MapReduce — Practice MCQs for CCAT

Correct Answer: C — Map and Reduce phases

MapReduce has Map phase (transforms data into key-value pairs) and Reduce phase (aggregates values by key).

Correct Answer: B — Key-value pairs

Map function processes input and emits intermediate key-value pairs for the Reduce phase.

Correct Answer: B — Between Map and Reduce phases

Shuffle and Sort transfers Map output to Reducers and sorts data by keys between phases.

Correct Answer: B — A mini-reducer that runs on Map output

Combiner is an optional local reducer that runs on Map output to reduce network transfer.

Correct Answer: B — Which Reducer gets which key

Partitioner determines which Reducer receives which key-value pairs, typically using hash of the key.

Correct Answer: A — Key-value pair

Map receives a key-value pair where key is typically offset and value is the line content.

Correct Answer: B — Defines how to read and split input files

InputFormat defines how input files are read and split into InputSplits for Map tasks.

Correct Answer: B — Number of input splits

Number of Map tasks equals number of input splits, which depends on input data size and block size.

Correct Answer: A — (word, 1) for each word

In Word Count, Map emits (word, 1) for each word occurrence; Reduce sums the counts per word.

Correct Answer: B — Runs duplicate tasks to handle slow nodes

Speculative execution runs backup copies of slow-running tasks to prevent stragglers from delaying jobs.

Correct Answer: B — Key and iterator of all values for that key

Reduce receives a key and an iterator over all values associated with that key after shuffle/sort.

Correct Answer: B — Defines how to write output

OutputFormat defines how Reduce output is written - format, location, and structure.

Correct Answer: B — Moving computation to where data resides

Data locality moves computation to nodes where data is stored rather than moving data over network.

Correct Answer: B — Reads input split and generates key-value pairs

RecordReader reads an InputSplit and generates key-value pairs for the Map function.

Correct Answer: B — 1

Default number of Reducers is 1, but can be configured based on data size and cluster capacity.

Correct Answer: B — Tracking job statistics and metrics

Counters track various statistics like input/output records, bytes processed, and custom metrics.

Correct Answer: B — Read-only data distribution to all nodes

DistributedCache distributes read-only files (like lookup tables) to all nodes before task execution.

Correct Answer: B — No Reduce phase

Map-only jobs set Reducers to 0, outputting Map results directly without reduce/aggregation.

Correct Answer: B — Resource management and job scheduling

JobTracker managed resources and scheduled jobs in Hadoop 1.x, replaced by YARN ResourceManager in 2.x.

Correct Answer: A — Sorts by value as well as key

Secondary sort allows sorting by both key and value, using composite keys and custom comparators.