Adding and removing nodes in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of adding a new node to a Redis cluster?

You have a Redis cluster with 3 nodes. You run the command to add a new node to the cluster. What will the cluster nodes list show after adding the new node but before resharding?

Redis

redis-cli --cluster add-node 192.168.1.4:7004 192.168.1.1:7000
redis-cli --cluster nodes 192.168.1.1:7000

A The new node appears in the cluster nodes list with no slots assigned.

B The new node appears with all slots assigned to it.

C The new node does not appear until resharding is done.

D The cluster nodes list shows an error because slots are not balanced.

💡 Hint

Think about what happens immediately after adding a node before moving slots.

When you add a new node to a Redis cluster, it joins the cluster but initially has no hash slots assigned. Slots must be manually resharded to the new node.

❓ query_result

intermediate

What happens when you remove a node that holds slots without resharding?

In a Redis cluster, you remove a node that currently holds hash slots without migrating its slots first. What will be the state of the cluster?

Redis

redis-cli --cluster del-node 192.168.1.2:7002 <node-id>

A The node is removed and slots are lost but cluster continues normally.

B The cluster automatically migrates slots to other nodes.

C The cluster enters an error state and refuses writes to affected slots.

D The command fails because slots must be migrated before removal.

💡 Hint

Consider what happens to data when slots are suddenly missing.

Removing a node that holds slots without migrating them causes the cluster to lose those slots, leading to an error state and inability to serve keys in those slots.

📝 Syntax

advanced

Which command correctly adds a node to a Redis cluster?

You want to add a new node at IP 10.0.0.5 port 7005 to an existing cluster with a node at 10.0.0.1 port 7000. Which command is syntactically correct?

A redis-cli --cluster add-node 10.0.0.5:7005 10.0.0.1:7000

B redis-cli --cluster add-node 10.0.0.1:7000 10.0.0.5:7005

C redis-cli --cluster add-node --node 10.0.0.5:7005 --cluster 10.0.0.1:7000

D redis-cli --add-node 10.0.0.5:7005 10.0.0.1:7000

💡 Hint

The syntax is: redis-cli --cluster add-node

The correct syntax for adding a node is to specify the new node address first, then an existing cluster node address.

❓ optimization

advanced

Best practice to minimize downtime when removing a Redis cluster node

You want to remove a node from a Redis cluster with minimal downtime. Which approach is best?

A Remove the node and rely on cluster auto-recovery to fix slots.

B Migrate slots from the node to be removed to other nodes, then remove the node.

C Remove the node immediately and then reshard slots manually.

D Stop the entire cluster, remove the node, then restart the cluster.

💡 Hint

Think about how to keep data available during node removal.

Migrating slots before removal ensures data is available on other nodes, preventing downtime.

🧠 Conceptual

expert

Why is it important to reshard slots after adding a new node to a Redis cluster?

After adding a new node to a Redis cluster, why must you reshard slots to the new node?

A To update the cluster configuration file automatically.

B Because Redis requires manual slot assignment for security reasons.

C Because the new node cannot join the cluster without slots.

D To balance the data load and improve cluster performance.

💡 Hint

Think about what happens if slots are unevenly distributed.

Resharding distributes hash slots to the new node, balancing data and load across the cluster.

Practice

1. What is the primary purpose of adding nodes to a Redis cluster?

easy

A. To increase the cluster's capacity and distribute data

B. To delete all data from the cluster

C. To backup the cluster data

D. To change the Redis version on all nodes

Solution

1. Step 1: Understand what adding nodes means

   Adding nodes means increasing the number of servers in the cluster to share the data load.

2. Step 2: Identify the effect of adding nodes

   More nodes mean more capacity and better data distribution across the cluster.

