Add some notes, and revert the id map, set the globalId first

common/src/main/java/org/apache/rocketmq/common/TopicQueueMappingDetail.java

@@ -63,13 +63,13 @@ public class TopicQueueMappingDetail extends TopicQueueMappingInfo {
                     && items.size() >= 1) {
                 LogicQueueMappingItem curr = items.get(items.size() - 1);
                 if (bname.equals(curr.getBname())) {
-                    tmpIdMap.put(curr.getQueueId(), globalId);
+                    tmpIdMap.put(globalId, curr.getQueueId());
                 }
             } else if (level == LEVEL_1
                     && items.size() >= 2) {
                 LogicQueueMappingItem prev = items.get(items.size() - 1);
                 if (bname.equals(prev.getBname())) {
-                    tmpIdMap.put(prev.getQueueId(), globalId);
+                    tmpIdMap.put(globalId, prev.getQueueId());
                 }
             }
         }

common/src/main/java/org/apache/rocketmq/common/TopicQueueMappingInfo.java

@@ -29,9 +29,9 @@ public class TopicQueueMappingInfo extends RemotingSerializable {
     int totalQueues;
     String bname;  //identify the hosted broker name
     //register to broker to construct the route
-    ConcurrentMap<Integer, Integer> currIdMap = new ConcurrentHashMap<Integer, Integer>();
+    ConcurrentMap<Integer/*logicId*/, Integer/*physicalId*/> currIdMap = new ConcurrentHashMap<Integer, Integer>();
     //register to broker to help detect remapping failure
-    protected ConcurrentMap<Integer, Integer> prevIdMap = new ConcurrentHashMap<Integer, Integer>();
+    ConcurrentMap<Integer/*logicId*/, Integer/*physicalId*/> prevIdMap = new ConcurrentHashMap<Integer, Integer>();
 
     public TopicQueueMappingInfo(String topic, int totalQueues, String bname) {
         this.topic = topic;

common/src/main/java/org/apache/rocketmq/common/protocol/route/TopicRouteData.java

@@ -33,7 +33,8 @@ public class TopicRouteData extends RemotingSerializable {
     private List<QueueData> queueDatas;
     private List<BrokerData> brokerDatas;
     private HashMap<String/* brokerAddr */, List<String>/* Filter Server */> filterServerTable;
-    private Map<String, TopicQueueMappingInfo> topicQueueMappingByBroker;
+    //It could be null or empty
+    private Map<String/*brokerName*/, TopicQueueMappingInfo> topicQueueMappingByBroker;
 
     public TopicRouteData() {
     }

namesrv/src/main/java/org/apache/rocketmq/namesrv/routeinfo/RouteInfoManager.java

@@ -163,7 +163,7 @@ public class RouteInfoManager {
                         TopicConfigAndMappingSerializeWrapper mappingSerializeWrapper = TopicConfigAndMappingSerializeWrapper.from(topicConfigWrapper);
 
                         Map<String, TopicQueueMappingInfo> topicQueueMappingInfoMap = mappingSerializeWrapper.getTopicQueueMappingInfoMap();
-
+                        //the topicQueueMappingInfoMap should never be null, but can be empty
                         for (Map.Entry<String, TopicQueueMappingInfo> entry : topicQueueMappingInfoMap.entrySet()) {
                             if (!topicQueueMappingInfoTable.containsKey(entry.getKey())) {
                                 topicQueueMappingInfoTable.put(entry.getKey(), new HashMap<String, TopicQueueMappingInfo>());