Github user aljoscha commented on a diff in the pull request: https://github.com/apache/flink/pull/4368#discussion_r129340400 --- 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. + */ + protected void recoverAndCommit(TXN transaction) { + commit(transaction); + } + + /** + * Abort a transaction. + */ + protected abstract void abort(TXN transaction); + + /** + * Abort a transaction that was rejected by a coordinator after a failure. + */ + protected void recoverAndAbort(TXN transaction) { + abort(transaction); + } + + // ------ entry points for above methods implementing {@CheckPointedFunction} and {@CheckpointListener} ------ + + @Override + public final void invoke(IN value) throws Exception { + invoke(currentTransaction, value); + } + + @Override + public final void notifyCheckpointComplete(long checkpointId) throws Exception { + // the following scenarios are possible here + // + // (1) there is exactly one transaction from the latest checkpoint that + // was triggered and completed. That should be the common case. + // Simply commit that transaction in that case. + // + // (2) there are multiple pending transactions because one previous + // checkpoint was skipped. That is a rare case, but can happen + // for example when: + // + // - the master cannot persist the metadata of the last + // checkpoint (temporary outage in the storage system) but + // could persist a successive checkpoint (the one notified here) + // + // - other tasks could not persist their status during + // the previous checkpoint, but did not trigger a failure because they + // could hold onto their state and could successfully persist it in + // a successive checkpoint (the one notified here) + // + // In both cases, the prior checkpoint never reach a committed state, but + // this checkpoint is always expected to subsume the prior one and cover all + // changes since the last successful one As a consequence, we need to commit + // all pending transactions. + // + // (3) Multiple transactions are pending, but the checkpoint complete notification + // relates not to the latest. That is possible, because notification messages + // can be delayed (in an extreme case till arrive after a succeeding checkpoint + // was triggered) and because there can be concurrent overlapping checkpoints + // (a new one is started before the previous fully finished). + // + // ==> There should never be a case where we have no pending transaction here + // + + Iterator<TransactionAndCheckpoint<TXN>> pendingTransactionsIterator = pendingCommitTransactions.iterator(); + checkState(pendingTransactionsIterator.hasNext(), "checkpoint completed, but no transaction pending"); + + List<TransactionAndCheckpoint<TXN>> remainingTransactions = new ArrayList<>(); + + for (TransactionAndCheckpoint<TXN> pendingTransaction : pendingCommitTransactions) { + if (pendingTransaction.checkpointId > checkpointId) { + remainingTransactions.add(pendingTransaction); + continue; + } + + LOG.info("{} - checkpoint {} complete, committing completed checkpoint transaction {}", + name(), checkpointId, pendingTransaction); + + // If this fails, there is actually a data loss + commit(pendingTransaction.transaction); + + LOG.debug("{} - committed checkpoint transaction {}", name(), pendingTransaction); + } + + pendingCommitTransactions.clear(); + for (TransactionAndCheckpoint<TXN> remainingTransaction : remainingTransactions) { + pendingCommitTransactions.add(remainingTransaction); + } + } + + @Override + public final void snapshotState(FunctionSnapshotContext context) throws Exception { + // this is like the pre-commit of a 2-phase-commit transaction + // we are ready to commit and remember the transaction + + checkState(currentTransaction != null, "bug: no transaction object when performing state snapshot"); + + long checkpointId = context.getCheckpointId(); + LOG.debug("{} - checkpoint {} triggered, flushing transaction '{}'", name(), context.getCheckpointId(), currentTransaction); + + preCommit(currentTransaction); + pendingCommitTransactions.add(new TransactionAndCheckpoint<>(currentTransaction, checkpointId)); + LOG.debug("{} - stored pending transactions {}", name(), pendingCommitTransactions); + + currentTransaction = beginTransaction(); + LOG.debug("{} - started new transaction '{}'", name(), currentTransaction); + + pendingCommitTransactionsState.clear(); + for (TransactionAndCheckpoint<TXN> pendingCommitTransaction : pendingCommitTransactions) { + pendingCommitTransactionsState.add(pendingCommitTransaction); + } + + pendingTransactionsState.clear(); + // in case of failure we might not be able to abort currentTransaction. Let's store it into the state + // so it can be aborted after a restart/crash + pendingTransactionsState.add(currentTransaction); + } + + @Override + public final void initializeState(FunctionInitializationContext context) throws Exception { + // when we are restoring state with pendingCommitTransactions, we don't really know whether the + // transactions were already committed, or whether there was a failure between + // completing the checkpoint on the master, and notifying the writer here. + + // (the common case is actually that is was already committed, the window + // between the commit on the master and the notification here is very small) + + // it is possible to not have any transactions at all if there was a failure before + // the first completed checkpoint, or in case of a scale-out event, where some of the + // new task do not have and transactions assigned to check) + + // we can have more than one transaction to check in case of a scale-in event, or + // for the reasons discussed in the 'notifyCheckpointComplete()' method. + + pendingTransactionsState = context.getOperatorStateStore().getListState(pendingTransactionsDescriptor); + pendingCommitTransactionsState = context.getOperatorStateStore().getListState(pendingCommitTransactionsDescriptor); + + int subtaskIndex = getRuntimeContext().getIndexOfThisSubtask(); + if (context.isRestored()) { + LOG.info("{} - restoring state (subtaskIndex={}).", name(), subtaskIndex); + + for (TransactionAndCheckpoint<TXN> recoveredTransaction : pendingCommitTransactionsState.get()) { + recoverAndCommit(recoveredTransaction.transaction); + LOG.debug("{} committed recovered transaction {} (subtaskIndex={})", name(), recoveredTransaction, subtaskIndex); + } + + // Explicitly abort transactions that could be not closed cleanly + for (TXN pendingTransaction : pendingTransactionsState.get()) { + recoverAndAbort(pendingTransaction); + LOG.debug("{} aborted recovered transaction {} (subtaskIndex={})", name(), pendingTransaction, subtaskIndex); + } + } else { + LOG.info("{} - no state to restore {} (subtaskIndex={}).", name(), subtaskIndex); + } + this.pendingCommitTransactions.clear(); + } + + @Override + public void open(Configuration parameters) throws Exception { --- End diff -- In a function that has `initializeState()` you can also put this code in there because `initializeState()` will always be called. Having both `open()` and `initializeState()` is tricky because of the ordering in which those two are called (this has lead to problems in the past when people where changing code and not completely aware of the ordering). For the record, currently the ordering is: 1) `initializeState()` 2) `open()`.
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