3. Final Answer:

   To increase the cluster's capacity and distribute data -> Option A

4. Quick Check:

   Adding nodes = More capacity and distribution [OK]

Hint: Adding nodes grows cluster capacity and data spread [OK]

Common Mistakes:

• Thinking adding nodes deletes data
• Confusing adding nodes with backup
• Assuming adding nodes upgrades Redis version

2. Which of the following is the correct syntax to add a node to a Redis cluster using redis-cli?

easy

A. redis-cli --cluster add-node :<port> :<port>

B. redis-cli --cluster remove-node :<port>

C. redis-cli --cluster create-node :<port>

D. redis-cli --cluster add :<port>

Solution

1. Step 1: Recall the command to add a node

   The correct command uses --cluster add-node followed by the new node and an existing node in the cluster.

2. Step 2: Match the syntax

   redis-cli --cluster add-node :<port> :<port> matches the correct syntax: redis-cli --cluster add-node new-node existing-node.

3. Final Answer:

   redis-cli --cluster add-node <new-node-ip>:<port> <existing-node-ip>:<port> -> Option A

4. Quick Check:

   Correct add-node syntax = redis-cli --cluster add-node :<port> :<port> [OK]

Hint: Add node command uses --cluster add-node with new and existing nodes [OK]

Common Mistakes:

• Using remove-node instead of add-node
• Using create-node which does not exist
• Omitting existing node IP in command

3. Given the command redis-cli --cluster del-node 192.168.1.5:7000 07c37dfeb2352e03a5a1e1f3a2f8b9a1f2e3d4c5, what will happen?

medium

A. A new node will be added at 192.168.1.5:7000

B. The node at 192.168.1.5:7000 will be removed from the cluster

C. The cluster will be created with the given node ID

D. The command will fail due to wrong syntax

Solution

1. Step 1: Understand the del-node command

   The command redis-cli --cluster del-node removes a node from the cluster using its IP and node ID.

2. Step 2: Analyze the given command

   The command specifies the node IP and the node ID, so it will remove that node from the cluster.

3. Final Answer:

   The node at 192.168.1.5:7000 will be removed from the cluster -> Option B

4. Quick Check:

   del-node removes specified node [OK]

Hint: del-node removes node by IP and node ID [OK]

Common Mistakes:

• Confusing del-node with add-node
• Thinking it adds a node
• Assuming command syntax is incorrect

4. You run redis-cli --cluster add-node 192.168.1.10:7003 192.168.1.11:7000 but get an error. What is the most likely cause?

medium

A. The new node is already part of the cluster

B. The existing node IP is incorrect

C. The command should be --cluster del-node instead

D. Redis server is not running on the new node

Solution

1. Step 1: Check prerequisites for adding a node

   The new node must be running Redis and reachable for the add-node command to succeed.

2. Step 2: Identify common causes of add-node failure

   If Redis is not running on the new node, the command will fail because it cannot connect.

3. Final Answer:

   Redis server is not running on the new node -> Option D

4. Quick Check:

   New node must run Redis to add [OK]

Hint: New node must be running Redis before adding [OK]

Common Mistakes:

• Assuming IP typo without checking server status
• Confusing add-node with del-node command
• Ignoring that Redis must run on new node

5. You want to remove a node from a Redis cluster but keep its data safe. Which sequence of commands should you use?

hard

A. Use redis-cli --cluster del-node then backup data manually

B. Directly stop the Redis server on the node and remove it

C. Use redis-cli --cluster reshard to move slots, then redis-cli --cluster del-node

D. Use redis-cli --cluster add-node to replace the node

Solution

1. Step 1: Understand data safety when removing nodes

   Removing a node without moving its data causes data loss.

2. Step 2: Use reshard to move data slots

   Resharding moves the data slots from the node to be removed to other nodes safely.

3. Step 3: Remove the node after resharding

   After data is moved, del-node safely removes the node without data loss.

4. Final Answer:

   Use redis-cli --cluster reshard to move slots, then redis-cli --cluster del-node -> Option C

5. Quick Check:

   Reshard then del-node = safe removal [OK]

Hint: Reshard slots before removing node to keep data safe [OK]

Common Mistakes:

• Removing node without moving data first
• Stopping server directly causing data loss
• Confusing add-node with removal process