Node decommissioning and scaling in Hadoop - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Node Decommissioning Purpose

Why is node decommissioning important in a Hadoop cluster?

A It automatically upgrades the node's hardware without downtime.

B It speeds up data processing by shutting down all nodes simultaneously.

C It allows safe removal of nodes without data loss by redistributing data blocks.

D It permanently deletes all data stored on the node before removal.

💡 Hint

Think about what happens to data when a node is removed.

Decommissioning ensures data blocks on the node are copied to other nodes to prevent data loss before the node is removed.

❓ Predict Output

intermediate

Output of Node Decommissioning Status Command

What is the output of the following command snippet when checking node decommissioning status?

Hadoop

hdfs dfsadmin -report | grep Decommissioning

A Decommissioning nodes: 0

B No decommissioning nodes found

C Error: Command not found

D

Decommissioning nodes: 2
Node1
Node2

💡 Hint

Consider what the command does and what it filters.

The command lists nodes currently being decommissioned, so it shows the count and their names.

❓ data_output

advanced

Data Block Distribution After Node Decommissioning

Given a Hadoop cluster with 5 nodes and replication factor 3, if one node is decommissioned, how many copies of each data block remain after decommissioning completes?

A 3 copies, redistributed to remaining 4 nodes

B 2 copies, because one node is removed

C 1 copy, on a single node only

D 4 copies, due to automatic replication increase

💡 Hint

Think about how replication factor affects data safety during node removal.

The replication factor remains the same; data blocks are copied to other nodes to maintain 3 copies.

🔧 Debug

advanced

Identifying Issue in Node Decommissioning Process

Which option shows a likely cause for a node stuck in decommissioning state indefinitely?

A Network issues preventing data replication to other nodes

B Node has no data blocks to replicate

C Decommissioning completed successfully

D Node is offline and removed from cluster

💡 Hint

Consider what could block data movement during decommissioning.

If the node cannot replicate data due to network problems, it will remain stuck in decommissioning.

🚀 Application

expert

Scaling Cluster by Adding Nodes

You want to scale your Hadoop cluster by adding 3 new nodes. Which step should you perform first to integrate them properly?

A Immediately start running MapReduce jobs on new nodes without configuration

B Add new nodes to the cluster configuration and start DataNode services on them

C Decommission existing nodes before adding new ones

D Remove old nodes from the cluster configuration

💡 Hint

Think about how new nodes join a Hadoop cluster.

New nodes must be added to the cluster config and have DataNode services started to join the cluster properly.

Practice

1. What is the main purpose of node decommissioning in a Hadoop cluster?

easy

A. To safely remove a node without losing data

B. To add more nodes to the cluster

C. To restart all nodes simultaneously

D. To backup data from the cluster

Solution

1. Step 1: Understand node decommissioning

   Node decommissioning is the process of removing a node from the cluster safely.

2. Step 2: Identify the purpose

   The goal is to ensure no data loss while removing the node.

3. Final Answer:

   To safely remove a node without losing data -> Option A

4. Quick Check:

   Node decommissioning = safe node removal [OK]

Hint: Decommission means safe removal without data loss [OK]

Common Mistakes:

• Confusing decommissioning with adding nodes
• Thinking it restarts nodes
• Assuming it backs up data

2. Which command is used to refresh the list of nodes after updating the exclude file for decommissioning?

easy

A. hdfs namenode -format

B. hdfs dfsadmin -refreshNodes

C. hdfs datanode -report

D. hdfs fsck /

Solution

1. Step 1: Identify the command for refreshing nodes

   After updating the exclude file, the cluster needs to refresh node status.

2. Step 2: Match the correct command

   The command hdfs dfsadmin -refreshNodes tells the NameNode to reload the exclude file.

3. Final Answer:

   hdfs dfsadmin -refreshNodes -> Option B

4. Quick Check:

   Refresh nodes command = dfsadmin -refreshNodes [OK]

Hint: Refresh nodes after exclude file update with dfsadmin -refreshNodes [OK]

Common Mistakes:

• Using namenode -format which erases metadata
• Confusing datanode -report with refresh
• Running fsck which checks file system health

3. Given the following exclude file content:
node1.example.com node3.example.com
and the cluster nodes are node1, node2, node3, node4.
After running hdfs dfsadmin -refreshNodes, which nodes will be decommissioned?

medium

A. All nodes

B. node2 and node4

C. node1, node2, node3

D. node1 and node3

Solution

1. Step 1: Understand exclude file role

   The exclude file lists nodes to be decommissioned.

2. Step 2: Identify nodes in exclude file

   Nodes node1.example.com and node3.example.com are listed, so these two will be decommissioned.

3. Final Answer:

   node1 and node3 -> Option D

4. Quick Check:

   Exclude file nodes = decommissioned nodes [OK]

Hint: Exclude file lists nodes to decommission [OK]

Common Mistakes:

• Assuming nodes not in exclude file are decommissioned
• Thinking all nodes are decommissioned
• Confusing node hostnames

4. You updated the exclude file to remove a node but forgot to run hdfs dfsadmin -refreshNodes. What is the likely result?

medium

A. The node remains active and is not decommissioned

B. The node is immediately decommissioned

C. The cluster shuts down

D. The exclude file is ignored and deleted

Solution

1. Step 1: Understand refreshNodes role

   The refreshNodes command tells the NameNode to reload the exclude file.

2. Step 2: Consequence of not running refresh

   If refreshNodes is not run, the NameNode does not know about the updated exclude file, so the node stays active.

3. Final Answer:

   The node remains active and is not decommissioned -> Option A

4. Quick Check:

   No refreshNodes = no decommission [OK]

Hint: Always run refreshNodes after exclude file update [OK]

Common Mistakes:

• Assuming node decommissions automatically
• Thinking cluster shuts down
• Believing exclude file deletes itself

5. You want to scale your Hadoop cluster by adding 3 new nodes and safely remove 2 old nodes. Which sequence of actions is correct?

hard

A. Add new nodes, restart cluster, update exclude file

B. Add new nodes, update exclude file with old nodes, run hdfs dfsadmin -refreshNodes

C. Update exclude file with old nodes, run hdfs dfsadmin -refreshNodes, add new nodes

D. Remove old nodes physically, then add new nodes

Solution

1. Step 1: Decommission old nodes first

   Update the exclude file with old nodes to remove and run refreshNodes to start decommissioning safely.

2. Step 2: Add new nodes after old nodes are decommissioned

   Once old nodes are safely removed, add new nodes to scale the cluster.

3. Final Answer:

   Update exclude file with old nodes, run hdfs dfsadmin -refreshNodes, add new nodes -> Option C

4. Quick Check:

   Decommission old nodes before adding new ones [OK]

Hint: Decommission old nodes before adding new ones [OK]

Common Mistakes:

• Adding new nodes before decommissioning old ones
• Physically removing nodes without decommissioning
• Restarting cluster unnecessarily