[GitHub] [incubator-druid] jon-wei commented on a change in pull request #7661: Fix compareAndSwap() in SQLMetadataConnector

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

jon-wei commented on a change in pull request #7661: Fix compareAndSwap() in SQLMetadataConnector
URL: https://github.com/apache/incubator-druid/pull/7661#discussion_r284426105
 
 

 ##########
 File path: extensions-core/postgresql-metadata-storage/src/main/java/org/apache/druid/metadata/storage/postgresql/PostgreSQLConnector.java
 ##########
 @@ -213,6 +221,45 @@ public Void withHandle(Handle handle) throws Exception
     );
   }
 
+  @Override
+  public boolean compareAndSwap(List<MetadataCASUpdate> updates)
+  {
+    try {
+      return super.compareAndSwap(updates);
+    }
+    catch (CallbackFailedException cfe) {
+      Throwable root = Throwables.getRootCause(cfe);
+      if (checkRootCauseForPSQLSerializationFailure(root)) {
+        return false;
+      } else {
+        throw cfe;
+      }
+    }
+  }
+
+  /**
+   * Used by compareAndSwap to check if the transaction was terminated because of concurrent updates.
+   *
+   * The parent implementation's compareAndSwap transaction has isolation level REPEATABLE_READ.
+   * In Postgres, such transactions will be canceled when another transaction commits a conflicting update:
+   * https://www.postgresql.org/docs/10/transaction-iso.html#XACT-REPEATABLE-READ
+   *
+   * When this occurs, we need to retry the transaction from the beginning: by returning false in compareAndSwap,
+   * the calling code will attempt retries.
+   */
+  private boolean checkRootCauseForPSQLSerializationFailure(
+      Throwable root
 
 Review comment:
   With the somewhat non-trivial boolean conditions I felt like it looked nicer as a separate method

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