Github user arunmahadevan commented on a diff in the pull request:
https://github.com/apache/storm/pull/939#discussion_r48323095
--- Diff: storm-core/src/jvm/backtype/storm/spout/CheckpointSpout.java ---
@@ -0,0 +1,280 @@
+/**
+ * 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 backtype.storm.spout;
+
+import backtype.storm.Config;
+import backtype.storm.state.KeyValueState;
+import backtype.storm.state.StateFactory;
+import backtype.storm.task.TopologyContext;
+import backtype.storm.topology.OutputFieldsDeclarer;
+import backtype.storm.topology.base.BaseRichSpout;
+import backtype.storm.tuple.Fields;
+import backtype.storm.tuple.Tuple;
+import backtype.storm.tuple.Values;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.Map;
+import static backtype.storm.spout.CheckPointState.State.COMMITTED;
+import static backtype.storm.spout.CheckPointState.State.COMMITTING;
+import static backtype.storm.spout.CheckPointState.State.PREPARING;
+
+/**
+ * Emits checkpoint tuples which is used to save the state of the {@link
backtype.storm.topology.IStatefulComponent}
+ * across the topology. If a topology contains Stateful bolts, Checkpoint
spouts are automatically added
+ * to the topology. There is only one Checkpoint task per topology.
+ * <p/>
+ * Checkpoint spout stores its internal state in a {@link KeyValueState}.
The state transitions are as follows.
+ * <p/>
+ * <pre>
+ * ROLLBACK(tx2)
+ * <------------- PREPARE(tx2)
COMMIT(tx2)
--- End diff --
Rollback can happen from 'preparing' state since all bolts might not have
prepared the data to commit. Once we get ack for prepare it indicates all bolts
have prepared the data to commit, the state changes from 'preparing' to
'committing' and a 'commit' message is sent. We cant rollback from 'committing'
state since some bolts might have already committed the data. In case of
failures, the commit message is sent again to make sure all bolts commit. If
some bolts already committed the prepared data, it ignores the message.
Will add more comments to make it clear.
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