Rack awareness in HDFS in Hadoop - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the rack ID for a DataNode in HDFS configuration.

Hadoop

dfs.datanode.hostname = [1]

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A/rack1/node1

Brack1-node1

Cnode1

D/rack1

Attempts:

3 left

💡 Hint

Common Mistakes

Using only node name without rack info

Omitting the leading slash in rack ID

The rack ID is specified as a path-like string such as '/rack1/node1' to indicate the rack and node location.

2fill in blank

medium

Complete the code to import the class used for rack awareness in HDFS Java code.

Hadoop

import org.apache.hadoop.net.[1];

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ARackAwareness

BRackPolicy

CRackResolver

DRackManager

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent classes like RackAwareness or RackManager

Confusing policy classes with resolver classes

The class RackResolver is used in Hadoop to resolve rack information for nodes.

3fill in blank

hard

Fix the error in the code to get the rack location of a DataNode using RackResolver.

Hadoop

String rack = RackResolver.[1](nodeName);

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AresolveRack

Bresolve

CgetRack

DgetRackLocation

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names that do not exist in RackResolver

Confusing method names with similar sounding ones

The correct method to get rack info is the static resolve() method on RackResolver.

4fill in blank

hard

Fill both blanks to create a rack-aware block placement policy object in HDFS.

Hadoop

BlockPlacementPolicy policy = new [1](conf, [2]);

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ABlockPlacementPolicyRackFaultTolerant

BBlockPlacementPolicyDefault

Cnull

DclusterMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using default policy instead of rack-aware policy

Passing null instead of cluster map

The BlockPlacementPolicyRackFaultTolerant class is used for rack-aware placement, and it requires a clusterMap object as a parameter.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping DataNodes to their rack locations.

Hadoop

Map<String, String> rackMap = new HashMap<>();
for (String node : nodes) {
    rackMap.put(node, [1].[2](node));
}
System.out.println(rackMap.[3]());

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ARackResolver

Bresolve

CkeySet

Dvalues

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names for rack resolution

Printing values instead of keys

We use RackResolver.resolve(node) to get rack info, and keySet() to print all node keys.

Practice

1. What is the main purpose of rack awareness in HDFS?

easy

A. To place data replicas on different racks to improve fault tolerance

B. To store all data replicas on the same rack for faster access

C. To reduce the number of data replicas stored in the cluster

D. To encrypt data before storing it in HDFS

Solution

1. Step 1: Understand rack awareness concept

   Rack awareness means placing data copies on different racks to avoid data loss if one rack fails.

2. Step 2: Identify the main goal

   The goal is to improve fault tolerance by spreading replicas across racks.

3. Final Answer:

   To place data replicas on different racks to improve fault tolerance -> Option A

4. Quick Check:

   Rack awareness = fault tolerance [OK]

Hint: Think about protecting data from rack failures [OK]

Common Mistakes:

• Assuming all replicas are on the same rack
• Confusing rack awareness with encryption
• Thinking rack awareness reduces replica count

2. Which of the following is the correct way to configure rack awareness in HDFS?

easy

A. Implement a script that maps nodes to racks and set net.topology.script.file.name

B. Disable rack awareness by setting dfs.rack.awareness.enabled to false

C. Set the property dfs.replication to 1

D. Use hdfs dfs -put command with rack option

Solution

1. Step 1: Identify rack awareness configuration

   Rack awareness requires a script that maps nodes to racks.

2. Step 2: Recognize the correct property

   The property net.topology.script.file.name points to this script.

3. Final Answer:

   Implement a script that maps nodes to racks and set net.topology.script.file.name -> Option A

4. Quick Check:

   Rack mapping script = net.topology.script.file.name [OK]

Hint: Rack awareness needs a node-to-rack mapping script [OK]

Common Mistakes:

• Setting replication to 1 disables replication, not rack awareness
• Using HDFS commands with rack options (not supported)
• Disabling rack awareness removes its benefits

3. Given a Hadoop cluster with 3 racks and replication factor 3, how does HDFS place replicas for a block?

medium

A. One replica on the local node, and the other two on different racks

B. Replicas are placed randomly without rack consideration

C. All 3 replicas on the same rack

D. One replica on the local node, one on a different rack, and one on the same rack but different node

Solution

1. Step 1: Recall HDFS replica placement policy

   HDFS places one replica on the local node, one replica on a different rack, and the third replica on the same rack but a different node to balance fault tolerance and network usage.

2. Step 2: Apply to 3 racks and replication factor 3

   One replica is local, one is on a different rack, and the third is on the same rack but different node.

3. Final Answer:

   One replica on the local node, one on a different rack, and one on the same rack but different node -> Option D

4. Quick Check:

   Replica placement = local + different rack + same rack different node [OK]

Hint: One local replica, one different rack, one same rack different node [OK]

Common Mistakes:

• Placing all replicas on the same rack
• Placing two replicas on the same rack as local
• Ignoring rack awareness in placement

4. You configured rack awareness but notice all replicas are stored on the same rack. What is the likely cause?

medium

A. Replication factor is set to 1

B. The rack awareness script is missing or misconfigured

C. HDFS does not support rack awareness

D. Data nodes are all physically on the same rack

Solution

1. Step 1: Check rack awareness setup

   If all replicas are on the same rack, the rack awareness script might be missing or incorrect, so HDFS cannot identify racks properly.

2. Step 2: Rule out other causes

   Replication factor 1 means only one replica, so multiple replicas on same rack is not possible. HDFS supports rack awareness. If all nodes are on the same rack physically, then placement is expected.

3. Final Answer:

   The rack awareness script is missing or misconfigured -> Option B

4. Quick Check:

   Missing script = replicas on same rack [OK]

Hint: Check rack script if replicas cluster on one rack [OK]

Common Mistakes:

• Assuming replication factor 1 causes multiple replicas
• Ignoring physical rack layout
• Believing HDFS lacks rack awareness support

5. In a cluster with only two racks and replication factor 3, how does HDFS handle rack awareness when placing replicas?

hard

A. Places replicas randomly ignoring rack awareness due to limited racks

B. Places all replicas on the same rack because there are only two racks

C. Places one replica on each rack and the third replica on any rack to maximize availability

D. Fails to store replicas because rack count is less than replication factor

Solution

1. Step 1: Understand rack awareness with limited racks

   When racks are fewer than replication factor, HDFS tries to spread replicas across racks as much as possible.

2. Step 2: Apply to 2 racks and replication factor 3

   HDFS places one replica on each rack, and the third replica on any rack to ensure three copies exist.

3. Final Answer:

   Places one replica on each rack and the third replica on any rack to maximize availability -> Option C

4. Quick Check:

   Limited racks = spread as much as possible [OK]

Hint: With fewer racks, spread replicas as much as possible [OK]

Common Mistakes:

• Thinking all replicas must be on different racks always
• Assuming failure if racks < replication factor
• Ignoring partial rack spreading