hadoop面试可能遇到的问题

本文引用自神之子《hadoop面试可能遇到的问题》

Q1. Name the most common InputFormats defined in Hadoop? Which one is default ? 
Following 2 are most common InputFormats defined in Hadoop 
- TextInputFormat
- KeyValueInputFormat
- SequenceFileInputFormat

Q2. What is the difference between TextInputFormatand KeyValueInputFormat class
TextInputFormat: It reads lines of text files and provides the offset of the line as key to the Mapper and actual line as Value to the mapper
KeyValueInputFormat: Reads text file and parses lines into key, val pairs. Everything up to the first tab character is sent as key to the Mapper and the remainder of the line is sent as value to the mapper.

Q3. What is InputSplit in Hadoop

When a hadoop job is run, it splits input files into chunks and assign each split to a mapper to process. This is called Input Split 

Q4. How is the splitting of file invoked in Hadoop Framework  

It is invoked by the Hadoop framework by running getInputSplit()method of the Input format class (like FileInputFormat) defined by the user 

Q5. Consider case scenario: In M/R system,

    - HDFS block size is 64 MB

    - Input format is FileInputFormat

    - We have 3 files of size 64K, 65Mb and 127Mb 

then how many input splits will be made by Hadoop framework?

Hadoop will make 5 splits as follows 

- 1 split for 64K files 

- 2  splits for 65Mb files 

- 2 splits for 127Mb file 

Q6. What is the purpose of RecordReader in Hadoop

The InputSplithas defined a slice of work, but does not describe how to access it. The RecordReaderclass actually loads the data from its source and converts it into (key, value) pairs suitable for reading by the Mapper. The RecordReader instance is defined by the InputFormat 

Q7. After the Map phase finishes, the hadoop framework does "Partitioning, Shuffle and sort". Explain what happens in this phase?

- Partitioning

Partitioning is the process of determining which reducer instance will receive which intermediate keys and values. Each mapper must determine for all of its output (key, value) pairs which reducer will receive them. It is necessary that for any key, regardless of which mapper instance generated it, the destination partition is the same

- Shuffle

After the first map tasks have completed, the nodes may still be performing several more map tasks each. But they also begin exchanging the intermediate outputs from the map tasks to where they are required by the reducers. This process of moving map outputs to the reducers is known as shuffling.

- Sort

Each reduce task is responsible for reducing the values associated with several intermediate keys. The set of intermediate keys on a single node is automatically sorted by Hadoop before they are presented to the Reducer 

Q9. If no custom partitioner is defined in the hadoop then how is data partitioned before its sent to the reducer  

The default partitioner computes a hash value for the key and assigns the partition based on this result 

Q10. What is a Combiner 

The Combiner is a "mini-reduce" process which operates only on data generated by a mapper. The Combiner will receive as input all data emitted by the Mapper instances on a given node. The output from the Combiner is then sent to the Reducers, instead of the output from the Mappers.

Q11. Give an example scenario where a cobiner can be used and where it cannot be used

There can be several examples following are the most common ones

- Scenario where you can use combiner

  Getting list of distinct words in a file

- Scenario where you cannot use a combiner

  Calculating mean of a list of numbers 

Q12. What is job tracker

Job Tracker is the service within Hadoop that runs Map Reduce jobs on the cluster

Q13. What are some typical functions of Job Tracker

The following are some typical tasks of Job Tracker

- Accepts jobs from clients

- It talks to the NameNode to determine the location of the data

- It locates TaskTracker nodes with available slots at or near the data

- It submits the work to the chosen Task Tracker nodes and monitors progress of each task by receiving heartbeat signals from Task tracker 

Q14. What is task tracker

Task Tracker is a node in the cluster that accepts tasks like Map, Reduce and Shuffle operations - from a JobTracker 

Q15. Whats the relationship between Jobs and Tasks in Hadoop

One job is broken down into one or many tasks in Hadoop. 

Q16. Suppose Hadoop spawned 100 tasks for a job and one of the task failed. What willhadoop do ?

It will restart the task again on some other task tracker and only if the task fails more than 4 (default setting and can be changed) times will it kill the job

Q17. Hadoop achieves parallelism by dividing the tasks across many nodes, it is possible for a few slow nodes to rate-limit the rest of the program and slow down the program. What mechanism Hadoop provides to combat this  

Speculative Execution 

Q18. How does speculative execution works in Hadoop  

Job tracker makes different task trackers process same input. When tasks complete, they announce this fact to the Job Tracker. Whichever copy of a task finishes first becomes the definitive copy. If other copies were executing speculatively, Hadoop tells the Task Trackers to abandon the tasks and discard their outputs. The Reducers then receive their inputs from whichever Mapper completed successfully, first. 

Q19. Using command line in Linux, how will you 

- see all jobs running in the hadoop cluster

- kill a job

- hadoop job -list

- hadoop job -kill jobid 

Q20. What is Hadoop Streaming  

Streaming is a generic API that allows programs written in virtually any language to be used asHadoop Mapper and Reducer implementations 

Q21. What is the characteristic of streaming API that makes it flexible run map reduce jobs in languages like perl, ruby, awk etc.  

Hadoop Streaming allows to use arbitrary programs for the Mapper and Reducer phases of a Map Reduce job by having both Mappers and Reducers receive their input on stdin and emit output (key, value) pairs on stdout.

Q22. Whats is Distributed Cache in Hadoop
Distributed Cache is a facility provided by the Map/Reduce framework to cache files (text, archives, jars and so on) needed by applications during execution of the job. The framework will copy the necessary files to the slave node before any tasks for the job are executed on that node.

Q23. What is the benifit of Distributed cache, why can we just have the file in HDFS and have the application read it  
This is because distributed cache is much faster. It copies the file to all trackers at the start of the job. Now if the task tracker runs 10 or 100 mappers or reducer, it will use the same copy of distributed cache. On the other hand, if you put code in file to read it from HDFS in the MR job then every mapper will try to access it from HDFS hence if a task tracker run 100 map jobs then it will try to read this file 100 times from HDFS. Also HDFS is not very efficient when used like this.

Q.24 What mechanism does Hadoop framework provides to synchronize changes made in Distribution Cache during runtime of the application  
This is a trick questions. There is no such mechanism. Distributed Cache by design is read only during the time of Job execution

Q25. Have you ever used Counters in Hadoop. Give us an example scenario
Anybody who claims to have worked on a Hadoop project is expected to use counters

Q26. Is it possible to provide multiple input to Hadoop? If yes then how can you give multiple directories as input to the Hadoop job  
Yes, The input format class provides methods to add multiple directories as input to a Hadoop job

Q27. Is it possible to have Hadoop job output in multiple directories. If yes then how  
Yes, by using Multiple Outputs class

Q28. What will a hadoop job do if you try to run it with an output directory that is already present? Will it
- overwrite it
- warn you and continue
- throw an exception and exit
The hadoop job will throw an exception and exit.

Q29. How can you set an arbitary number of mappers to be created for a job in Hadoop  
This is a trick question. You cannot set it

Q30. How can you set an arbitary number of reducers to be created for a job in Hadoop  
You can either do it progamatically by using method setNumReduceTasksin the JobConfclass or set it up as a configuration setting