Mirrored queues for redundancy in RabbitMQ - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Mirrored Queues in RabbitMQ

What is the primary purpose of using mirrored queues in RabbitMQ?

A To increase message throughput by parallel processing

B To replicate messages across multiple nodes for high availability

C To encrypt messages during transmission

D To compress messages to save storage space

💡 Hint

Think about what redundancy means in message queuing.

Mirrored queues replicate messages across nodes to ensure availability if one node fails.

💻 Command Output

intermediate

Check Mirrored Queue Status

What is the output of the following RabbitMQ command when checking the status of a mirrored queue named task_queue?

rabbitmqctl list_queues name policy

A Error: queue not found

B task_queue lazy

C task_queue none

D task_queue all

💡 Hint

Mirrored queues use the all policy by default.

The all policy means the queue is mirrored across all nodes.

❓ Configuration

advanced

Configuring a Mirrored Queue Policy

Which of the following commands correctly sets a policy to mirror all queues on all nodes in RabbitMQ?

A rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}'

B rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"nodes"}'

C rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"lazy"}'

D rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"exactly", "ha-params":2}'

💡 Hint

The ha-mode must be set to all to mirror on all nodes.

The command sets a policy named ha-all to mirror all queues matching .* on all nodes.

❓ Troubleshoot

advanced

Troubleshooting Mirrored Queue Failover

You notice that after a node failure, messages are lost from a mirrored queue. Which of the following is the most likely cause?

A The RabbitMQ service was restarted on all nodes simultaneously

B The queue was not configured with a mirrored policy

C The messages were published with <code>delivery_mode=1</code> (non-persistent)

D The consumers were not connected to the master node

💡 Hint

Consider message durability and persistence in RabbitMQ.

Non-persistent messages (delivery_mode=1) are not saved to disk and can be lost on node failure even with mirrored queues.

🔀 Workflow

expert

Steps to Create and Verify a Mirrored Queue

Arrange the steps in the correct order to create a mirrored queue named logs and verify its mirroring status in RabbitMQ.

A 1,2,3,4

B 2,1,4,3

C 1,2,4,3

D 2,4,1,3

💡 Hint

Policies must be set before queue declaration to apply mirroring.

First set the mirroring policy, then declare the queue so it inherits the policy, verify the policy, then publish messages.

Practice

1. What is the main purpose of using mirrored queues in RabbitMQ?

easy

A. To keep copies of messages on multiple servers for safety

B. To speed up message delivery by duplicating messages

C. To reduce the number of messages stored in the queue

D. To encrypt messages automatically during transmission

Solution

1. Step 1: Understand mirrored queues concept

   Mirrored queues replicate messages across multiple RabbitMQ nodes to ensure no message is lost if one node fails.

2. Step 2: Identify the main benefit

   This replication provides redundancy and safety, not speed or encryption.

3. Final Answer:

   To keep copies of messages on multiple servers for safety -> Option A

4. Quick Check:

   Mirroring = Redundancy and safety [OK]

Hint: Mirrored queues copy messages for safety, not speed or encryption [OK]

Common Mistakes:

• Thinking mirroring speeds up delivery
• Confusing mirroring with encryption
• Assuming mirroring reduces message count

2. Which command correctly sets a policy to mirror all queues in RabbitMQ?

easy

A. rabbitmqctl set_policy ha-all .* '{"ha-mode":"all"}'

B. rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}'

C. rabbitmqctl set_policy ha-all ".*" '{ha-mode:all}'

D. rabbitmqctl set_policy ha-all ".*" '{"ha-mode":all}'

Solution

1. Step 1: Review correct syntax for set_policy

   The command requires the policy name, pattern in quotes, and JSON string with keys and values in double quotes.

2. Step 2: Identify correct JSON format and quoting

   rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}' uses correct JSON with double quotes around keys and values and quotes the pattern ".*" properly.

