[GitHub] [hudi] stayrascal commented on a change in pull request #4141: [HUDI-2815] Support partial update for streaming change logs

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stayrascal commented on a change in pull request #4141:
URL: https://github.com/apache/hudi/pull/4141#discussion_r788331396



##########
File path: hudi-common/src/main/java/org/apache/hudi/common/model/PartialUpdateWithLatestAvroPayload.java
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@@ -0,0 +1,78 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hudi.common.model;
+
+import org.apache.hudi.common.util.Option;
+
+import org.apache.avro.Schema;
+import org.apache.avro.generic.GenericRecord;
+import org.apache.avro.generic.IndexedRecord;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Objects;
+import java.util.Properties;
+
+import static org.apache.hudi.avro.HoodieAvroUtils.bytesToAvro;
+
+/**
+ * The only difference with {@link DefaultHoodieRecordPayload} is that support update partial fields
+ * in latest record which value is not null to existing record instead of all fields.
+ *
+ * <p> Assuming a {@link GenericRecord} has three fields: a int , b int, c int. The first record value: 1, 2, 3.
+ * The second record value is: 4, 5, null, the field c value is null. After call the combineAndGetUpdateValue method,
+ * we will get final record value: 4, 5, 3, field c value will not be overwritten because its value is null in latest record.
+ */
+public class PartialUpdateWithLatestAvroPayload extends DefaultHoodieRecordPayload {
+

Review comment:
       Hi @danny0405 , if I didn't understand wrongly, the `preCombine` method will be called during deduplicate record by `flushBucket` or `flushRemaining` in `SteamWriteFucntion`, which will only deduplicate the records(new created/updated record) in the buffer. 
   
   If we only overwrite `preCombine`, which will only update/merge the records in the buffer, but if there is a record with same recordKey existed in base/log file, the record will be overwrote by the  new merged record from buffer, right?
   
   For example, in COW mode, we might still need to overwrite `combineAndGetUpdateValue` method, because it will be called by `HoodieMergeHandle.write(GenericRecord oldRecord)`, this method will merge the new merged record with old records.
   ```
   public void write(GenericRecord oldRecord) {
       String key = KeyGenUtils.getRecordKeyFromGenericRecord(oldRecord, keyGeneratorOpt);
       boolean copyOldRecord = true;
       if (keyToNewRecords.containsKey(key)) {
         // If we have duplicate records that we are updating, then the hoodie record will be deflated after
         // writing the first record. So make a copy of the record to be merged
         HoodieRecord<T> hoodieRecord = new HoodieRecord<>(keyToNewRecords.get(key));
         try {
           Option<IndexedRecord> combinedAvroRecord =
               hoodieRecord.getData().combineAndGetUpdateValue(oldRecord,
                 useWriterSchema ? tableSchemaWithMetaFields : tableSchema,
                   config.getPayloadConfig().getProps());
   
           if (combinedAvroRecord.isPresent() && combinedAvroRecord.get().equals(IGNORE_RECORD)) {
             // If it is an IGNORE_RECORD, just copy the old record, and do not update the new record.
             copyOldRecord = true;
           } else if (writeUpdateRecord(hoodieRecord, oldRecord, combinedAvroRecord)) {
             /*
              * ONLY WHEN 1) we have an update for this key AND 2) We are able to successfully
              * write the the combined new
              * value
              *
              * We no longer need to copy the old record over.
              */
             copyOldRecord = false;
           }
           writtenRecordKeys.add(key);
         } catch (Exception e) {
           throw new HoodieUpsertException("Failed to combine/merge new record with old value in storage, for new record {"
               + keyToNewRecords.get(key) + "}, old value {" + oldRecord + "}", e);
         }
       }
   ```




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