Modified the Monitor InformationFetcher class to purge stale data

core/src/main/java/org/apache/accumulo/core/conf/Property.java

@@ -951,6 +951,15 @@ Each key is the name of the pool (can be assigned any string). Each value is a J
       The resources that are used by default can be seen in \
       `accumulo/server/monitor/src/main/resources/templates/default.ftl`.
       """, "2.0.0"),
+  MONITOR_PURGE_STATE_THRESHOLD("monitor.purge.state.threshold", "10m", PropertyType.TIMEDURATION,
+      """
+          The Monitor contains an internal data structure that contains all of the data used in the UI. \
+          This property controls how long the Monitor should retain that data structure after the last \
+          connection has disconnected from the Monitor. Keeping the data structure will allow for an \
+          immediate display of the last known state. Removing the data structure will cause the UI \
+          to wait for new data to be fetched to compute the known state.
+          """,
+      "4.0.0"),
   MONITOR_FETCH_TIMEOUT("monitor.fetch.timeout", "5m", PropertyType.TIMEDURATION, """
       The Monitor fetches information for display in a set of background threads. This property \
       controls the amount of time that process should wait before cancelling any remaining \

server/monitor/src/main/java/org/apache/accumulo/monitor/next/InformationFetcher.java

@@ -78,6 +78,7 @@
 import org.apache.accumulo.core.tabletscan.thrift.ActiveScan;
 import org.apache.accumulo.core.tabletscan.thrift.TabletScanClientService;
 import org.apache.accumulo.core.trace.TraceUtil;
+import org.apache.accumulo.core.util.Timer;
 import org.apache.accumulo.core.util.UtilWaitThread;
 import org.apache.accumulo.core.util.compaction.ExternalCompactionUtil;
 import org.apache.accumulo.core.util.threads.ThreadPools;
@@ -660,10 +661,11 @@ public void newConnectionEvent() {
 
   // Protect against NPE and wait for initial data gathering
   private SystemInformation getSummary() throws InterruptedException {
-    while (summaryRef.get() == null) {
+    SystemInformation summary;
+    while ((summary = summaryRef.get()) == null) {
       Thread.sleep(100);
     }
-    return summaryRef.get();
+    return summary;
   }
 
   /**
@@ -690,13 +692,12 @@ public void onRemoval(@Nullable ServerId server, @Nullable MetricResponse respon
     if (server == null) {
       return;
     }
-    try {
-      getSummary().processError(server);
-      LOG.info("{} has been evicted", server);
-    } catch (InterruptedException e) {
-      Thread.currentThread().interrupt();
-      LOG.warn("{} could not be evicted", server, e);
+    final SystemInformation currentSummary = summaryRef.get();
+    if (currentSummary == null) {
+      return;
     }
+    currentSummary.processError(server);
+    LOG.info("{} has been evicted", server);
   }
 
   /**
@@ -792,26 +793,54 @@ private void fetchTabletInformation(SystemInformation summary, UpdateTasks futur
   public void run() {
 
     long lastRunTime = 0;
+    final Timer noConnectionTimer = Timer.startNew();
+    final long clearStateThreshold =
+        ctx.getConfiguration().getTimeInMillis(Property.MONITOR_FETCH_TIMEOUT);
+    final Duration clearStateDuration = Duration.ofMillis(clearStateThreshold);
+    final long minimumRefreshTimeMs = 5000;
     while (true) {
 
+      // Only refresh internal data structure every 5s (old monitor logic).
+      while (NanoTime.millisElapsed(lastRunTime, NanoTime.now()) < minimumRefreshTimeMs) {
+        LOG.trace("Waiting for the 5s refresh interval");
+        try {
+          Thread.sleep(250);
+        } catch (InterruptedException e) {
+          Thread.currentThread().interrupt();
+          throw new IllegalStateException(
+              "Thread " + Thread.currentThread().getName() + " interrupted", e);
+        }
+      }
+
       // Don't fetch new data if there are no connections.
-      // On an initial connection, no data may be displayed.
-      // If a connection has not been made in a while, stale data may be displayed.
-      // Only refresh every 5s (old monitor logic).
-      while (!newConnectionEvent.get() && connectionCount.get() == 0
-          && NanoTime.millisElapsed(lastRunTime, NanoTime.now()) > 5000) {
+      // When summaryRef is not set, then the REST endpoint will wait
+      // until data is retrieved. summaryRef is not set on initial
+      // connection or when there has been no connection for the configured duration.
+      noConnectionTimer.restart();
+      while (!newConnectionEvent.get() && connectionCount.get() == 0) {
+        LOG.trace("Waiting for a connection, connections: {}", connectionCount.get());
         try {
-          Thread.sleep(100);
+          Thread.sleep(250);
         } catch (InterruptedException e) {
           Thread.currentThread().interrupt();
           throw new IllegalStateException(
               "Thread " + Thread.currentThread().getName() + " interrupted", e);
         }
+        // If a connection has not been made in the configured duration,
+        // then clear the summaryRef so that stale data is not displayed.
+        if (this.summaryRef.get() != null && noConnectionTimer.hasElapsed(clearStateDuration)) {
+          LOG.debug("Clearing internal summary state due to no connection for {} ms",
+              clearStateThreshold);
+          SystemInformation oldSummary = summaryRef.getAndSet(null);
+          if (oldSummary != null) {
+            oldSummary.clear();
+          }
+        }
       }
       // reset the connection event flag
       newConnectionEvent.compareAndExchange(true, false);
 
-      LOG.info("Fetching information from servers");
+      LOG.info("Fetching information from servers, connection count: {}", connectionCount.get());
       long fetchCycleStart = System.currentTimeMillis();
 
       final UpdateTasks futures = new UpdateTasks();