3. Final Answer:

   rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}' -> Option B

4. Quick Check:

   Correct JSON and quoting = rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}' [OK]

Hint: Use double quotes inside JSON and quote pattern with ".*" [OK]

Common Mistakes:

• Missing quotes around JSON keys or values
• Not quoting the pattern string
• Using single quotes inside JSON instead of double quotes

3. Given this policy command:
rabbitmqctl set_policy ha-two ".*" '{"ha-mode":"exactly","ha-params":2}'
What will happen when a queue is created?

medium

A. The queue will be mirrored on one node only

B. The queue will be mirrored on all nodes

C. The queue will be mirrored on exactly two nodes

D. The queue will not be mirrored at all

Solution

1. Step 1: Understand ha-mode 'exactly' with ha-params

   The policy sets ha-mode to 'exactly' with ha-params 2, meaning the queue is mirrored on exactly 2 nodes.

2. Step 2: Apply policy effect on new queues

   Any queue matching ".*" will be mirrored on exactly two nodes as per the policy.

3. Final Answer:

   The queue will be mirrored on exactly two nodes -> Option C

4. Quick Check:

   ha-mode exactly + ha-params 2 = mirror on 2 nodes [OK]

Hint: ha-mode 'exactly' with ha-params sets mirror count [OK]

Common Mistakes:

• Confusing 'all' with 'exactly'
• Ignoring ha-params value
• Assuming no mirroring without ha-mode 'all'

4. You set this policy:
rabbitmqctl set_policy ha-all ".*" '{"ha-mode":"all"}'
But queues are not mirrored. What is the likely problem?

medium

A. The policy requires a restart of the queue or server to apply

B. The pattern ".*" does not match queue names

C. The policy was set but RabbitMQ was not restarted

D. The JSON syntax is incorrect

Solution

1. Step 1: Check policy application behavior

   Policies apply only to new queues or queues restarted after policy set.

2. Step 2: Identify why existing queues are not mirrored

   Existing queues created before policy need to be deleted or restarted to apply mirroring.

3. Final Answer:

   The policy requires a restart of the queue or server to apply -> Option A

4. Quick Check:

   Policy applies only on new or restarted queues [OK]

Hint: Policies apply only to new or restarted queues [OK]

Common Mistakes:

• Thinking RabbitMQ restart is needed
• Assuming pattern ".*" never matches
• Believing JSON syntax is wrong when it is correct

5. You want to ensure a queue named task_queue is mirrored on at least 3 nodes but no more than 5 using a range specification in ha-params. Which policy configuration achieves this?

hard

A. rabbitmqctl set_policy ha-range "^task_queue$" '{"ha-mode":"nodes","ha-params":["node1","node2","node3"]}'

B. rabbitmqctl set_policy ha-minmax ".*" '{"ha-mode":"exactly","ha-params":3}'

C. rabbitmqctl set_policy ha-minmax "^task_queue$" '{"ha-mode":"nodes","ha-params":["node1","node2","node3","node4","node5"]}'

D. rabbitmqctl set_policy ha-minmax "^task_queue$" '{"ha-mode":"exactly","ha-params":"3-5"}'

Solution

1. Step 1: Understand ha-mode 'exactly' with range ha-params

   ha-mode 'exactly' with ha-params as string like '3-5' dynamically mirrors the queue on a number of replicas between 3 and 5.

2. Step 2: Identify correct command

   rabbitmqctl set_policy ha-minmax "^task_queue$" '{"ha-mode":"exactly","ha-params":"3-5"}' matches the queue exactly and uses range specification.

3. Step 3: Check other options

   B uses fixed exactly 3 on all queues; C uses fixed nodes list of 5; D uses fixed nodes list of 3.

4. Final Answer:

   rabbitmqctl set_policy ha-minmax "^task_queue$" '{"ha-mode":"exactly","ha-params":"3-5"}' -> Option D

5. Quick Check:

   ha-mode exactly + ha-params "3-5" = dynamic range 3-5 [OK]

Hint: Use ha-mode 'exactly' with ha-params "3-5" for range [OK]

Common Mistakes:

• Using 'exactly' with fixed integer instead of range string
• Not matching queue name exactly
• Using 'nodes' mode with fixed list instead of range