Message ordering guarantees in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the message ordering type that ensures messages are received in the exact order they were sent.

HLD

message_ordering = "[1]"

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

Aat-least-once

Bunordered

CFIFO

Dat-most-once

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing ordering with delivery guarantees like at-least-once.

FIFO (First In First Out) ordering guarantees that messages are received in the order they were sent.

2fill in blank

medium

Complete the code to select the message ordering guarantee that allows messages to be delivered more than once but preserves order.

HLD

message_guarantee = "[1]"

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

Aat-least-once

Bunordered

Cbest-effort

Dexactly-once

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing exactly-once which is harder to achieve.

At-least-once delivery may deliver duplicates but can preserve message order if combined with FIFO.

3fill in blank

hard

Fix the error in the code to correctly represent a system that guarantees no message reordering.

HLD

if system_guarantee == "[1]":
    process_messages_in_order()

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

Aunordered

BFIFO

Cat-most-once

Dbest-effort

Attempts:

3 left

💡 Hint

Common Mistakes

Using unordered or best-effort which do not guarantee order.

FIFO guarantees no message reordering, so the system should check for FIFO to process messages in order.

4fill in blank

hard

Fill both blanks to complete the code that filters messages to ensure only those with correct ordering and delivery guarantees are processed.

HLD

if message.ordering == "[1]" and message.delivery == "[2]":
    process(message)

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

AFIFO

Bat-least-once

Cunordered

Dexactly-once

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing unordered with exactly-once which is rare and complex.

Processing messages with FIFO ordering and at-least-once delivery ensures order is preserved and messages are delivered at least once.

5fill in blank

hard

Fill all three blanks to complete the code that creates a message processing pipeline with ordering, delivery, and deduplication steps.

HLD

pipeline = ["[1]", "[2]", "[3]"]

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

Aordering

Bdeduplication

Cdelivery

Dencryption

Attempts:

3 left

💡 Hint

Common Mistakes

Placing deduplication before delivery or mixing encryption which is unrelated.

The pipeline first ensures ordering, then delivery guarantees, and finally deduplication to handle duplicates.

Practice

1. Which message ordering guarantee ensures messages from the same sender arrive in the order they were sent?

easy

A. Random ordering

B. Total ordering

C. No ordering guarantee

D. FIFO (First In, First Out) ordering

Solution

1. Step 1: Understand FIFO ordering

   FIFO means messages from one sender arrive in the exact order they were sent.

2. Step 2: Compare with other options

   Total ordering orders all messages globally, not just per sender. No ordering or random ordering do not guarantee order.

3. Final Answer:

   FIFO (First In, First Out) ordering -> Option D

4. Quick Check:

   Per-sender order = FIFO [OK]

Hint: FIFO orders per sender, total orders globally [OK]

Common Mistakes:

• Confusing total ordering with per-sender ordering
• Thinking no ordering guarantees any order
• Assuming random ordering preserves send order

2. Which of the following is the correct way to describe total ordering in message systems?

easy

A. All messages from all senders are delivered in the same global order.

B. Messages from each sender arrive in order, but different senders' messages may be unordered.

C. Messages can arrive in any order without restrictions.

D. Messages are delivered in reverse order of sending.

Solution

1. Step 1: Define total ordering

   Total ordering means all messages, regardless of sender, are delivered in one global sequence.

2. Step 2: Eliminate incorrect options

   Messages from each sender arrive in order, but different senders' messages may be unordered. describes FIFO, C means no ordering, D is reverse order which is not standard.

3. Final Answer:

   All messages from all senders are delivered in the same global order. -> Option A

4. Quick Check:

   Total ordering = global sequence [OK]

Hint: Total ordering means one global message sequence [OK]

Common Mistakes:

• Mixing FIFO with total ordering
• Assuming messages can be unordered in total ordering
• Confusing reverse order with total order

3. Consider a messaging system where two senders, S1 and S2, send messages as follows:
S1: M1, M2
S2: N1, N2
If the system guarantees FIFO ordering only, which of the following sequences is possible at the receiver?

medium

A. [M2, M1, N1, N2]

B. [N1, M2, M1, N2]

C. [M1, N1, M2, N2]

D. [N2, N1, M1, M2]

Solution

1. Step 1: Understand FIFO per sender

   Messages from each sender must arrive in order: S1's M1 before M2, S2's N1 before N2.

2. Step 2: Check sequences for FIFO compliance

   [M1, N1, M2, N2] respects order for both senders. Options A, B, D break order for at least one sender.

3. Final Answer:

   [M1, N1, M2, N2] -> Option C

4. Quick Check:

   Sender order preserved = FIFO [OK]

Hint: Check each sender's message order only for FIFO [OK]

Common Mistakes:

• Mixing messages from different senders' order
• Assuming total order with FIFO guarantee
• Ignoring sender-specific order

4. A system claims to provide total ordering but sometimes delivers messages out of order. Which of the following is the most likely cause?

medium

A. Using FIFO queues per sender without global coordination

B. Messages are duplicated at the receiver

C. Messages are sent in parallel but processed sequentially

D. Network delays causing message reordering without coordination

Solution

1. Step 1: Understand total ordering requirements

   Total ordering requires global coordination to maintain one sequence for all messages.

2. Step 2: Identify cause of out-of-order delivery

   Using only FIFO per sender (no global coordination) can cause messages from different senders to arrive unordered, violating total order.

3. Final Answer:

   Using FIFO queues per sender without global coordination -> Option A

4. Quick Check:

   Total order needs global coordination [OK]

Hint: Total order needs global coordination, FIFO alone is insufficient [OK]

Common Mistakes:

• Blaming network delays without considering coordination
• Confusing message duplication with ordering
• Assuming parallel sending guarantees order

5. You are designing a chat application where message order is critical only per user, but global order is not required. Which message ordering guarantee should you choose to optimize for performance?

hard

A. Total ordering to ensure all messages are globally ordered

B. FIFO ordering per sender to maintain each user's message order

C. No ordering guarantee to maximize throughput

D. Random ordering to balance load

Solution

1. Step 1: Analyze application requirements

   Chat needs messages from each user in order, but no need for global order across users.

2. Step 2: Choose ordering guarantee for performance

   FIFO per sender meets the requirement and is more efficient than total ordering, which is costly.

3. Final Answer:

   FIFO ordering per sender to maintain each user's message order -> Option B

4. Quick Check:

   Per-user order only = FIFO [OK]

Hint: Choose FIFO if only per-sender order matters [OK]

Common Mistakes:

• Choosing total order unnecessarily, hurting performance
• Ignoring ordering needs and picking no guarantee
• Confusing random order with any guarantee