Github user pnowojski commented on a diff in the pull request: https://github.com/apache/flink/pull/4368#discussion_r129527454 --- Diff: flink-streaming-java/src/main/java/org/apache/flink/streaming/api/functions/sink/TwoPhaseCommitSinkFunction.java --- @@ -0,0 +1,342 @@ +/* + * 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.flink.streaming.api.functions.sink; + +import org.apache.flink.annotation.PublicEvolving; +import org.apache.flink.api.common.state.ListState; +import org.apache.flink.api.common.state.ListStateDescriptor; +import org.apache.flink.api.common.typeinfo.TypeHint; +import org.apache.flink.api.common.typeinfo.TypeInformation; +import org.apache.flink.configuration.Configuration; +import org.apache.flink.runtime.state.CheckpointListener; +import org.apache.flink.runtime.state.FunctionInitializationContext; +import org.apache.flink.runtime.state.FunctionSnapshotContext; +import org.apache.flink.streaming.api.checkpoint.CheckpointedFunction; + +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + +import javax.annotation.Nullable; + +import java.io.Serializable; +import java.util.ArrayList; +import java.util.Iterator; +import java.util.List; +import java.util.Objects; + +import static java.util.Objects.requireNonNull; + +/** + * This is a recommended base class for all of the {@link SinkFunction} that intend to implement exactly-once semantic. + * It does that by implementing two phase commit algorithm on top of the {@link CheckpointedFunction} and + * {@link CheckpointListener}. User should provide custom TXN (transaction handle) and implement abstract methods + * handling this transaction handle. + * + * @param <IN> Input type for {@link SinkFunction} + * @param <TXN> Transaction to store all of the information required to handle a transaction (must be Serializable) + */ +@PublicEvolving +public abstract class TwoPhaseCommitSinkFunction<IN, TXN extends Serializable> + extends RichSinkFunction<IN> + implements CheckpointedFunction, CheckpointListener { + + private static final Logger LOG = LoggerFactory.getLogger(TwoPhaseCommitSinkFunction.class); + + protected final ListStateDescriptor<TransactionAndCheckpoint<TXN>> pendingCommitTransactionsDescriptor; + protected final ListStateDescriptor<TXN> pendingTransactionsDescriptor; + + protected final List<TransactionAndCheckpoint<TXN>> pendingCommitTransactions = new ArrayList<>(); + + @Nullable + protected TXN currentTransaction; + protected ListState<TXN> pendingTransactionsState; + protected ListState<TransactionAndCheckpoint<TXN>> pendingCommitTransactionsState; + + public TwoPhaseCommitSinkFunction(Class<TXN> txnClass) { + this( + TypeInformation.of(txnClass), + TypeInformation.of(new TypeHint<TransactionAndCheckpoint<TXN>>() {})); + } + + public TwoPhaseCommitSinkFunction( + TypeInformation<TXN> txnTypeInformation, + TypeInformation<TransactionAndCheckpoint<TXN>> txnAndCheckpointTypeInformation) { + this( + new ListStateDescriptor<>("pendingTransactions", txnTypeInformation), + new ListStateDescriptor<>("pendingCommitTransactions", txnAndCheckpointTypeInformation)); + } + + public TwoPhaseCommitSinkFunction( + ListStateDescriptor<TXN> pendingTransactionsDescriptor, + ListStateDescriptor<TransactionAndCheckpoint<TXN>> pendingCommitTransactionsDescriptor) { + this.pendingTransactionsDescriptor = requireNonNull(pendingTransactionsDescriptor, "pendingTransactionsDescriptor is null"); + this.pendingCommitTransactionsDescriptor = requireNonNull(pendingCommitTransactionsDescriptor, "pendingCommitTransactionsDescriptor is null"); + } + + // ------ methods that should be implemented in child class to support two phase commit algorithm ------ + + /** + * Write value within a transaction. + */ + protected abstract void invoke(TXN transaction, IN value) throws Exception; + + /** + * Method that starts a new transaction. + * + * @return newly created transaction. + */ + protected abstract TXN beginTransaction() throws Exception; + + /** + * Pre commit previously created transaction. Pre commit must make all of the necessary steps to prepare the + * transaction for a commit that might happen in the future. After this point the transaction might still be + * aborted, but underlying implementation must ensure that commit calls on already pre committed transactions + * will always succeed. + * + * <p>Usually implementation involves flushing the data. + */ + protected abstract void preCommit(TXN transaction) throws Exception; + + /** + * Commit a pre-committed transaction. If this method fail, Flink application will be + * restarted and {@link TwoPhaseCommitSinkFunction#recoverAndCommit(Serializable)} will be called again for the + * same transaction. + */ + protected abstract void commit(TXN transaction); + + /** + * Invoked on recovered transactions after a failure. Must eventually succeed. If it fails, Flink application will + * be restarted and it will be invoked again. If it does not succeed it means a data loss will occur. --- End diff -- It is a just warning, that if it doesn't succeed eventually, there will be a data loss. But since it is using a user code for recovering and committing a transaction, it is the user that must ensure that "commits will always eventually succeed,". I rephrased a little bit this java doc.
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