Answer / Neeraj Rastogi

When a DataNode fails, the NameNode notices the absence of its heartbeats and marks the affected blocks as missing. The NameNode then schedules replication of these blocks to other available DataNodes. If necessary, the NameNode may also redirect client read and write requests to other DataNodes that have replicas of the desired data.

Web-ui shows that half of the datanodes are in decommissioning mode. What does that mean? Is it safe to remove those nodes from the network?

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What are the four basic parameters of a mapper?

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Why would nosql be better than using a sql database? And how much better is it?

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Why does one remove or add nodes in a Hadoop cluster frequently?

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After the Map phase finishes, the Hadoop framework does 'Partitioning, Shuffle and sort'. Explain what happens in this phase?

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Does hadoop follows the unix pattern?

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Which object can be used to get the progress of a particular job

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How to add/delete a Node to the existing cluster?

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Who are ‘Data Scientists’?

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What is the difference between an inputsplit and a block?

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What is Mapper in Hadoop?

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When and how to create hadoop archive?

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