Friday, 24 October 2014

How to load a file in DistributedCache in Hadoop MapReduce

We can load an extra file using Distributed Cache.To do that we need to configure the Distributed Cache with needed file in Driver Class

Configuration conf = new Configuration();
FileSystem fs = FileSystem.get(conf);
Path cachefile = new Path("path/to/file");
FileStatus[] list = fs.globStatus(cachefile);
for (FileStatus status : list) {
 DistributedCache.addCacheFile(status.getPath().toUri(), conf);
And in Reducers setup() or Mappers Setup() we will be able to read this file.
public void setup(Context context) throws IOException{
 Configuration conf = context.getConfiguration();
 FileSystem fs = FileSystem.get(conf);
 URI[] cacheFiles = DistributedCache.getCacheFiles(conf);
 Path getPath = new Path(cacheFiles[0].getPath());  
 BufferedReader bf = new BufferedReader(new InputStreamReader(;
 String setupData = null;
 while ((setupData = bf.readLine()) != null) {
   System.out.println("Setup Line in reducer "+setupData);
You can give 0,1,... if you supplied more than 1 cache file
Path getPath = new Path(cacheFiles[1].getPath());  

Happy Hadooping ....

Monday, 29 September 2014

Comments On CCD-410 Sample Dumps

What do you think of these three questions mentioned in site CCD-410 Practice Exam Questions Demo 100% Pass-Guaranteed or Your Money Back!!!


What happens in a MapReduce job when you set the number of reducers to one?

A. A single reducer gathers and processes all the output from all the mappers. The output is written in as many separate files as there are mappers.
B. A single reducer gathers and processes all the output from all the mappers. The output is written to a single file in HDFS.
C. Setting the number of reducers to one creates a processing bottleneck, and since the number of reducers as specified by the programmer is used as a reference value only, the MapReduceruntime provides a default setting for the number of reducers.
D. Setting the number of reducers to one is invalid, and an exception is thrown.



In the standard word count MapReduce algorithm, why might using a combiner reduce theoverall Job running time?

A. Because combiners perform local aggregation of word counts, thereby allowing the mappers to process input data faster.
B. Because combinersperform local aggregation of word counts, thereby reducing the number of mappers that need to run.
C. Because combiners perform local aggregation of word counts, and then transfer that data toreducers without writing the intermediate data to disk.
D. Because combiners perform local aggregation of word counts, thereby reducing the number of key-value pairs that need to be snuff let across the network to the reducers.



You have user profile records in your OLTP database,that you want to join with weblogs you have already ingested into HDFS.How will you obtain these user records?

A. HDFS commands
B. Pig load
C. Sqoop import
D. Hive

Answer :B

Correct Answers

QUESTION 3: Answer B
QUESTION 4: Answer D
QUESTION 5: Answer C

See reviews on correct answer

Can we change the default key-value input seperator in Hadoop MapReduce

Previous Post

Yes, We can change it using "" property in Driver class.

There may be cases where we need to take each line with specific delimiter.

one	first line
two	second line
Inorder to read a file like this we will be using KeyValueTextInputFormat.class as it takes the line with TAB  as default seperator.

So while printing each line in map() , the key will be "one" and value will be "first line".

What if we need other delimiters instead of TAB delimiter

one,first line
two,second line

Here also we need to get key as "one" and value as "first line".

It is possible by adding an extra configuration along with KeyValueTextInputFormat to change the default seperator.

//New API
Configuration conf = new Configuration();
conf.set("", ","); 
Job job = new Job(conf);

Monday, 22 September 2014

Matrix Multiplication in Hadoop MapReduce

Matrix multiplication is common and important algebraic operation.

Matrix multiplication is applied to file of format:MatrixName,row,col,element


Find code : GitHub

Monday, 8 September 2014

How To Set Counters In Hadoop MapReduce

Counters are a useful channel for gathering statistics about the job: for quality control or for application level-statistics.Lets see an example where Counters count the no of keys processed in reducer.

3 key points to set

1. Define counter in Driver class

static enum UpdateCount{

2. Increment or set counter in Reducer


3. Get counter in Driver class 

c = job.getCounters().findCounter(UpdateCount.CNT).getValue();

Full code :  GitHub

You will be able to see the counters in console also.

