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Top 100 Hadoop Complex Interview Questions (Part 2 of 4)

1.     If a data Node is full how it’s identified?
When data is stored in datanode, then the metadata of that data will be stored in the Namenode. So Namenode will identify if the data node is full.

2.     If datanodes increase, then do we need to upgrade Namenode?
While installing the Hadoop system, Namenode is determined based on the size of the clusters. Most of the time, we do not need to upgrade the Namenode because it does not store the actual data, but just the metadata, so such a requirement rarely arise.

3.     Are job tracker and task trackers present in separate machines?
Yes, job tracker and task tracker are present in different machines. The reason is job tracker is a single point of failure for the Hadoop MapReduce service. If it goes down, all running jobs are halted.

4.     When we send a data to a node, do we allow settling in time, before sending another data to that node?
Yes, we do.

5.     Does hadoop always require digital data to process?
Yes.  Hadoop always require digital data to be processed.

6.     On what basis Namenode will decide which datanode to write on?
As the Namenode has the metadata (information) related to all the data nodes, it knows which datanode is free.

7.     Doesn’t Google have its very own version of DFS?
Yes, Google owns a DFS known as “Google File System (GFS)”  developed by Google Inc. for its own use.

8.     Who is a ‘user’ in HDFS?
A user is like you or me, who has some query or who needs some kind of data.

9.     Is client the end user in HDFS?
No, Client is an application which runs on your machine, which is used to interact with the Namenode (job tracker) or datanode (task tracker).

10.What is the communication channel between client and namenode/datanode?
The mode of communication is SSH.

11.What is a rack?
Rack is a storage area with all the datanodes put together. These datanodes can be physically located at different places. Rack is a physical collection of datanodes which are stored at a single location. There can be multiple racks in a single location.

12.On what basis data will be stored on a rack?
When the client is ready to load a file into the cluster, the content of the file will be divided into blocks. Now the client consults the Namenode and gets 3 datanodes for every block of the file which indicates where the block should be stored. While placing the datanodes, the key rule followed is “for every block of data, two copies will exist in one rack, third copy in a different rack“. This rule is known as “Replica Placement Policy“.

13.Do we need to place 2nd and 3rd data in rack 2 only?
Yes, this is to avoid datanode failure.

14.What if rack 2 and datanode fails?
If both rack2 and datanode present in rack 1 fails then there is no chance of getting data from it. In order to avoid such situations, we need to replicate that data more number of times instead of replicating only thrice. This can be done by changing the value in replication factor which is set to 3 by default.

15.What is a Secondary Namenode? Is it a substitute to the Namenode?
The secondary Namenode constantly reads the data from the RAM of the Namenode and writes it into the hard disk or the file system. It is not a substitute to the Namenode, so if the Namenode fails, the entire Hadoop system goes down.

16.What is the difference between Gen1 and Gen2 Hadoop with regards to the Namenode?
In Gen 1 Hadoop, Namenode is the single point of failure. In Gen 2 Hadoop, we have what is known as Active and Passive Namenodes kind of a structure. If the active Namenode fails, passive Namenode takes over the charge.

17.What is MapReduce?
Map Reduce is the ‘heart‘ of Hadoop that consists of two parts – ‘map’ and ‘reduce’. Maps and reduces are programs for processing data. ‘Map’ processes the data first to give some intermediate output which is further processed by ‘Reduce’ to generate the final output. Thus, MapReduce allows for distributed processing of the map and reduction operations.

18.Can you explain how do ‘map’ and ‘reduce’ work?
Namenode takes the input and divide it into parts and assign them to data nodes. These datanodes process the tasks assigned to them and make a key-value pair and returns the intermediate output to the Reducer. The reducer collects this key value pairs of all the datanodes and combines them and generates the final output.

19.What is ‘Key value pair’ in HDFS?
Key value pair is the intermediate data generated by maps and sent to reduces for generating the final output.

20.What is the difference between MapReduce engine and HDFS cluster?
HDFS cluster is the name given to the whole configuration of master and slaves where data is stored. Map Reduce Engine is the programming module which is used to retrieve and analyze data.

21.Is map like a pointer?
No, Map is not like a pointer.

22.Do we require two servers for the Namenode and the datanodes?
Yes, we need two different servers for the Namenode and the datanodes. This is because Namenode requires highly configurable system as it stores information about the location details of all the files stored in different datanodes and on the other hand, datanodes require low configuration system.

23.Why are the number of splits equal to the number of maps?
The number of maps is equal to the number of input splits because we want the key and value pairs of all the input splits.

24.Is a job split into maps?
No, a job is not split into maps. Spilt is created for the file. The file is placed on datanodes in blocks. For each split,  a map is needed.

25.Which are the two types of ‘writes’ in HDFS?
There are two types of writes in HDFS: posted and non-posted write. Posted Write is when we write it and forget about it, without worrying about the acknowledgement. It is similar to our traditional Indian post. In a Non-posted Write, we wait for the acknowledgement. It is similar to the today’s courier services. Naturally, non-posted write is more expensive than the posted write. It is much more expensive, though both writes are asynchronous.

26.Why ‘Reading‘ is done in parallel and ‘Writing‘ is not in HDFS?
Reading is done in parallel because by doing so we can access the data fast. But we do not perform the write operation in parallel. The reason is that if we perform the write operation in parallel, then it might result in data inconsistency. For example, you have a file and two nodes are trying to write data into the file in parallel, then the first node does not know what the second node has written and vice-versa. So, this makes it confusing which data to be stored and accessed.

27.Can Hadoop be compared to NOSQL database like Cassandra?
Though NOSQL is the closet technology that can be compared to Hadoop, it has its own pros and cons. There is no DFS in NOSQL. Hadoop is not a database. It’s a filesystem (HDFS) and distributed programming framework (MapReduce).

28.How can I install Cloudera VM in my system?
When you enrol for the hadoop course at Edureka, you can download the Hadoop Installation steps.pdf file from our dropbox. This will be shared with you by an e-mail.

29.Which are the three modes in which Hadoop can be run?
The three modes in which Hadoop can be run are:
standalone (local) mode
Pseudo-distributed mode
Fully distributed mode


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