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Unlike performing joins against other files on HDFS, HBase tables are mutable. Hence random reads using precisely the same row key may return different results (ie one job is updating a table while another is reading from it). HCatalog's integration with HBase introduces the notion of snapshots, which guarantees consistent reads over a HBase Table during the lifetime of a MR job. Snapshots can also be shared, guaranteeing consistency over a DAG of MR jobs. In the context of the UDA use case problem snapshots guarantee that retroactive updates do not affect jobs that are running concurrently.

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