HoustonPutman commented on code in PR #909:
URL: https://github.com/apache/solr/pull/909#discussion_r921461000


##########
solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java:
##########
@@ -246,6 +245,138 @@ public boolean canBeRemoved() {
     }
   }
 
+  /**
+   * A ConcurrentHashMap of active watcher by collection name
+   *
+   * <p>Each watcher DocCollectionWatch also contains the latest DocCollection 
(state) observed
+   */
+  private static class DocCollectionWatches {
+    private final ConcurrentHashMap<String, StatefulCollectionWatch>
+        statefulWatchesByCollectionName = new ConcurrentHashMap<>();
+
+    /**
+     * Gets the DocCollection (state) of the collection which the 
corresponding watch last observed
+     *
+     * @param collection the collection name to get DocCollection on
+     * @return The last observed DocCollection(state). if null, that means 
there's no such
+     *     collection.
+     */
+    private DocCollection getDocCollection(String collection) {
+      StatefulCollectionWatch watch =
+          statefulWatchesByCollectionName.get(collection);
+      return watch != null ? watch.currentState : null;
+    }
+
+    /**
+     * Gets the active collections (collections that exist) being watched
+     *
+     * @return an immutable set of active collection names
+     */
+    private Set<String> activeCollections() {
+      return statefulWatchesByCollectionName.entrySet().stream()
+          .filter(
+              (Entry<String, StatefulCollectionWatch> entry) ->
+                  entry.getValue().currentState != null)
+          .map(Entry::getKey)
+          .collect(Collectors.toUnmodifiableSet());
+    }
+
+    /**
+     * Gets the count of active collections (collections that exist) being 
watched
+     *
+     * @return the count of active collections
+     */
+    private long activeCollectionCount() {
+      return statefulWatchesByCollectionName.entrySet().stream()
+          .filter(
+              (Entry<String, StatefulCollectionWatch> entry) ->
+                  entry.getValue().currentState != null)
+          .count();
+    }
+
+    private Set<String> watchedCollections() {
+      return 
Collections.unmodifiableSet(statefulWatchesByCollectionName.keySet());
+    }
+
+    private Set<Entry<String, StatefulCollectionWatch>>
+        watchedCollectionEntries() {
+      return 
Collections.unmodifiableSet(statefulWatchesByCollectionName.entrySet());
+    }
+
+    /**
+     * Updates the latest observed DocCollection (state) of the {@link 
StatefulCollectionWatch} if
+     * the collection is being watched
+     *
+     * @param collection the collection name
+     * @param newState the new DocCollection (state) observed
+     * @return whether an active watch exists for such collection
+     */
+    private boolean updateDocCollection(String collection, DocCollection 
newState) {
+      StatefulCollectionWatch finalWatch =
+          statefulWatchesByCollectionName.computeIfPresent(
+              collection,
+              (col, watch) -> {
+                DocCollection oldState = watch.currentState;
+                if (oldState == null && newState == null) {
+                  // OK, the collection not yet exist in ZK or already deleted
+                } else if (oldState == null) {
+                  if (log.isDebugEnabled()) {
+                    log.debug("Add data for [{}] ver [{}]", collection, 
newState.getZNodeVersion());
+                  }
+                  watch.currentState = newState;
+                } else if (newState == null) {
+                  log.debug("Removing cached collection state for [{}]", 
collection);
+                  watch.currentState = null;
+                } else { // both new and old states are non-null
+                  int oldCVersion =
+                      oldState.getPerReplicaStates() == null
+                          ? -1
+                          : oldState.getPerReplicaStates().cversion;
+                  int newCVersion =
+                      newState.getPerReplicaStates() == null
+                          ? -1
+                          : newState.getPerReplicaStates().cversion;
+                  if (oldState.getZNodeVersion() < newState.getZNodeVersion()
+                      || oldCVersion < newCVersion) {
+                    watch.currentState = newState;
+                    if (log.isDebugEnabled()) {
+                      log.debug(
+                          "Updating data for [{}] from [{}] to [{}]",
+                          collection,
+                          oldState.getZNodeVersion(),
+                          newState.getZNodeVersion());
+                    }
+                  }
+                }
+                return watch;
+              });
+      return finalWatch != null;
+    }

Review Comment:
   ```suggestion
        * @return whether the state has changed for the watched collection
        */
       private boolean updateDocCollection(String collection, DocCollection 
newState) {
         AtomicBoolean stateHasChanged = new AtomicBoolean(false);
         statefulWatchesByCollectionName.computeIfPresent(
             collection,
             (col, watch) -> {
               DocCollection oldState = watch.currentState;
               if (oldState == null && newState == null) {
                 // OK, the collection not yet exist in ZK or already deleted
               } else if (oldState == null) {
                 if (log.isDebugEnabled()) {
                   log.debug("Add data for [{}] ver [{}]", collection, 
newState.getZNodeVersion());
                 }
                 watch.currentState = newState;
               } else if (newState == null) {
                 log.debug("Removing cached collection state for [{}]", 
collection);
                 watch.currentState = null;
               } else { // both new and old states are non-null
                 int oldCVersion =
                     oldState.getPerReplicaStates() == null
                         ? -1
                         : oldState.getPerReplicaStates().cversion;
                 int newCVersion =
                     newState.getPerReplicaStates() == null
                         ? -1
                         : newState.getPerReplicaStates().cversion;
                 if (oldState.getZNodeVersion() < newState.getZNodeVersion()
                     || oldCVersion < newCVersion) {
                   watch.currentState = newState;
                   if (log.isDebugEnabled()) {
                     log.debug(
                         "Updating data for [{}] from [{}] to [{}]",
                         collection,
                         oldState.getZNodeVersion(),
                         newState.getZNodeVersion());
                   }
                 }
               }
               stateHasChanged.set(oldState != watch.currentState);
               return watch;
             });
   
         return stateHasChanged.get();
       }
   ```



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