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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. +--> +<section> +<h2><a name="Distributed_Lock_Manager"></a>Distributed Lock Manager</h2> +<p>Distributed locks are used to synchronize accesses shared resources. Most applications today use ZooKeeper to model distributed locks.</p> +<p>The simplest way to model a lock using ZooKeeper is (See ZooKeeper leader recipe for an exact and more advanced solution)</p> +<ul> + +<li>Each process tries to create an emphemeral node</li> +<li>If the node is successfully created, the process acquires the lock</li> +<li>Otherwise, it will watch the ZNode and try to acquire the lock again if the current lock holder disappears</li> +</ul> +<p>This is good enough if there is only one lock. But in practice, an application will need many such locks. Distributing and managing the locks among difference process becomes challenging. Extending such a solution to many locks will result in:</p> +<ul> + +<li>Uneven distribution of locks among nodes; the node that starts first will acquire all the locks. Nodes that start later will be idle.</li> +<li>When a node fails, how the locks will be distributed among remaining nodes is not predicable.</li> +<li>When new nodes are added the current nodes don't relinquish the locks so that new nodes can acquire some locks</li> +</ul> +<p>In other words we want a system to satisfy the following requirements.</p> +<ul> + +<li>Distribute locks evenly among all nodes to get better hardware utilization</li> +<li>If a node fails, the locks that were acquired by that node should be evenly distributed among other nodes</li> +<li>If nodes are added, locks must be evenly re-distributed among nodes.</li> +</ul> +<p>Helix provides a simple and elegant solution to this problem. Simply specify the number of locks and Helix will ensure that above constraints are satisfied.</p> +<p>To quickly see this working run the <code>lock-manager-demo</code> script where 12 locks are evenly distributed among three nodes, and when a node fails, the locks get re-distributed among remaining two nodes. Note that Helix does not re-shuffle the locks completely, instead it simply distributes the locks relinquished by dead node among 2 remaining nodes evenly.</p><hr /><section> +<h3><a name="Short_Version"></a>Short Version</h3> +<p>This version starts multiple threads within the same process to simulate a multi node deployment. Try the long version to get a better idea of how it works.</p> + +<div class="source"><pre class="prettyprint"><code>git clone https://git-wip-us.apache.org/repos/asf/helix.git +cd helix +git checkout tags/helix-1.0.3 +mvn clean install package -DskipTests +cd recipes/distributed-lock-manager/target/distributed-lock-manager-pkg/bin +chmod +x * +./lock-manager-demo +</code></pre></div><section> +<h4><a name="Output"></a>Output</h4> + +<div class="source"><pre class="prettyprint"><code>./lock-manager-demo +STARTING localhost_12000 +STARTING localhost_12002 +STARTING localhost_12001 +STARTED localhost_12000 +STARTED localhost_12002 +STARTED localhost_12001 +localhost_12001 acquired lock:lock-group_3 +localhost_12000 acquired lock:lock-group_8 +localhost_12001 acquired lock:lock-group_2 +localhost_12001 acquired lock:lock-group_4 +localhost_12002 acquired lock:lock-group_1 +localhost_12002 acquired lock:lock-group_10 +localhost_12000 acquired lock:lock-group_7 +localhost_12001 acquired lock:lock-group_5 +localhost_12002 acquired lock:lock-group_11 +localhost_12000 acquired lock:lock-group_6 +localhost_12002 acquired lock:lock-group_0 +localhost_12000 acquired lock:lock-group_9 +lockName acquired By +====================================== +lock-group_0 localhost_12002 +lock-group_1 localhost_12002 +lock-group_10 localhost_12002 +lock-group_11 localhost_12002 +lock-group_2 localhost_12001 +lock-group_3 localhost_12001 +lock-group_4 localhost_12001 +lock-group_5 localhost_12001 +lock-group_6 localhost_12000 +lock-group_7 localhost_12000 +lock-group_8 localhost_12000 +lock-group_9 localhost_12000 +Stopping localhost_12000 +localhost_12000 Interrupted +localhost_12001 acquired lock:lock-group_9 +localhost_12001 acquired lock:lock-group_8 +localhost_12002 acquired lock:lock-group_6 +localhost_12002 acquired lock:lock-group_7 +lockName acquired By +====================================== +lock-group_0 localhost_12002 +lock-group_1 localhost_12002 +lock-group_10 localhost_12002 +lock-group_11 localhost_12002 +lock-group_2 localhost_12001 +lock-group_3 localhost_12001 +lock-group_4 localhost_12001 +lock-group_5 localhost_12001 +lock-group_6 localhost_12002 +lock-group_7 localhost_12002 +lock-group_8 localhost_12001 +lock-group_9 localhost_12001 + +</code></pre></div><hr /></section></section><section> +<h3><a name="Long_version"></a>Long version</h3> +<p>This provides more details on how to setup the cluster and where to plugin application code.</p><section> +<h4><a name="Start_ZooKeeper"></a>Start ZooKeeper</h4> + +<div class="source"><pre class="prettyprint"><code>./start-standalone-zookeeper 2199 +</code></pre></div></section><section> +<h4><a name="Create_a_Cluster"></a>Create a Cluster</h4> + +<div class="source"><pre class="prettyprint"><code>./helix-admin --zkSvr localhost:2199 --addCluster lock-manager-demo +</code></pre></div></section><section> +<h4><a name="Create_a_Lock_Group"></a>Create a Lock Group</h4> +<p>Create a lock group and specify the number of locks in the lock group.</p> + +<div class="source"><pre class="prettyprint"><code>./helix-admin --zkSvr localhost:2199 --addResource lock-manager-demo lock-group 6 OnlineOffline --mode AUTO_REBALANCE +</code></pre></div></section><section> +<h4><a name="Start_the_Nodes"></a>Start the Nodes</h4> +<p>Create a Lock class that handles the callbacks.</p> + +<div class="source"><pre class="prettyprint"><code>public class Lock extends StateModel { + private String lockName; + + public Lock(String lockName) { + this.lockName = lockName; + } + + public void lock(Message m, NotificationContext context) { + System.out.println(" acquired lock:"+ lockName ); + } + + public void release(Message m, NotificationContext context) { + System.out.println(" releasing lock:"+ lockName ); + } + +} +</code></pre></div> +<p>and a LockFactory that creates Locks</p> + +<div class="source"><pre class="prettyprint"><code>public class LockFactory extends StateModelFactory<Lock> { + /* Instantiates the lock handler, one per lockName */ + public Lock create(String lockName) { + return new Lock(lockName); + } +} +</code></pre></div> +<p>At node start up, simply join the cluster and Helix will invoke the appropriate callbacks on the appropriate Lock instance. One can start any number of nodes and Helix detects that a new node has joined the cluster and re-distributes the locks automatically.</p> + +<div class="source"><pre class="prettyprint"><code>public class LockProcess { + public static void main(String args) { + String zkAddress= "localhost:2199"; + String clusterName = "lock-manager-demo"; + //Give a unique id to each process, most commonly used format hostname_port + String instanceName ="localhost_12000"; + ZKHelixAdmin helixAdmin = new ZKHelixAdmin(zkAddress); + //configure the instance and provide some metadata + InstanceConfig config = new InstanceConfig(instanceName); + config.setHostName("localhost"); + config.setPort("12000"); + admin.addInstance(clusterName, config); + //join the cluster + HelixManager manager; + manager = HelixManagerFactory.getHelixManager(clusterName, + instanceName, + InstanceType.PARTICIPANT, + zkAddress); + manager.getStateMachineEngine().registerStateModelFactory("OnlineOffline", modelFactory); + manager.connect(); + Thread.currentThread.join(); + } +} +</code></pre></div></section><section> +<h4><a name="Start_the_Controller"></a>Start the Controller</h4> +<p>The controller can be started either as a separate process or can be embedded within each node process</p><section> +<h5><a name="Separate_Process"></a>Separate Process</h5> +<p>This is recommended when number of nodes in the cluster > 100. For fault tolerance, you can run multiple controllers on different boxes.