NDB Cluster Configuration Files occurs unnecessarily, this can in turn lead to the unnecessary loss of a node group and as thus to a failure of the cluster. Consider a setup where there are 4 data nodes A, B, C, and D running on 2 host computers  host1  and host2 , and that these data nodes make up 2 node groups, as shown in the following table: Suppose the heartbeats are transmitted in the order A->B->C->D->A. In this case, the loss of the heartbeat between the hosts causes node B to declare node A dead and node C to declare node B dead. This results in loss of Node Group 0, and so the cluster fails. On the other hand, if the order of transmission is A->B->D->C->A (and all other conditions remain as previously stated), the loss of the heartbeat causes nodes A and D to be declared dead; in this case, each node group has one surviving node, and the cluster survives. The  HeartbeatOrder  configuration parameter makes the order of heartbeat transmission user- configurable. The default value for  HeartbeatOrder  is zero; allowing the default value to be used on all data nodes causes the order of heartbeat transmission to be determined by  NDB . If this parameter is used, it must be set to a nonzero value (maximum 65535) for every data node in the cluster, and this value must be unique for each data node; this causes the heartbeat transmission to proceed from data node to data node in the order of their  HeartbeatOrder  values from lowest to highest (and then directly from the data node having the highest  HeartbeatOrder  to the data node having the lowest value, to complete the circle). The values need not be consecutive. For example, to force the heartbeat transmission order A->B->D->C->A in the scenario outlined previously, you could set the HeartbeatOrder  values as shown here: To use this parameter to change the heartbeat transmission order in a running NDB Cluster, you must first set  HeartbeatOrder  for each data node in the cluster in the global configuration ( config.ini ) file (or files). To cause the change to take effect, you must perform either of the following: You can use  DUMP 908  to observe the effect of this parameter in the data node logs. • ConnectCheckIntervalDelay •2 rolling restarts of the cluster in succession.  All nodes must be restarted in the same order in both rolling restarts . •A complete shutdown and restart of the entire cluster. Table 23.10 Four data nodes A, B, C, D running on two host computers host1, host2; each data node belongs to one of two node groups. Table 23.11 HeartbeatOrder values to force a heartbeat transition order of A->B->D->C->A. Node HeartbeatOrder  Value A 10 B 20 C 30 D 25 Node Group Nodes Running on  host1 Nodes Running on  host2 Node Group 0: Node A Node B Node Group 1: Node C Node D 4106 Version (or later) NDB 8.0.13