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In Hadoop Name node and data node do communicate using Heartbeat. Therefore Heartbeat is the signal that is sent by the datanode to the namenode after the regular interval to time to indicate its presence, i.e. to indicate that it is alive.
If after a certain time of heartbeat Name Node does not receive any response from Data Node, then that particular Data Node used to be declared as dead.The default heartbeat interval is 3 seconds. If the DataNode in HDFS does not send heartbeat to NameNode in ten minutes, then NameNode considers the DataNode to be out of service and the Blocks replicas hosted by that DataNode to be unavailable. The NameNode then schedules the creation of new replicas of those blocks on other DataNodes.
NameNode that receives the Heartbeats from a DataNode also carries information like total storage capacity, the fraction of storage in use, and the number of data transfers currently in progress. For the NameNode’s block allocation and load balancing decisions, we use these statistics.
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A Heartbeat is a signal from Datanode to Namenode to indicate that it is alive. In HDFS, absence of heartbeat indicates that there is some problem and then Namenode, Datanode can not perform any computation.
Based on Apache documentation:-
1) dfs.heartbeat.interval has the default value as 3 which is taken as a unit of second.
2) dfs.blockreport.intervalMec has the default value as 21600000 which is taken as a unit of milliseconds. -
In Hadoop, Namenode and Datanode are two physically separated machines, therefore Heartbeat is the signal that is sent by the datanode to the namenode after the regular interval to time to indicate its presence, i.e. to indicate that it is alive.
- In case Namenode does not receive the heartbeat from a Datanode in a certain amount of time(within 10 mins), Namenode then considers that datanode as a dead machine.
- Datanode along with heartbeat also sends the block report to Namenode, block report typically contains the list of all the blocks on a datanode.
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In Hadoop, Datanodes sends heartbeats to Namenodes to apprise that its that it is alive and working, in this heartbeats the data nodes also send the information like the total disk space, total space in use and the data transfers in the process, It helps namenode to perform load balancing.
The heartbeat interval is 3 seconds by default which is configured in property dfs.heartbeat.interval. If the namenode doesn’t get any signal, it waits for 10 minutes and considers the datanode as dead. When namenode declare any datanode dead, it initiates the replication of blocks stored on the dead datanode to other working nodes for ensuring Data availibility.
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A Heartbeat is a signal from Datanodeto Name node to indicate that Data node is alive. Data node sends the heartbeat to the Name Node.
The heartbeat interval is 3 seconds by default which is configured in property dfs.heartbeat.interval. If the namenode doesn’t get any signal, it waits for 10 minutes and considers the datanode as dead. When namenode declare any datanode dead, it initiates the replication of blocks stored on the dead datanode to other working nodes for ensuring data availability.
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HeartBeat is a signal which is sent by DataNode to the NameNode about its health which includes Node Capacity in terms of used and unused disk space and performance measures of the particular DataNode.
Usually, HeartBeat is sent to NameNode in every 3 seconds, if the NameNode does not receive any signal from the DataNode for about 10 min then the DataNode will be declared as dead which is of no use.
Hence once the heartbeat stops sending a signal to NameNode, then NameNode perform certain tasks such as replicating the blocks present in DataNode to other DataNodes to make the data is highly available and ensuring data reliability.
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