</p> + +<div class="source"><pre class="prettyprint"><code>./run-helix-controller --zkSvr localhost:2199 --cluster lock-manager-demo 2>&1 > /tmp/controller.log & +</code></pre></div></section><section> +<h5><a name="Embedded_Within_the_Node_Process"></a>Embedded Within the Node Process</h5> +<p>This is recommended when the number of nodes in the cluster is less than 100. To start a controller from each process, simply add the following lines to MyClass</p> + +<div class="source"><pre class="prettyprint"><code>public class LockProcess { + public static void main(String args) { + String zkAddress= "localhost:2199"; + String clusterName = "lock-manager-demo"; + // . + // . + manager.connect(); + HelixManager controller; + controller = HelixControllerMain.startHelixController(zkAddress, + clusterName, + "controller", + HelixControllerMain.STANDALONE); + Thread.currentThread.join(); + } +} +</code></pre></div></section></section></section></section> + </main> + </div> + </div> + <hr/> + <footer> + <div class="container-fluid"> + <div class="row-fluid"> +<div class="row span16"><div>Apache Helix, Apache, the Apache feather logo, and the Apache Helix project logos are trademarks of The Apache Software Foundation. + All other marks mentioned may be trademarks or registered trademarks of their respective owners.</div> + <a href="https://helix.apache.org/1.0.3-docs/privacy-policy.html";>Privacy Policy</a> + </div> + </div> + </div> + </footer> +<script> + if(anchors) { + anchors.add(); + } +</script> + </body> +</html> \ No newline at end of file
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See the License for the +specific language governing permissions and limitations +under the License. +--> +<section> +<h2><a name="RabbitMQ_Consumer_Group"></a>RabbitMQ Consumer Group</h2> +<p><a class="externalLink" href="http://www.rabbitmq.com/";>RabbitMQ</a> is well-known open source software the provides robust messaging for applications.</p> +<p>One of the commonly implemented recipes using this software is a work queue. <a class="externalLink" href="http://www.rabbitmq.com/tutorials/tutorial-four-java.html";>http://www.rabbitmq.com/tutorials/tutorial-four-java.html</a> describes the use case where</p> +<ul> + +<li>A producer sends a message with a routing key</li> +<li>The message is routed to the queue whose binding key exactly matches the routing key of the message</li> +<li>There are multiple consumers and each consumer is interested in processing only a subset of the messages by binding to the interested keys</li> +</ul> +<p>The example provided <a class="externalLink" href="http://www.rabbitmq.com/tutorials/tutorial-four-java.html";>here</a> describes how multiple consumers can be started to process all the messages.</p> +<p>While this works, in production systems one needs the following:</p> +<ul> + +<li>Ability to handle failures: when a consumer fails, another consumer must be started or the other consumers must start processing these messages that should have been processed by the failed consumer</li> +<li>When the existing consumers cannot keep up with the task generation rate, new consumers will be added. The tasks must be redistributed among all the consumers</li> +</ul> +<p>In this recipe, we demonstrate handling of consumer failures and new consumer additions using Helix.</p> +<p>Mapping this usecase to Helix is pretty easy as the binding key/routing key is equivalent to a partition.</p> +<p>Let's take an example. Lets say the queue has 6 partitions, and we have 2 consumers to process all the queues. +What we want is all 6 queues to be evenly divided among 2 consumers. +Eventually when the system scales, we add more consumers to keep up. This will make each consumer process tasks from 2 queues. +Now let's say that a consumer failed, reducing the number of active consumers to 2. This means each consumer must process 3 queues.