If a data node fails the job tracker and name node will detect the failure. After that, all tasks are re-scheduled on the failed node and then name node will replicate the user data to another node.
In Hadoop’s distributed file system, HDFS (Hadoop Distributed File System), when a data node fails, the following actions typically take place:
- Replication Mechanism: HDFS replicates data across multiple data nodes to ensure fault tolerance. By default, Hadoop replicates each block of data three times (the replication factor is configurable). When a data node fails, the system detects the loss of replicas associated with that node.
- Block Replication: HDFS automatically triggers the replication of the lost block(s) to maintain the desired replication factor. The NameNode (which manages metadata) identifies the lost replicas and initiates the process of creating new replicas on other available data nodes.
- Redistribution of Work: The HDFS architecture ensures that data is distributed across multiple nodes. When a data node fails, the NameNode redistributes the tasks associated with that node to other healthy nodes. This redistribution ensures that the processing of data can continue without interruption.
- Heartbeat Mechanism: Hadoop uses a heartbeat mechanism to detect the health of data nodes. If a data node fails to send a heartbeat signal within a specified time interval, the system marks it as dead, and the above steps are initiated.
It’s important to note that the replication and redistribution processes happen automatically in the background, providing fault tolerance and ensuring data availability even in the presence of hardware failures.