Heart Beat Contents

The contents of the heartbeat message are:

• Progress report of tasks currently running on sender TaskTracker.
• Lists of completed or failed tasks.
• State of resources – virtual memory, disk space, etc.
• A Boolean flag (acceptNewTasks) indicating whether the sender TaskTracker should be accept new tasks *

The NameNode does not directly call DataNodes. It uses replies to heartbeats to send instructions to the DataNodes. The instructions include commands to:

• replicate blocks to other nodes;
• remove local block replicas;
• re-register or to shut down the node;
• send an immediate block report.