Publisher confirms in RabbitMQ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Publisher Confirms in RabbitMQ?

Publisher Confirms is a RabbitMQ feature that lets a message sender know when a message has been safely received and stored by the broker. It helps ensure messages are not lost.

beginner

How does a publisher enable confirms in RabbitMQ?

The publisher enables confirms by putting the channel into confirm mode using the command channel.confirmSelect(). This tells RabbitMQ to send acknowledgments for published messages.

beginner

What does a positive confirmation (ack) from RabbitMQ mean?

A positive confirmation (ack) means RabbitMQ has received and stored the message safely. The publisher can then consider the message delivered.

intermediate

What should a publisher do if it receives a negative confirmation (nack)?

If the publisher receives a negative confirmation (nack), it means the message was not processed correctly. The publisher should retry sending the message or handle the failure accordingly.

intermediate

Why are Publisher Confirms preferred over transactions in RabbitMQ?

Publisher Confirms are faster and more efficient than transactions because they allow asynchronous acknowledgments without blocking the publisher, improving performance.

How does a publisher enable Publisher Confirms in RabbitMQ?

A By calling <code>channel.confirmSelect()</code>

B By starting a transaction with <code>channel.txSelect()</code>

C By setting <code>autoAck=true</code>

D By declaring a durable queue

The publisher enables confirms by calling channel.confirmSelect() to put the channel into confirm mode.

What does a positive confirmation (ack) from RabbitMQ indicate?

A Message was lost

B Message was expired

C Message was rejected

D Message was received and stored safely

A positive confirmation means RabbitMQ has safely received and stored the message.

What should a publisher do when it receives a negative confirmation (nack)?

A Ignore it

B Retry sending the message or handle failure

C Close the connection immediately

D Delete the queue

A negative confirmation means the message was not processed correctly, so the publisher should retry or handle the failure.

Why are Publisher Confirms preferred over transactions in RabbitMQ?

A They allow asynchronous acknowledgments and improve performance

B They block the publisher

C They require manual message deletion

D They are slower but safer

Publisher Confirms allow asynchronous acknowledgments without blocking, making them faster and more efficient than transactions.

Which RabbitMQ feature helps ensure messages are not lost after publishing?

A Exclusive queues

B Auto-delete queues

C Publisher Confirms

D Message TTL

Publisher Confirms help the publisher know when messages are safely stored, reducing message loss.

Explain how Publisher Confirms work in RabbitMQ and why they are useful.

Think about how the publisher knows the message is safe.

Describe the steps a publisher should take when using Publisher Confirms to handle message delivery.

Consider the message lifecycle from sending to confirmation.

Practice

1. What does enabling publisher confirms in RabbitMQ do?

easy

A. It automatically retries sending messages on failure.

B. It encrypts messages before sending.

C. It tells you if RabbitMQ accepted your message.

D. It queues messages without sending them.

Solution

1. Step 1: Understand the purpose of publisher confirms

   Publisher confirms notify the sender if the message was accepted by RabbitMQ.

2. Step 2: Compare with other options

   Retrying, encrypting, or queuing without sending are unrelated to confirms.

3. Final Answer:

   It tells you if RabbitMQ accepted your message. -> Option C

4. Quick Check:

   Publisher confirms = message acceptance notification [OK]

Hint: Publisher confirms confirm message acceptance by RabbitMQ [OK]

Common Mistakes:

• Confusing confirms with message encryption
• Thinking confirms retry messages automatically
• Assuming confirms delay message sending

2. Which method must be called on a RabbitMQ channel to enable publisher confirms?

easy

A. enableConfirms()

B. setConfirmMode()

C. startConfirm()

D. confirmSelect()

Solution

1. Step 1: Recall the correct method name

   The method to enable confirms on a channel is confirmSelect().

2. Step 2: Verify other options

   Other method names like enableConfirms() or startConfirm() do not exist in RabbitMQ API.

3. Final Answer:

   confirmSelect() -> Option D

4. Quick Check:

   Enable confirms = confirmSelect() [OK]

Hint: Enable confirms by calling confirmSelect() on the channel [OK]

Common Mistakes:

• Using incorrect method names like enableConfirms()
• Forgetting to call confirmSelect() before publishing
• Confusing confirmSelect() with waitForConfirms()

3. Given this code snippet, what will waitForConfirms() return if the message was accepted?

channel.confirmSelect();
channel.basicPublish(exchange, routingKey, props, body);
boolean confirmed = channel.waitForConfirms();
System.out.println(confirmed);

medium

A. false

B. true

C. null

D. Exception thrown

Solution

1. Step 1: Understand waitForConfirms() behavior

   This method returns true if RabbitMQ accepted the message, false otherwise.

2. Step 2: Given the message was accepted

   The variable confirmed will be true, so printing it outputs 'true'.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Message accepted = waitForConfirms() returns true [OK]

Hint: waitForConfirms() returns true if message accepted [OK]

Common Mistakes:

• Expecting waitForConfirms() to return false on success
• Assuming it returns null or throws exception on success
• Confusing waitForConfirms() with asynchronous callbacks

4. What is wrong with this code snippet for enabling publisher confirms?

channel.basicPublish(exchange, routingKey, props, body);
channel.confirmSelect();
boolean confirmed = channel.waitForConfirms();

medium

A. confirmSelect() must be called before basicPublish()

B. waitForConfirms() cannot be called after publishing

C. basicPublish() requires a callback parameter

D. confirmSelect() should be called after waitForConfirms()

Solution

1. Step 1: Check order of method calls

   confirmSelect() must be called before publishing to enable confirms.

2. Step 2: Identify the error

   In the snippet, confirmSelect() is called after basicPublish(), so confirms are not enabled when publishing.

3. Final Answer:

   confirmSelect() must be called before basicPublish() -> Option A

4. Quick Check:

   Enable confirms before publishing [OK]

Hint: Call confirmSelect() before publishing messages [OK]

Common Mistakes:

• Calling confirmSelect() after publishing
• Thinking waitForConfirms() disables confirms
• Believing basicPublish() needs a callback for confirms

5. You want to ensure your application only proceeds after RabbitMQ confirms message acceptance. Which sequence of calls correctly implements this using publisher confirms?

hard

A. channel.confirmSelect(); channel.basicPublish(...); channel.waitForConfirms();

B. channel.basicPublish(...); channel.waitForConfirms(); channel.confirmSelect();

C. channel.waitForConfirms(); channel.confirmSelect(); channel.basicPublish(...);

D. channel.basicPublish(...); channel.confirmSelect(); channel.waitForConfirms();

Solution

1. Step 1: Enable confirms before publishing

   Call confirmSelect() first to enable confirms on the channel.

2. Step 2: Publish the message and wait for confirmation

   After basicPublish(...), call waitForConfirms() to block until RabbitMQ confirms acceptance.

3. Final Answer:

   channel.confirmSelect(); channel.basicPublish(...); channel.waitForConfirms(); -> Option A

4. Quick Check:

   Enable confirms, publish, then wait [OK]

Hint: Enable confirms, publish, then waitForConfirms() [OK]

Common Mistakes:

• Calling confirmSelect() after publishing
• Waiting for confirms before publishing
• Not calling waitForConfirms() at all