause of duplicates in 'Exactly once'

   Duplicates usually happen if 'At least once' is used without deduplication.

2. Step 2: Evaluate other options

   'At most once' causes losses not duplicates, improper ack handling or dropping messages cause losses or retries but exactly once needs deduplication atop at least once.

3. Final Answer:

   The system uses 'At least once' delivery without deduplication -> Option A

4. Quick Check:

   Duplicates mean missing deduplication in 'At least once' [OK]

Hint: Duplicates in 'Exactly once' mean missing deduplication [OK]

Common Mistakes:

• Assuming 'At most once' causes duplicates
• Ignoring deduplication step
• Confusing message loss with duplication

5. You are designing a payment processing system that must never lose or duplicate transactions. Which message delivery guarantee should you choose and why?

hard

A. At most once, because it avoids duplicates

B. At least once, because it ensures no message loss

C. Exactly once, because it guarantees no loss and no duplicates

D. None, because message delivery guarantees are not relevant

Solution

1. Step 1: Identify system requirements

   Payment processing must avoid losing or duplicating transactions.

2. Step 2: Match guarantee to requirements

   'Exactly once' ensures messages are delivered once without loss or duplication, fitting the need.

3. Final Answer:

   Exactly once, because it guarantees no loss and no duplicates -> Option C

4. Quick Check:

   Payment systems need exactly once delivery [OK]

Hint: No loss or duplicates means choose 'Exactly once' [OK]

Common Mistakes:

• Choosing 'At most once' and risking loss
• Choosing 'At least once' and risking duplicates
• Ignoring delivery guarantees in critical systems