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While partial updates are intended to update existing records, there will be certain cases which require support for first-time partial update i.e update to a record which doesn’t exist. To account for such cases, the design needs to have a provision to optionally provide a default record which can be PUT in the absence of an existing record. The update can then be applied on top of the default record.

The approach introduces UpdateMessage class which captures the update and an optional default. sendUpdateTo operator which sends updates to a table is designed to take a key (which uniquely identifies a record) and the UpdateMessage as the value.

SendUpdateToTableOperatorImpl is the implementation of the send-update-to-table operator and whether this operator supports first-time partial update entirely depends on the Remote store's implementation of the TableWriteFunction. This approach introduces a RecordNotFoundException which is a custom exception to be thrown in the updateAsync method of TableWriteFunction if the update fails due to an existing record not present for the key. If SendUpdateToTableOperatorImpl operator encounters this exception, it attempts to PUT a default (if one is provided) and then applies an update on top of it. is designed

Code Block

language	java
title	UpdateMessage
collapse	true

/**
 * Represents an update and an optional default record to be inserted for a key,
 * if the update is applied to a non-existent record.
 *
 * @param <U> type of the update record
 * @param <V> type of the default record
 */
public final class UpdateMessage<U, V> {
  private final U update;
  @Nullable private final V defaultValue;

  public static <U, V> UpdateMessage<U, V> of(U update, @Nullable V defaultValue) {
    return new UpdateMessage<>(update, defaultValue);
  }

  public static <U, V> UpdateMessage<U, V> of(U update) {
    return new UpdateMessage<>(update, null);
  }

  private UpdateMessage(U update, V defaultValue) {
    this.update = update;
    this.defaultValue = defaultValue;
  }

  public U getUpdate() {
    return update;
  }

  public V getDefault() {
    return defaultValue;
  }
}

Code Block

language	java
title	SendUpdateToTableOperatorImpl
collapse	true

/*
 * 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.samza.operators.impl;

import java.util.Collection;
import java.util.Collections;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import org.apache.samza.SamzaException;
import org.apache.samza.context.Context;
import org.apache.samza.operators.KV;
import org.apache.samza.operators.UpdateMessage;
import org.apache.samza.operators.spec.OperatorSpec;
import org.apache.samza.operators.spec.SendUpdateToTableOperatorSpec;
import org.apache.samza.table.ReadWriteTable;
import org.apache.samza.table.RecordNotFoundException;
import org.apache.samza.table.batching.CompactBatchProvider;
import org.apache.samza.table.remote.RemoteTable;
import org.apache.samza.task.MessageCollector;
import org.apache.samza.task.TaskCoordinator;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


/**
 * Implementation of a send-update-stream-to-table operator that applies updates to existing records
 * in the table. If there is no pre-existing record, based on Table's implementation it might attempt to write a
 * default record and then applies an update.
 *
 * @param <K> the type of the record key
 * @param <V> the type of the record value
 * @param <U> the type of the update applied to this table
 */
public class SendUpdateToTableOperatorImpl<K, V, U>
    extends OperatorImpl<KV<K, UpdateMessage<U, V>>, KV<K, UpdateMessage<U, V>>> {
  private static final Logger LOG = LoggerFactory.getLogger(SendUpdateToTableOperatorImpl.class);

  private final SendUpdateToTableOperatorSpec<K, V, U> sendUpdateToTableOpSpec;
  private final ReadWriteTable<K, V, U> table;

  public SendUpdateToTableOperatorImpl(SendUpdateToTableOperatorSpec<K, V, U>  sendUpdateToTableOpSpec, Context context) {
    this.sendUpdateToTableOpSpec = sendUpdateToTableOpSpec;
    this.table = context.getTaskContext().getTable(sendUpdateToTableOpSpec.getTableId());
    if (context.getTaskContext().getTable(sendUpdateToTableOpSpec.getTableId()) instanceof RemoteTable) {
      RemoteTable<K, V, U> remoteTable = (RemoteTable<K, V, U>) table;
      if (remoteTable.getBatchProvider() instanceof CompactBatchProvider) {
        throw new SamzaException("Batching is not supported with Compact Batches for partial updates");
      }
    }
  }

  @Override
  protected void handleInit(Context context) {
  }

  @Override
  protected CompletionStage<Collection<KV<K, UpdateMessage<U, V>>>> handleMessageAsync(KV<K, UpdateMessage<U, V>> message,
      MessageCollector collector, TaskCoordinator coordinator) {
    final CompletableFuture<Void> updateFuture = table.updateAsync(message.getKey(), message.getValue().getUpdate(),
        sendUpdateToTableOpSpec.getArgs());

    return updateFuture
        .handle((result, ex) -> {
          if (ex == null) {
            // success, no need to Put a default value
            return false;
          } else if (ex.getCause() instanceof RecordNotFoundException && message.getValue().getDefault() != null) {
            // If update fails for a given key due to a RecordDoesNotExistException exception thrown and a default is
            // provided, then attempt to PUT a default record for the key and then apply the update
            return true;
          } else {
            throw new SamzaException("Update failed with exception: ", ex);
          }
        })
        .thenCompose(shouldPutDefault -> {
          if (shouldPutDefault) {
            final CompletableFuture<Void> putFuture = table.putAsync(message.getKey(), message.getValue().getDefault(),
                sendUpdateToTableOpSpec.getArgs());
            return putFuture
                .exceptionally(ex -> {
                  LOG.warn("PUT default failed due to an exception. Ignoring the exception and proceeding with update. "
                          + "The exception encountered is: ", ex);
                  return null;
                })
                .thenCompose(res -> table.updateAsync(message.getKey(), message.getValue().getUpdate(),
                sendUpdateToTableOpSpec.getArgs()));
          } else {
            return CompletableFuture.completedFuture(null);
          }
        }).thenApply(result -> Collections.singleton(message));
  }

  @Override
  protected void handleClose() {
    table.close();
  }

  @Override
  protected OperatorSpec<KV<K, UpdateMessage<U, V>>, KV<K, UpdateMessage<U, V>>> getOperatorSpec() {
    return sendUpdateToTableOpSpec;
  }
}

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