Github user tzulitai commented on a diff in the pull request:
https://github.com/apache/flink/pull/4239#discussion_r128166034
--- Diff:
flink-streaming-java/src/main/java/org/apache/flink/streaming/api/functions/sink/TwoPhaseCommitSinkFunction.java
---
@@ -0,0 +1,317 @@
+/*
+ * 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.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);
+
+ @Nullable
+ protected TXN currentTransaction;
+
+ protected final List<TransactionAndCheckpoint<TXN>>
pendingCommitTransactions = new ArrayList<>();
+
+ protected ListState<TXN> pendingTransactionsState;
+
+ protected ListState<TransactionAndCheckpoint<TXN>>
pendingCommitTransactionsState;
+
+ // ------ 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 (non Pravega sink) 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
--- End diff --
Lets move this comment block as a Javadoc on the method.
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
