Hadoop Distributed File Systmes

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HDFS is built around the idea that the most efficient data processing pattern is a write once, read-many-times pattern.

Can I have any real time example how the HDFS write once and ready many times? I wanted to understand this core concepts in deep.

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HDFS applications need a write-once-read-many access model for files. A file once created, written, and closed need not be changed. This assumption simplifies data coherency issues and enables high throughput data access. A MapReduce application or a web crawler application fits perfectly with this model. (Source: HDFS Design)

HDFS is built around the idea that files are rarely updated. Rather, they are read as data for some calculation, and possibly additional data is appended to files from time to time. For example, an airline reservation system would not be suitable for a DFS, even if the data were very large, because the data is changed so frequently. (Source: Mining of Massive Datasets)

Also see Why HDFS is write once and read multiple times?