how #communication between #datanodes happens in #hadoop

 In Hadoop, the communication between DataNodes happens in the following way:

1. Heartbeats: DataNodes periodically send heartbeats to the NameNode to indicate that they are alive and functioning properly. The frequency of these heartbeats can be configured by the administrator.

2. Block Reports: DataNodes send block reports to the NameNode at startup and periodically afterward to report the list of blocks that they are currently storing. These block reports help the NameNode to maintain an up-to-date map of the cluster's data.

3. Replication: When a DataNode detects that a block has become under-replicated (i.e., there are not enough copies of the block), it will request additional copies from other DataNodes that have replicas of the same block. This process is known as replication.

4. Data Transfer: When a client wants to read or write a file, it first contacts the NameNode to get the location of the file's blocks. The client can then directly contact the DataNodes that are storing the blocks to read or write the data. The DataNodes communicate with each other to transfer data as needed to maintain the desired level of replication.