HDFS high availability in Hadoop - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is HDFS High Availability (HA)?

HDFS High Availability means having two NameNodes: one active and one standby. This setup helps keep the system running even if one NameNode fails.

beginner

Why do we need a standby NameNode in HDFS HA?

The standby NameNode takes over if the active NameNode fails. This prevents downtime and data loss.

intermediate

What is the role of ZooKeeper in HDFS HA?

ZooKeeper helps manage which NameNode is active by coordinating failover automatically.

intermediate

What is a Quorum Journal Manager (QJM) in HDFS HA?

QJM is a shared storage system that keeps edit logs from the active NameNode. Both NameNodes use it to stay in sync.

intermediate

How does automatic failover work in HDFS HA?

If the active NameNode fails, ZooKeeper detects it and switches the standby NameNode to active, so the system keeps working without manual help.

What does HDFS High Availability primarily prevent?

A System downtime due to NameNode failure

B Data replication across DataNodes

C Slow data processing

D Network congestion

HDFS HA prevents downtime by having a standby NameNode ready to take over if the active one fails.

Which component helps coordinate failover in HDFS HA?

A ZooKeeper

B DataNode

C ResourceManager

D NameNode

ZooKeeper manages which NameNode is active and handles automatic failover.

What is stored in the Quorum Journal Manager?

A User data files

B Configuration files

C Backup snapshots

D Edit logs from the active NameNode

QJM stores edit logs to keep both NameNodes synchronized.

How many NameNodes are active at the same time in HDFS HA?

A Two

B One

C None

D Depends on cluster size

Only one NameNode is active; the other is standby.

What happens if the active NameNode fails in an HA setup?

A Data is lost

B The cluster stops working

C The standby NameNode becomes active automatically

D Manual restart is required

The standby NameNode automatically takes over to keep the system running.

Explain how HDFS High Availability works to keep the system running without downtime.

Think about how two NameNodes share work and how the system switches between them.

Describe the role of ZooKeeper and Quorum Journal Manager in HDFS High Availability.

Focus on coordination and data sharing between NameNodes.

Practice

1. What is the main purpose of HDFS high availability (HA)?

easy

A. To speed up data processing by parallel tasks

B. To avoid downtime by using two active NameNodes

C. To increase storage capacity by adding more DataNodes

D. To encrypt data stored in HDFS automatically

Solution

1. Step 1: Understand HDFS HA concept

   HDFS HA uses two NameNodes to ensure one is always available if the other fails.

2. Step 2: Identify the main goal

   The goal is to avoid downtime, not to increase storage or speed processing.

3. Final Answer:

   To avoid downtime by using two active NameNodes -> Option B

4. Quick Check:

   HDFS HA = Avoid downtime [OK]

Hint: Remember: HA means no downtime with two NameNodes [OK]

Common Mistakes:

• Confusing HA with storage scaling
• Thinking HA speeds up processing
• Assuming HA encrypts data

2. Which component is essential for automatic failover in HDFS high availability?

easy

A. ZooKeeper

B. HDFS DataNode

C. YARN ResourceManager

D. MapReduce JobTracker

Solution

1. Step 1: Identify failover mechanism in HDFS HA

   ZooKeeper coordinates and manages automatic failover between NameNodes.

2. Step 2: Exclude unrelated components

   DataNode stores data, ResourceManager manages resources, JobTracker is for MapReduce jobs, none handle failover.

3. Final Answer:

   ZooKeeper -> Option A

4. Quick Check:

   Failover = ZooKeeper [OK]

Hint: ZooKeeper manages failover in HDFS HA [OK]

Common Mistakes:

• Choosing DataNode as failover manager
• Confusing YARN with failover
• Selecting MapReduce components

3. Given the following simplified HDFS HA configuration snippet, what is the role of dfs.nameservices?

dfs.nameservices=cluster1

medium

A. Lists all DataNodes in the cluster

B. Specifies the IP address of the active NameNode

C. Defines the logical name for the HDFS cluster used by clients

D. Sets the replication factor for data blocks

Solution

1. Step 1: Understand dfs.nameservices meaning

   It defines a logical name for the HDFS cluster, grouping NameNodes under one namespace.

2. Step 2: Eliminate other options

   It does not specify IPs, DataNodes, or replication factors.

3. Final Answer:

   Defines the logical name for the HDFS cluster used by clients -> Option C

4. Quick Check:

   dfs.nameservices = cluster name [OK]

Hint: dfs.nameservices names the cluster logically [OK]

Common Mistakes:

• Thinking it sets IP addresses
• Confusing with DataNode list
• Mixing with replication settings

4. You configured HDFS HA but the failover does not happen automatically. Which configuration is most likely missing?

medium

A. Setting dfs.ha.automatic-failover.enabled to true

B. Increasing the replication factor to 5

C. Adding more DataNodes to the cluster

D. Disabling the NameNode checkpoint

Solution

1. Step 1: Identify failover configuration

   Automatic failover requires dfs.ha.automatic-failover.enabled set to true.

2. Step 2: Exclude unrelated settings

   Replication factor, DataNodes count, and checkpoint settings do not control failover.

3. Final Answer:

   Setting dfs.ha.automatic-failover.enabled to true -> Option A

4. Quick Check:

   Auto failover enabled = true [OK]

Hint: Enable dfs.ha.automatic-failover.enabled for auto failover [OK]

Common Mistakes:

• Changing replication factor to fix failover
• Adding DataNodes expecting failover fix
• Disabling checkpoint thinking it helps failover

5. You want to configure HDFS HA with two NameNodes: nn1 and nn2. Which configuration snippet correctly sets the failover proxy provider for cluster1?

hard

A. dfs.replication=3

B. dfs.nameservices=nn1,nn2

C. dfs.ha.automatic-failover.enabled=false

D. dfs.client.failover.proxy.provider.cluster1=org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider

Solution

1. Step 1: Understand failover proxy provider setting

   This property tells the client how to switch between NameNodes during failover.

2. Step 2: Check correctness of options

   dfs.client.failover.proxy.provider.cluster1=org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider

   correctly sets the proxy provider for cluster1.

   dfs.nameservices=nn1,nn2

   wrongly lists NameNodes as nameservices.

   dfs.ha.automatic-failover.enabled=false

   disables failover.

   dfs.replication=3

   sets replication, unrelated to failover.

3. Final Answer:

   dfs.client.failover.proxy.provider.cluster1=org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider -> Option D

4. Quick Check:

   Failover proxy provider =

   dfs.client.failover.proxy.provider.cluster1=org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider

   [OK]

Hint: Set failover proxy provider for cluster1 correctly [OK]

Common Mistakes:

• Confusing nameservices with NameNodes list
• Disabling automatic failover by mistake
• Mixing replication with failover settings