Fix: added a buffer to LSManager (#1286)

* added a buffer to LSManager

* Added the out of sync buffer when we are in sync

src/main/java/com/iota/iri/service/snapshot/impl/LocalSnapshotManagerImpl.java

@@ -26,6 +26,12 @@ public class LocalSnapshotManagerImpl implements LocalSnapshotManager {
      * due.
      */
     private static final int LOCAL_SNAPSHOT_RESCAN_INTERVAL = 10000;
+
+    /**
+     * To prevent jumping back and forth in and out of sync, there is a buffer in between.
+     * Only when the latest milestone and latest snapshot differ more than this number, we fall out of sync
+     */
+    private static final int LOCAL_SNAPSHOT_SYNC_BUFFER = 5;
 
     /**
      * Logger for this class allowing us to dump debug and status messages.
@@ -51,6 +57,11 @@ public class LocalSnapshotManagerImpl implements LocalSnapshotManager {
      * Configuration with important snapshot related parameters.
      */
     private SnapshotConfig config;
+
+    /**
+     * If this node is currently seen as in sync
+     */
+    private boolean isInSync;
 
     /**
      * Holds a reference to the {@link ThreadIdentifier} for the monitor thread.
@@ -85,6 +96,8 @@ public LocalSnapshotManagerImpl init(SnapshotProvider snapshotProvider, Snapshot
         this.snapshotService = snapshotService;
         this.transactionPruner = transactionPruner;
         this.config = config;
+
+        this.isInSync = false;
 
         return this;
     }
@@ -117,10 +130,7 @@ public void shutdown() {
      */
     private void monitorThread(LatestMilestoneTracker latestMilestoneTracker) {
         while (!Thread.currentThread().isInterrupted()) {
-            int localSnapshotInterval = latestMilestoneTracker.isInitialScanComplete() &&
-                    snapshotProvider.getLatestSnapshot().getIndex() == latestMilestoneTracker.getLatestMilestoneIndex()
-                    ? config.getLocalSnapshotsIntervalSynced()
-                    : config.getLocalSnapshotsIntervalUnsynced();
+            int localSnapshotInterval = getSnapshotInterval(isInSync(latestMilestoneTracker));
 
             int latestSnapshotIndex = snapshotProvider.getLatestSnapshot().getIndex();
             int initialSnapshotIndex = snapshotProvider.getInitialSnapshot().getIndex();
@@ -136,4 +146,46 @@ private void monitorThread(LatestMilestoneTracker latestMilestoneTracker) {
             ThreadUtils.sleep(LOCAL_SNAPSHOT_RESCAN_INTERVAL);
         }
     }
+
+    /**
+     * A snapshot is taken in an interval. 
+     * This interval changes based on the state of the node.
+     * 
+     * @param inSync if this node is in sync
+     * @return the current interval in which we take local snapshots
+     */
+    private int getSnapshotInterval(boolean inSync) {
+        return inSync
+                ? config.getLocalSnapshotsIntervalSynced()
+                : config.getLocalSnapshotsIntervalUnsynced();
+    }
+
+    /**
+     * A node is defined in sync when the latest snapshot milestone index and the latest milestone index are equal.
+     * In order to prevent a bounce between in and out of sync, a buffer is added when a node became in sync.
+     * 
+     * This will always return false if we are not done scanning milestone candidates during initialization.
+     * 
+     * @param latestMilestoneTracker tracker we use to determine milestones
+     * @return <code>true</code> if we are in sync, otherwise <code>false</code>
+     */
+    private boolean isInSync(LatestMilestoneTracker latestMilestoneTracker) {
+        if (!latestMilestoneTracker.isInitialScanComplete()) {
+            return false;
+        }
+
+        int latestIndex = latestMilestoneTracker.getLatestMilestoneIndex();
+        int latestSnapshot = snapshotProvider.getLatestSnapshot().getIndex();
+
+        // If we are out of sync, only a full sync will get us in
+        if (!isInSync && latestIndex == latestSnapshot) {
+            isInSync = true;
+
+        // When we are in sync, only dropping below the buffer gets us out of sync 
+        } else if (latestSnapshot < latestIndex - LOCAL_SNAPSHOT_SYNC_BUFFER) {
+            isInSync = false;
+        }
+
+        return isInSync;
+    }
 }