[GitHub] [hudi] alexeykudinkin commented on a diff in pull request #5627: [HUDI-3350][HUDI-3351] Support HoodieMerge API and Spark engine-specific HoodieRecord

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

alexeykudinkin commented on code in PR #5627:
URL: https://github.com/apache/hudi/pull/5627#discussion_r910521355


##########
hudi-common/src/main/java/org/apache/hudi/common/model/HoodieMerge.java:
##########
@@ -0,0 +1,38 @@
+/*
+ * 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.avro.Schema;
+import org.apache.hudi.common.util.Option;
+
+import java.io.IOException;
+import java.io.Serializable;
+import java.util.Properties;
+
+/**
+ * HoodieMerge defines how to merge two records. It is a stateless component.
+ * It can implement the merging logic of HoodieRecord of different engines
+ * and avoid the performance consumption caused by the serialization/deserialization of Avro payload.
+ */
+public interface HoodieMerge extends Serializable {

Review Comment:
   "Merge" by itself is a very opaque term and when we call it `HoodieMerge` w/o contextualizing it it becomes hard to understand what it actually relates to.
   
   I'd suggest to instead be a little more verbose in the name to make sure it's very clear what this component is intended to do, for ex something like `HoodieRecordMergingEngine`



##########
hudi-common/src/main/java/org/apache/hudi/common/model/HoodieMerge.java:
##########
@@ -0,0 +1,38 @@
+/*
+ * 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.avro.Schema;
+import org.apache.hudi.common.util.Option;
+
+import java.io.IOException;
+import java.io.Serializable;
+import java.util.Properties;
+
+/**
+ * HoodieMerge defines how to merge two records. It is a stateless component.
+ * It can implement the merging logic of HoodieRecord of different engines
+ * and avoid the performance consumption caused by the serialization/deserialization of Avro payload.
+ */
+public interface HoodieMerge extends Serializable {
+  
+  HoodieRecord preCombine(HoodieRecord older, HoodieRecord newer);

Review Comment:
   And for us to actually be able to do that we have to enforce following properties onto the `merge` implementations:
   
    - It'd be [associative operation](https://en.wikipedia.org/wiki/Associative_property): `f(a, f(b, c)) = f(f(a, b), c)` (which we can translate as having 3 versions A, B, C of the single record, both orders of operations applications have to yield the same result)



##########
hudi-common/src/main/java/org/apache/hudi/common/model/HoodieMerge.java:
##########
@@ -0,0 +1,38 @@
+/*
+ * 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.avro.Schema;
+import org.apache.hudi.common.util.Option;
+
+import java.io.IOException;
+import java.io.Serializable;
+import java.util.Properties;
+
+/**
+ * HoodieMerge defines how to merge two records. It is a stateless component.
+ * It can implement the merging logic of HoodieRecord of different engines
+ * and avoid the performance consumption caused by the serialization/deserialization of Avro payload.
+ */
+public interface HoodieMerge extends Serializable {
+  
+  HoodieRecord preCombine(HoodieRecord older, HoodieRecord newer);

Review Comment:
   So, historically, this has been 2 different methods with (potentially) different semantics:
     - `preCombine` is de-duplicating the input batch (before inserting it into the table
     - `combineAndGet` is used to merge persisted version with the incoming (that could have been previously de-duplicated
   
   I also don't see a reason for us to get hung up on this historical context and we should try to unify these historically (potentially) divergent methods into 1 providing a single avenue of merging records either inside a batch (when de-duping) or when combining persisted one with the incoming.



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