</p> +<p>We showcase how such a dynamic application can be developed using Helix. Even though we use RabbitMQ as the pub/sub system one can extend this solution to other pub/sub systems.</p><section> +<h3><a name="Try_It"></a>Try It</h3> + +<div class="source"><pre class="prettyprint"><code>git clone https://git-wip-us.apache.org/repos/asf/helix.git +cd helix +git checkout tags/helix-1.0.3 +mvn clean install package -DskipTests +cd recipes/rabbitmq-consumer-group/bin +chmod +x * +export HELIX_PKG_ROOT=`pwd`/helix-core/target/helix-core-pkg +export HELIX_RABBITMQ_ROOT=`pwd`/recipes/rabbitmq-consumer-group/ +chmod +x $HELIX_PKG_ROOT/bin/* +chmod +x $HELIX_RABBITMQ_ROOT/bin/* +</code></pre></div><section> +<h4><a name="Install_RabbitMQ"></a>Install RabbitMQ</h4> +<p>Setting up RabbitMQ on a local box is straightforward. You can find the instructions here +<a class="externalLink" href="http://www.rabbitmq.com/download.html";>http://www.rabbitmq.com/download.html</a></p></section><section> +<h4><a name="Start_ZK"></a>Start ZK</h4> +<p>Start ZooKeeper at port 2199</p> + +<div class="source"><pre class="prettyprint"><code>$HELIX_PKG_ROOT/bin/start-standalone-zookeeper 2199 +</code></pre></div></section><section> +<h4><a name="Setup_the_Consumer_Group_Cluster"></a>Setup the Consumer Group Cluster</h4> +<p>This will setup the cluster by creating a “rabbitmq-consumer-group” cluster and adds a “topic” with “6” queues.</p> + +<div class="source"><pre class="prettyprint"><code>$HELIX_RABBITMQ_ROOT/bin/setup-cluster.sh localhost:2199 +</code></pre></div></section><section> +<h4><a name="Add_Consumers"></a>Add Consumers</h4> +<p>Start 2 consumers in 2 different terminals. Each consumer is given a unique ID.</p> + +<div class="source"><pre class="prettyprint"><code>//start-consumer.sh zookeeperAddress (e.g. localhost:2181) consumerId , rabbitmqServer (e.g. localhost) +$HELIX_RABBITMQ_ROOT/bin/start-consumer.sh localhost:2199 0 localhost +$HELIX_RABBITMQ_ROOT/bin/start-consumer.sh localhost:2199 1 localhost + +</code></pre></div></section><section> +<h4><a name="Start_the_Helix_Controller"></a>Start the Helix Controller</h4> +<p>Now start a Helix controller that starts managing the “rabbitmq-consumer-group” cluster.</p> + +<div class="source"><pre class="prettyprint"><code>$HELIX_RABBITMQ_ROOT/bin/start-cluster-manager.sh localhost:2199 +</code></pre></div></section><section> +<h4><a name="Send_Messages_to_the_Topic"></a>Send Messages to the Topic</h4> +<p>Start sending messages to the topic. This script randomly selects a routing key (1-6) and sends the message to topic. +Based on the key, messages gets routed to the appropriate queue.</p> + +<div class="source"><pre class="prettyprint"><code>$HELIX_RABBITMQ_ROOT/bin/send-message.sh localhost 20 +</code></pre></div> +<p>After running this, you should see all 20 messages being processed by 2 consumers.</p></section><section> +<h4><a name="Add_Another_Consumer"></a>Add Another Consumer</h4> +<p>Once a new consumer is started, Helix detects it. In order to balance the load between 3 consumers, it deallocates 1 partition from the existing consumers and allocates it to the new consumer. We see that +each consumer is now processing only 2 queues. +Helix makes sure that old nodes are asked to stop consuming before the new consumer is asked to start consuming for a given partition. But the transitions for each partition can happen in parallel.</p> + +<div class="source"><pre class="prettyprint"><code>$HELIX_RABBITMQ_ROOT/bin/start-consumer.sh localhost:2199 2 localhost +</code></pre></div> +<p>Send messages again to the topic</p> + +<div class="source"><pre class="prettyprint"><code>$HELIX_RABBITMQ_ROOT/bin/send-message.sh localhost 100 +</code></pre></div> +<p>You should see that messages are now received by all 3 consumers.