Final answer:
The NameNode in Hadoop handles DataNode failures by marking the DataNode as offline, triggering data replication to maintain the replication factor, and reconciling data blocks upon DataNode recovery to ensure data consistency and system reliability.
Step-by-step explanation:
The NameNode in Hadoop's HDFS architecture is responsible for maintaining the file system directory tree and the metadata for all files and directories. When a DataNode fails, the NameNode is tasked with handling the failure to ensure data reliability and availability. In the event of a DataNode failure, the following steps take place:
- The NameNode marks the failed DataNode as offline and does not forward any new IO requests to it.
- Data replication is triggered. Each block of data that was hosted on the failed DataNode is copied from an existing DataNode that has a replica of that block, to another healthy DataNode. This ensures that the replication factor (the number of copies of each block across the DataNodes) remains consistent with the user-defined value.
- The system continues to function without any data loss, although there might be some temporary decrease in data availability or performance until the replication process is completed.
Additionally, on restart, if the failed DataNode comes back online, it reports to the NameNode with a list of all blocks it holds. The NameNode then reconciles this list with its metadata and makes decisions on which data blocks need to be replicated or deleted to maintain the consistency and replication factor of the file system.