Node decommissioning and scaling in Hadoop - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is node decommissioning in Hadoop?

Node decommissioning is the process of safely removing a node from a Hadoop cluster without losing data or interrupting running jobs. It ensures data blocks are replicated elsewhere before the node is taken offline.

beginner

Why is node decommissioning important before scaling down a Hadoop cluster?

Decommissioning ensures that data stored on the node is copied to other nodes, preventing data loss. It also allows running tasks to finish or move, maintaining cluster stability during scaling down.

intermediate

What happens during the node decommissioning process in Hadoop?

The node is marked for decommissioning, the cluster replicates its data blocks to other nodes, running tasks are completed or moved, and finally the node is removed from the cluster.

beginner

How does Hadoop handle scaling up the cluster?

Scaling up means adding new nodes to the cluster. Hadoop automatically detects new nodes, balances data across them, and uses them for storage and processing to improve performance.

intermediate

What is the role of the 'exclude' file in Hadoop node decommissioning?

The 'exclude' file lists nodes to be decommissioned. Hadoop reads this file to know which nodes to remove safely from the cluster during decommissioning.

What is the first step in safely removing a node from a Hadoop cluster?

A Delete data on the node

B Turn off the node immediately

C Mark the node for decommissioning

D Add new nodes to the cluster

The first step is to mark the node for decommissioning so Hadoop can start replicating data and moving tasks.

Which file in Hadoop specifies nodes to be decommissioned?

A include file

B exclude file

C config file

D hosts file

The exclude file lists nodes that should be decommissioned from the cluster.

What does scaling up a Hadoop cluster involve?

A Adding nodes

B Deleting data

C Stopping all jobs

D Removing nodes

Scaling up means adding new nodes to increase cluster capacity.

Why must data be replicated before node decommissioning?

A To reduce cluster size

B To speed up the node

C To delete old data

D To prevent data loss

Data replication ensures no data is lost when a node is removed.

What happens to running tasks on a node during decommissioning?

A They finish or move to other nodes

B They are immediately stopped

C They continue running without change

D They are deleted

Running tasks are allowed to finish or are moved to other nodes to maintain job integrity.

Explain the process and importance of node decommissioning in a Hadoop cluster.

Think about what happens before a node is removed to keep data safe and jobs running.

Describe how scaling up and scaling down work in Hadoop clusters.

Consider what happens when you want to grow or shrink your cluster.

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