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https://issues.apache.org/jira/browse/STORM-1175?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15068483#comment-15068483
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ASF GitHub Bot commented on STORM-1175:
---------------------------------------
Github user Parth-Brahmbhatt commented on a diff in the pull request:
https://github.com/apache/storm/pull/939#discussion_r48279460
--- 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 --
why Rollback can only happen from Prepare and not from Committing? in that
case why do we have Committing state?
> State store for windowing operations
> ------------------------------------
>
> Key: STORM-1175
> URL: https://issues.apache.org/jira/browse/STORM-1175
> Project: Apache Storm
> Issue Type: Sub-task
> Reporter: Arun Mahadevan
> Assignee: Arun Mahadevan
>
> Windowing operations should be able to save the results of partial
> computations in a state store. This should persist across restarts and bolts
> should be able to look up the values from the store.
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