[GitHub] [helix] qqu0127 commented on a diff in pull request #2289: Improve AssignmentMetadataStore with double-checked locking

[GitBox <gi...@apache.org>]

qqu0127 commented on code in PR #2289:
URL: https://github.com/apache/helix/pull/2289#discussion_r1028524870


##########
helix-core/src/main/java/org/apache/helix/controller/rebalancer/waged/AssignmentMetadataStore.java:
##########
@@ -61,36 +62,42 @@ protected AssignmentMetadataStore(BucketDataAccessor bucketDataAccessor, String
     _bestPossiblePath = String.format(BEST_POSSIBLE_TEMPLATE, clusterName, ASSIGNMENT_METADATA_KEY);
   }
 
-  public synchronized Map<String, ResourceAssignment> getBaseline() {
+  public Map<String, ResourceAssignment> getBaseline() {
     // Return the in-memory baseline. If null, read from ZK. This is to minimize reads from ZK
     if (_globalBaseline == null) {
-      try {
-        HelixProperty baseline =
-            _dataAccessor.compressedBucketRead(_baselinePath, HelixProperty.class);
-        _globalBaseline = splitAssignments(baseline);
-      } catch (ZkNoNodeException ex) {
-        // Metadata does not exist, so return an empty map
-        _globalBaseline = new HashMap<>();
+      // double-checked locking
+      synchronized (this) {
+        if (_globalBaseline == null) {
+          _globalBaseline = fetchAssignmentOrDefault(_baselinePath);
+        }
       }
     }
     return _globalBaseline;
   }
 
-  public synchronized Map<String, ResourceAssignment> getBestPossibleAssignment() {
+  public Map<String, ResourceAssignment> getBestPossibleAssignment() {
     // Return the in-memory baseline. If null, read from ZK. This is to minimize reads from ZK
     if (_bestPossibleAssignment == null) {
-      try {
-        HelixProperty baseline =
-            _dataAccessor.compressedBucketRead(_bestPossiblePath, HelixProperty.class);
-        _bestPossibleAssignment = splitAssignments(baseline);
-      } catch (ZkNoNodeException ex) {
-        // Metadata does not exist, so return an empty map
-        _bestPossibleAssignment = new HashMap<>();
+      // double-checked locking
+      synchronized (this) {
+        if (_bestPossibleAssignment == null) {
+          _bestPossibleAssignment = fetchAssignmentOrDefault(_bestPossiblePath);
+        }
       }
     }
     return _bestPossibleAssignment;
   }
 
+  private Map<String, ResourceAssignment> fetchAssignmentOrDefault(String path) {
+    try {

Review Comment:
   Thanks for the comment, but why this has to be synchronized? it's a read operation



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