Github user d2r commented on a diff in the pull request: https://github.com/apache/storm/pull/845#discussion_r45789625 --- Diff: storm-core/src/jvm/backtype/storm/blobstore/KeySequenceNumber.java --- @@ -0,0 +1,227 @@ +/** + * 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.blobstore; + +import backtype.storm.nimbus.NimbusInfo; +import backtype.storm.utils.Utils; +import org.apache.curator.framework.CuratorFramework; +import org.apache.zookeeper.CreateMode; +import org.apache.zookeeper.ZooDefs; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + +import java.nio.ByteBuffer; +import java.util.TreeSet; +import java.util.Map; +import java.util.List; + +/** + * Class hands over the key sequence number which implies the number of updates made to a blob. + * The information regarding the keys and the sequence number which represents the number of updates are + * stored within the zookeeper in the following format. + * /storm/blobstore/key_name/nimbushostport-sequencenumber + * Example: + * If there are two nimbodes with nimbus.seeds:leader,non-leader are set, + * then the state inside the zookeeper is eventually stored as: + * /storm/blobstore/key1/leader:8080-1 + * /storm/blobstore/key1/non-leader:8080-1 + * indicates that a new blob with the name key1 has been created on the leader + * nimbus and the non-leader nimbus syncs after a call back is triggered by attempting + * to download the blob and finally updates its state inside the zookeeper. + * + * A watch is placed on the /storm/blobstore/key1 and the znodes leader:8080-1 and + * non-leader:8080-1 are ephemeral which implies that these nodes exist only until the + * connection between the corresponding nimbus and the zookeeper persist. If in case the + * nimbus crashes the node disappears under /storm/blobstore/key1. + * + * The sequence number for the keys are handed over based on the following scenario: + * Lets assume there are three nimbodes up and running, one being the leader and the other + * being the non-leader. + * + * 1. Create is straight forward. + * Check whether the znode -> /storm/blobstore/key1 has been created or not. It implies + * the blob has not been created yet. If not created, it creates it and updates the zookeeper + * states under /storm/blobstore/key1 and /storm/blobstoremaxkeysequencenumber/key1. + * The znodes it creates on these nodes are /storm/blobstore/key1/leader:8080-1, + * /storm/blobstore/key1/non-leader:8080-1 and /storm/blobstoremaxkeysequencenumber/key1/1. + * The later holds the global sequence number across all nimbodes more like a static variable --- End diff -- Really nice documentation here! `later` -> `latter`
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