YARN
Distinguish ResourceManager, NodeManager, and ApplicationMaster roles, and understand why resource containers do not guarantee latency.
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Combiner
Understand mapper-local combining, optional repeated execution, and why merging sums and counts is safer than averaging partial means.
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MapReduce
Follow input splits through mapping, keyed shuffle, and reduction, and separate successful task output from repeated external effects.
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Hive Metastore
Separate Hive Metastore metadata from table rows, check partition registration, and account for backend availability and connector behavior.
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HDFS
Understand how HDFS separates metadata from block storage, repairs surviving replicas, and differs from a backup or a transaction layer.
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What Hadoop Left Behind
Read inherited Hadoop systems through storage, catalogs, replica repair, mapper-local combining, retry boundaries, and workload migration.