Fair dispatch with prefetch in RabbitMQ - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the purpose of prefetch in RabbitMQ

What is the main purpose of setting the prefetch count in RabbitMQ when using fair dispatch?

A To ensure a consumer receives only one message at a time until it acknowledges it

B To limit the number of messages sent over the network at once

C To increase the message queue size on the broker

D To automatically reject messages that take too long to process

💡 Hint

Think about how RabbitMQ controls message flow to consumers to balance workload.

Setting prefetch=1 tells RabbitMQ to send only one unacknowledged message at a time to a consumer. This helps distribute messages fairly among consumers by not overwhelming any single one.

💻 Command Output

intermediate

Effect of prefetch count on message delivery

Given two consumers connected to the same RabbitMQ queue with prefetch=1, what will be the output behavior when 4 messages are published?

RabbitMQ

Consumer1: prefetch=1
Consumer2: prefetch=1
Publish 4 messages to queue

A Consumer1 receives all 4 messages before Consumer2 gets any

B Both consumers receive 2 messages each simultaneously

C Messages are sent randomly without regard to prefetch settings

D Messages are distributed one by one alternately between Consumer1 and Consumer2

💡 Hint

Consider how prefetch=1 affects message dispatch to multiple consumers.

With prefetch=1, RabbitMQ sends one message to a consumer and waits for its acknowledgment before sending another. This causes messages to be distributed fairly, alternating between consumers.

❓ Configuration

advanced

Configuring prefetch count in RabbitMQ consumer code

Which of the following code snippets correctly sets the prefetch count to 1 in a RabbitMQ consumer using Python's pika library?

A

channel.set_qos(prefetch=1)
channel.consume('task_queue', callback)

B channel.basic_consume(queue='task_queue', on_message_callback=callback, prefetch_count=1)

C

channel.basic_qos(prefetch_count=1)
channel.basic_consume(queue='task_queue', on_message_callback=callback)

D

channel.qos(prefetch=1)
channel.basic_consume('task_queue', callback)

💡 Hint

Look for the correct method name and parameter for setting prefetch in pika.

The basic_qos method with prefetch_count=1 sets the prefetch count correctly. The basic_consume method does not accept a prefetch parameter.

❓ Troubleshoot

advanced

Diagnosing message imbalance despite prefetch=1

You set prefetch=1 for two consumers on the same queue, but one consumer receives most messages while the other is idle. What is the most likely cause?

A The idle consumer is not acknowledging messages, causing RabbitMQ to stop sending it new messages

B Prefetch count must be set to 0 to enable fair dispatch

C RabbitMQ does not support fair dispatch with multiple consumers

D The queue is configured as exclusive, so only one consumer can receive messages

💡 Hint

Think about how unacknowledged messages affect message flow.

If a consumer does not acknowledge messages, RabbitMQ will not send it more messages, causing imbalance. Prefetch=1 only limits unacknowledged messages but does not force acknowledgments.

🔀 Workflow

expert

Order of steps to implement fair dispatch with prefetch in RabbitMQ

Arrange the following steps in the correct order to implement fair dispatch with prefetch=1 in a RabbitMQ consumer application.

A 1,3,2,4

B 1,2,3,4

C 3,1,2,4

D 2,1,3,4

💡 Hint

Consider the logical order of setup before starting consumption.

You must first create the connection and channel, then set prefetch count, define the callback, and finally start consuming messages.