NIFI-3933:

- When monitoring heartbeats use the connected nodes as the basis for the check. This addresses the case when a node is terminated and no corresponding heartbeats exist.

This closes #1838.

nifi-nar-bundles/nifi-framework-bundle/nifi-framework/nifi-framework-cluster-protocol/src/main/java/org/apache/nifi/cluster/coordination/heartbeat/HeartbeatMonitor.java

@@ -58,13 +58,6 @@ public interface HeartbeatMonitor {
      */
     void purgeHeartbeats();
 
-    /**
-     * Returns when the heartbeats were purged last.
-     *
-     * @return when the heartbeats were purged last
-     */
-    long getPurgeTimestamp();
-
     /**
      * @return the address that heartbeats should be sent to when this node is elected coordinator.
      */

nifi-nar-bundles/nifi-framework-bundle/nifi-framework/nifi-framework-cluster/src/main/java/org/apache/nifi/cluster/coordination/heartbeat/AbstractHeartbeatMonitor.java

@@ -281,6 +281,13 @@ public abstract class AbstractHeartbeatMonitor implements HeartbeatMonitor {
      */
     protected abstract Map<NodeIdentifier, NodeHeartbeat> getLatestHeartbeats();
 
+    /**
+     * Returns when the heartbeats were purged last.
+     *
+     * @return when the heartbeats were purged last
+     */
+    protected abstract long getPurgeTimestamp();
+
     /**
      * This method does nothing in the abstract class but is meant for
      * subclasses to override in order to provide functionality when the monitor