Raghu Angadi wrote:
A heartBeat is also an RPC. When you pause Namenode for 30 sec the datanode's heartbeat thread just waits for 30 sec for its heartbeat RPC to return. Note that when you pause Namenode, the RPCs to it don't fail immediately. During this wait, DNs can perform other transactions like serving data to clients.
If the heartbeat were just telling the NN that the DN is alive, you could do it with a UDP that didn't block the DN. If, however, the DN needs to know/care that the NN is up, then you do need to care about the state of the namenode. But you don't have to do it blocking; looking for a UDP back some time later is all you need to do.
