GrantPSpencer commented on code in PR #2805:
URL: https://github.com/apache/helix/pull/2805#discussion_r1628525748


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meta-client/src/main/java/org/apache/helix/metaclient/impl/zk/ZkMetaClient.java:
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@@ -207,16 +208,37 @@ public void set(String key, T data, int version) {
 
   @Override
   public T update(String key, DataUpdater<T> updater) {
-    org.apache.zookeeper.data.Stat stat = new org.apache.zookeeper.data.Stat();
-    // TODO: add retry logic for ZkBadVersionException.
-    try {
-      T oldData = _zkClient.readData(key, stat);
-      T newData = updater.update(oldData);
-      set(key, newData, stat.getVersion());
-      return newData;
-    } catch (ZkException e) {
-      throw translateZkExceptionToMetaclientException(e);
-    }
+    boolean retry;
+    T updatedData = null;
+    do {
+      retry = false;
+      try {
+        ImmutablePair<T, Stat> tup = getDataAndStat(key);
+        Stat stat = tup.right;
+        T oldData = tup.left;
+        T newData = updater.update(oldData);
+        set(key, newData, stat.getVersion());
+        updatedData = newData;
+      } catch (MetaClientBadVersionException badVersionException) {
+        // Retry on bad version
+        retry = true;
+      } catch (MetaClientNoNodeException noNodeException) {
+        // If node does not exist, attempt to create it - pass null to updater
+        T newData = updater.update(null);
+        if (newData != null) {
+          try {
+            create(key, newData);
+            updatedData = newData;
+          } catch (MetaClientNodeExistsException nodeExistsException) {
+            // If node now exists, then retry update
+            retry = true;
+          } catch (ZkException e) {
+            throw translateZkExceptionToMetaclientException(e);
+          }
+        }
+      }
+    } while (retry);

Review Comment:
   Added MAX_RETRY_ATTEMPTS to limit updater attempts. 
   Just want to clarify wording. Would total allowed attempts be the initial 
attempt + retry attempts after first failure? So 1 + 3 = 4 total attempts to 
update the znode. Or just 3 total attempts?
   



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