Saturday, 23 August 2014

Example for Apriori Algorithm

Lets take a store data
To start with Apriori follow the below steps.
Step 1: Initially we need to find Item 1 Frequent Dataset
book 6
chalk 3
eraser 6
pen 8
pencil 7
Ink 1
We will say that an item set is frequent if it appears in at least 3 transactions of the itemset: the value 3 is the support threshold.
Support count = 3
So the items less that support count can be discarded form F1 frequent Dataset.
so our new set will be
book 6
chalk 3
eraser 6
pen 8
pencil 7
Step 2: We need to generate size 2 frequent item pair sets by joining L1 set
eg:{book} U {chalk} => {book,chalk} and so on..



Once the transactions are joined we need to identify the no occurence of the above data items in original transaction(That will be the support count of C2)
{book,chalk} 2
{book,eraser} 2
{book,pen} 4
{book,pencil} 5

{chalk,eraser} 2
{chalk,pen} 2
{chalk,pencil} 0

{eraser,pen} 5
{eraser,pencil} 3

{pen,pencil} 5
Transactions less that support count can be discarded form C2 frequent Dataset
{book,pen} 4
{book,pencil} 5
{eraser,pen} 5
{eraser,pencil} 3
{pen,pencil} 5
To find C3 loop through L2
eg: {book,pen} U {book,pencil} => {book,pen,pencil}
{book,pen,pencil} 3
{chalk,eraser,pen} 2
{eraser,pen,pencil} 2
Transactions less that support count can be discarded form C3 frequent Dataset
{book,pen,pencil} 3
There are no transaction to join further.
So our Frequent item sets are
book 6
chalk 3
eraser 6
pen 8
pencil 7

{book,pen} 4
{book,pencil} 5
{eraser,pen} 5
{eraser,pencil} 3
{pen,pencil} 5

{book,pen,pencil} 3
Step 3: We need to generate Strong Assosiaction  Rules for frequent Set using L1,L2and L3

Say confidence is 60% and Support count is 3.So we have to find the Transactions with no.of item 3 (ie L3 as support count = 3) and which has a confidence >=60.Now we can identify L3 set
{book,pen,pencil} 3

Finding Ruleset
{book,pen} => pencil
{book,pencil} => pen
{pen,pencil} => book

pencil => {book,pen}
pen => {book,pencil}
book => {pen,pencil}

Now we need to find the confidence of each transaction
eg: {book,pen} => pencil
           = support Cnt{book,pen}/ support count({pen})

Therefore rules having confidence greater than and equal to 60 are
book,pen=>pencil 75.0
book,pencil=>pen 60.0
pen,pencil=>book 60.0
These are the strongest rules.
If a customer buys book and pen he have a tendency to buy a pencil too. Like wise if he buys book and pencil he may buy pen too.

Calculating Mean in Hadoop MapReduce

Given a csv files we will find mean of each column (Optimized approach)


 Takes each input line and calculate the sum and stores the no of lines it sumed.Then sum get stored in a hash map with key as column Id.cleanup emits the sum and total line count inorder to take the overall mean.As we know each map only gets a block of input data.So while summing up we need to know how many elements summed up.

//Calculating sum
if (sumVal.isEmpty()) {
 //if sumval is empty add elements to sumval
 } else {
//calculating sum
 double sum = 0;
 for (Integer colId : mapLine.keySet()) {
  double val1 = mapLine.get(colId);
  double val2 = sumVal.get(colId);
  * calculating sum
 sum = val1 + val2;
 sumVal.put(colId, sum);


 Sums of the values for each key.

Reducer calculates 2 sums.
  1. Sums the values for each key and 
  2. Sums total no.of linecount

for (TwovalueWritable value : values) {
 //Taking sum of values and total number of lines 
 sum += value.getSum();
 total += value.getTotalCnt();
 //sum contains total sum of all elements in each column
 //total contains total no of elements in each column
 mean = sum / total;
 context.write(key, valEmit);

This approach helps in avoiding a large no of communication with reducer.Reducer needs only to sum up few values from mapper.
Say we have only 3 mappers and 4 columns in input set.Reducer only want to wait for 4 values from each mapper(no.of columns also considered)

Complete code : GitHub Link