MapReduce at ETHZ - ETH Zurich | Flashcards & Summaries

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# Lernmaterialien für MapReduce an der ETHZ - ETH Zurich

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What does a reduce function do on a high level?

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It takes as input a key with an associated list of values and returns a list of values or key-value pairs. Often the type of the final key and value are the same as of the intermediate data.

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Are the input key-value pairs of mappers sorted by key?

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Yes

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Do MapReduce splits always correspond to HDFS blocks?

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No, since splits respects logical record boundaries, they might contain data from multiple HDFS blocks.

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Is one single Reducer applied to all values associated with the same key?

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Yes, this is the principle behind partitioning: one Reducer is responsible for all values associated with a particular key.

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In Hadoop MapReduce do the key-value pairs a Reducer outputs have to be of the same type as its input pairs?

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Yes

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What is a combiner in MapReduce?

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A combiner processes the output data from the Mapper before passing it to the Reducer and is optional. Its there so less shuffling has to be done which can reduce network congestion.

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What does a map function do on a high level?

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It takes as input a key-value pair and emits a list of key-value pairs.

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Does each mapper have to generate the same number of key/value pairs as its input had?

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Yes

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Is the JobTracker or the TaskTracker responsible for scheduling mappers and reducers and making sure all nodes are correctly running?

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JobTracker

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What's the core principle of Hadoop?

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Bring the query to the data. (same as short-circuiting, datanode and tasktracker/regionserver on the same machine)

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Mapper and Reducer are functions that are passed to the map and reduce function respectively. What is the default behaviour of the MapReduce job if the Mapper and Reducer are not changed?

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Mapper and Reducers are identity functions by default and there is always a shuffling phase that sorts by input key so the whole job performs sorting by input key.

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What is the role of the Context object that is passed to the map and reduce function respectively?

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It allows the Mapper/Reducer to interact with the rest of the Hadoop system. The context can be used to set parameters to mappers and reducers running on different nodes.

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Q:

What does a reduce function do on a high level?

A:

It takes as input a key with an associated list of values and returns a list of values or key-value pairs. Often the type of the final key and value are the same as of the intermediate data.

Q:

Are the input key-value pairs of mappers sorted by key?

A:

Yes

Q:

Do MapReduce splits always correspond to HDFS blocks?

A:

No, since splits respects logical record boundaries, they might contain data from multiple HDFS blocks.

Q:

Is one single Reducer applied to all values associated with the same key?

A:

Yes, this is the principle behind partitioning: one Reducer is responsible for all values associated with a particular key.

Q:

In Hadoop MapReduce do the key-value pairs a Reducer outputs have to be of the same type as its input pairs?

A:

Yes

Q:

What is a combiner in MapReduce?

A:

A combiner processes the output data from the Mapper before passing it to the Reducer and is optional. Its there so less shuffling has to be done which can reduce network congestion.

Q:

What does a map function do on a high level?

A:

It takes as input a key-value pair and emits a list of key-value pairs.

Q:

Does each mapper have to generate the same number of key/value pairs as its input had?

A:

Yes

Q:

Is the JobTracker or the TaskTracker responsible for scheduling mappers and reducers and making sure all nodes are correctly running?

A:

JobTracker

Q:

What's the core principle of Hadoop?

A:

Bring the query to the data. (same as short-circuiting, datanode and tasktracker/regionserver on the same machine)

Q:

Mapper and Reducer are functions that are passed to the map and reduce function respectively. What is the default behaviour of the MapReduce job if the Mapper and Reducer are not changed?

A:

Mapper and Reducers are identity functions by default and there is always a shuffling phase that sorts by input key so the whole job performs sorting by input key.

Q:

What is the role of the Context object that is passed to the map and reduce function respectively?

A:

It allows the Mapper/Reducer to interact with the rest of the Hadoop system. The context can be used to set parameters to mappers and reducers running on different nodes.

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