Github user spdrnl commented on a diff in the pull request:
https://github.com/apache/flink/pull/1620#discussion_r54090368
--- Diff:
flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/operators/GenericExactlyOnceSink.java
---
@@ -0,0 +1,173 @@
+/**
+ * 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
+ * <p/>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p/>
+ * 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.runtime.operators;
+
+import org.apache.flink.api.common.ExecutionConfig;
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.api.java.tuple.Tuple;
+import org.apache.flink.api.java.typeutils.TypeExtractor;
+import org.apache.flink.core.memory.DataInputView;
+import org.apache.flink.runtime.state.AbstractStateBackend;
+import org.apache.flink.runtime.state.StateHandle;
+import org.apache.flink.streaming.api.operators.AbstractStreamOperator;
+import org.apache.flink.streaming.api.operators.OneInputStreamOperator;
+import org.apache.flink.streaming.api.watermark.Watermark;
+import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
+import org.apache.flink.streaming.runtime.tasks.StreamTaskState;
+
+import java.io.IOException;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+/**
+ * Generic Sink that emits its input elements into an arbitrary backend.
This sink is integrated with the checkpointing
+ * mechanism to provide exactly once semantics.
+ *
+ * Incoming records are stored within a {@link
org.apache.flink.runtime.state.AbstractStateBackend}, and only committed if a
+ * checkpoint is completed. Should a job fail while the data is being
committed, no exactly once guarantee can be made.
+ * @param <IN>
+ */
+public abstract class GenericExactlyOnceSink<IN extends Tuple> extends
AbstractStreamOperator<IN> implements OneInputStreamOperator<IN, IN> {
+ private AbstractStateBackend.CheckpointStateOutputView out;
+ private TypeSerializer<IN> serializer;
+ protected TypeInformation<IN> typeInfo;
+
+ private ExactlyOnceState state = new ExactlyOnceState();
+
+ /**
+ * Saves a handle in the state.
+ * @param checkpointId
+ * @throws IOException
+ */
+ private void saveHandleInState(final long checkpointId) throws
IOException {
+ //only add handle if a new OperatorState was created since the
last snapshot/notify
+ if (out != null) {
+ out.writeByte(0); //EOF-byte
+ StateHandle<DataInputView> handle =
out.closeAndGetHandle();
+ if (state.pendingHandles.containsKey(checkpointId)) {
+
state.pendingHandles.get(checkpointId).add(handle);
+ } else {
+ ArrayList<StateHandle<DataInputView>> list =
new ArrayList<>();
+ list.add(handle);
+ state.pendingHandles.put(checkpointId, list);
+ }
+ out = null;
+ }
+ }
+
+ @Override
+ public StreamTaskState snapshotOperatorState(final long checkpointId,
final long timestamp) throws Exception {
+ StreamTaskState taskState =
super.snapshotOperatorState(checkpointId, timestamp);
+ saveHandleInState(checkpointId);
+ taskState.setFunctionState(state);
+ return taskState;
+ }
+
+ @Override
+ public void restoreState(StreamTaskState state, long recoveryTimestamp)
throws Exception {
+ super.restoreState(state, recoveryTimestamp);
+ this.state = (ExactlyOnceState) state.getFunctionState();
+ out = null;
+ }
+
+ @Override
+ public void notifyOfCompletedCheckpoint(long checkpointId) throws
Exception {
--- End diff --
Apart from Flink specifics it is possible to get Cassandra to behave
idempotent with the right index/table structure. See and example below, the eid
field is used as part of the index to identity unique records.
```
cqlsh:mykeyspace> CREATE TABLE IF NOT EXISTS timeline (
eid uuid,
timesegment bigint,
from_url text,
to_url text,
PRIMARY KEY ((timesegment) , eid));
cqlsh:mykeyspace> INSERT INTO timeline (eid, timesegment, from_url, to_url)
VALUES(50554d6e-29bb-11e5-b345-feff819cdc9f, 1, 'a', 'b') ;
cqlsh:mykeyspace> SELECT * FROM timeline;
timesegment | eid | from_url | to_url
-------------+--------------------------------------+----------+--------
1 | 50554d6e-29bb-11e5-b345-feff819cdc9f | a | b
(1 rows)
cqlsh:mykeyspace> INSERT INTO timeline (eid, timesegment, from_url, to_url)
VALUES(50554d6e-29bb-11e5-b345-feff819cdc9f, 1, 'a', 'b') ;
cqlsh:mykeyspace> SELECT * FROM timeline;
timesegment | eid | from_url | to_url
-------------+--------------------------------------+----------+--------
1 | 50554d6e-29bb-11e5-b345-feff819cdc9f | a | b
(1 rows)
cqlsh:mykeyspace> INSERT INTO timeline (eid, timesegment, from_url, to_url)
VALUES(uuid(), 1, 'a', 'b') ;
cqlsh:mykeyspace> SELECT * FROM timeline;
timesegment | eid | from_url | to_url
-------------+--------------------------------------+----------+--------
1 | 50554d6e-29bb-11e5-b345-feff819cdc9f | a | b
1 | 35761e5e-06b3-4ce6-9038-c9014e498281 | a | b
```
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