keith-turner commented on code in PR #4733:
URL: https://github.com/apache/accumulo/pull/4733#discussion_r1687143518


##########
server/manager/src/main/java/org/apache/accumulo/manager/Manager.java:
##########
@@ -960,39 +957,92 @@ private void 
checkForHeldServer(SortedMap<TServerInstance,TabletServerStatus> ts
       }
     }
 
-    private long balanceTablets() {
-
-      Map<DataLevel,Set<KeyExtent>> partitionedMigrations =
+    /**
+     * balanceTablets() balances tables by DataLevel. Return the current set 
of migrations
+     * partitioned by DataLevel
+     */
+    private Map<DataLevel,Set<KeyExtent>> partitionMigrations(final 
Set<KeyExtent> migrations) {
+      final Map<DataLevel,Set<KeyExtent>> partitionedMigrations =
           new HashMap<>(DataLevel.values().length);
-      migrationsSnapshot().forEach(ke -> {
-        partitionedMigrations.computeIfAbsent(DataLevel.of(ke.tableId()), f -> 
new HashSet<>())
-            .add(ke);
+      // populate to prevent NPE
+      for (DataLevel dl : DataLevel.values()) {
+        partitionedMigrations.put(dl, new HashSet<>());
+      }
+      migrations.forEach(ke -> {
+        partitionedMigrations.get(DataLevel.of(ke.tableId())).add(ke);
       });
+      return partitionedMigrations;
+    }
+
+    /**
+     * Given the current tserverStatus map and a DataLevel, return a view of 
the tserverStatus map
+     * that only contains entries for tables in the DataLevel
+     */
+    private SortedMap<TServerInstance,TabletServerStatus> 
createTServerStatusView(
+        final DataLevel dl, final 
SortedMap<TServerInstance,TabletServerStatus> status) {
+      final SortedMap<TServerInstance,TabletServerStatus> 
tserverStatusForLevel = new TreeMap<>();
+      status.forEach((tsi, tss) -> {
+        final TabletServerStatus copy = tss.deepCopy();
+        final Map<String,TableInfo> oldTableMap = copy.getTableMap();
+        final Map<String,TableInfo> newTableMap =
+            new HashMap<>(dl == DataLevel.USER ? oldTableMap.size() : 1);
+        if (dl == DataLevel.ROOT) {
+          if (oldTableMap.containsKey(RootTable.NAME)) {
+            newTableMap.put(RootTable.NAME, oldTableMap.get(RootTable.NAME));
+          }
+        } else if (dl == DataLevel.METADATA) {
+          if (oldTableMap.containsKey(MetadataTable.NAME)) {
+            newTableMap.put(MetadataTable.NAME, 
oldTableMap.get(MetadataTable.NAME));
+          }
+        } else if (dl == DataLevel.USER) {
+          oldTableMap.forEach((table, info) -> {
+            if (!table.equals(RootTable.NAME) && 
!table.equals(MetadataTable.NAME)) {
+              newTableMap.put(table, info);
+            }
+          });

Review Comment:
   There is a good chance that a lot of tservers are only hosting user tables 
and there is nothing to do, maybe could do the following to optimize this case.
   
   
   ```suggestion
         if(!oldTableMap.containsKey(MetadataTable.NAME) && 
!oldTableMap.containsKey(RootTable.NAME)){
             newTableMap = oldTableMap;
         } else {
             oldTableMap.forEach((table, info) -> {
               if (!table.equals(RootTable.NAME) && 
!table.equals(MetadataTable.NAME)) {
                 newTableMap.put(table, info);
               }
             });
         }
   ```



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