</p></section><section> +<h4><a name="Stop_a_Consumer"></a>Stop a Consumer</h4> +<p>In any terminal press CTRL^C and notice that Helix detects the consumer failure and distributes the 2 partitions that were processed by failed consumer to the remaining 2 active consumers.</p></section></section><section> +<h3><a name="How_does_this_work.3F"></a>How does this work?</h3> +<p>Find the entire code <a class="externalLink" href="https://git-wip-us.apache.org/repos/asf?p=helix.git;a=tree;f=recipes/rabbitmq-consumer-group/src/main/java/org/apache/helix/recipes/rabbitmq";>here</a>.</p><section> +<h4><a name="Cluster_Setup"></a>Cluster Setup</h4> +<p>This step creates ZNode on ZooKeeper for the cluster and adds the state model. We use online offline state model since there is no need for other states. The consumer is either processing a queue or it is not.</p> +<p>It creates a resource called “rabbitmq-consumer-group” with 6 partitions. The execution mode is set to AUTO_REBALANCE. This means that the Helix controls the assignment of partition to consumers and automatically distributes the partitions evenly among the active consumers. When a consumer is added or removed, it ensures that a minimum number of partitions are shuffled.</p> + +<div class="source"><pre class="prettyprint"><code>zkclient = new ZkClient(zkAddr, ZkClient.DEFAULT_SESSION_TIMEOUT, + ZkClient.DEFAULT_CONNECTION_TIMEOUT, new ZNRecordSerializer()); +ZKHelixAdmin admin = new ZKHelixAdmin(zkclient); + +// add cluster +admin.addCluster(clusterName, true); + +// add state model definition +StateModelConfigGenerator generator = new StateModelConfigGenerator(); +admin.addStateModelDef(clusterName, "OnlineOffline", + new StateModelDefinition(generator.generateConfigForOnlineOffline())); + +// add resource "topic" which has 6 partitions +String resourceName = "rabbitmq-consumer-group"; +admin.addResource(clusterName, resourceName, 6, "OnlineOffline", "AUTO_REBALANCE"); +</code></pre></div></section></section><section> +<h3><a name="Starting_the_Consumers"></a>Starting the Consumers</h3> +<p>The only thing consumers need to know is the ZooKeeper address, cluster name and consumer ID. It does not need to know anything else.</p> + +<div class="source"><pre class="prettyprint"><code>_manager = HelixManagerFactory.getZKHelixManager(_clusterName, + _consumerId, + InstanceType.PARTICIPANT, + _zkAddr); + +StateMachineEngine stateMach = _manager.getStateMachineEngine(); +ConsumerStateModelFactory modelFactory = + new ConsumerStateModelFactory(_consumerId, _mqServer); +stateMach.registerStateModelFactory("OnlineOffline", modelFactory); + +_manager.connect(); +</code></pre></div> +<p>Once the consumer has registered the state model and the controller is started, the consumer starts getting callbacks (onBecomeOnlineFromOffline) for the partition it needs to host. All it needs to do as part of the callback is to start consuming messages from the appropriate queue. Similarly, when the controller deallocates a partitions from a consumer, it fires onBecomeOfflineFromOnline for the same partition. +As a part of this transition, the consumer will stop consuming from a that queue.</p> + +<div class="source"><pre class="prettyprint"><code>@Transition(to = "ONLINE", from = "OFFLINE") +public void onBecomeOnlineFromOffline(Message message, NotificationContext context) { + LOG.debug(_consumerId + " becomes ONLINE from OFFLINE for " + _partition); + if (_thread == null) { + LOG.debug("Starting ConsumerThread for " + _partition + "..."); + _thread = new ConsumerThread(_partition, _mqServer, _consumerId); + _thread.start(); + LOG.debug("Starting ConsumerThread for " + _partition + " done"); + + } +} + +@Transition(to = "OFFLINE", from = "ONLINE") +public void onBecomeOfflineFromOnline(Message message, NotificationContext context) + throws InterruptedException { + LOG.debug(_consumerId + " becomes OFFLINE from ONLINE for " + _partition); + if (_thread != null) { + LOG.debug("Stopping " + _consumerId + " for " + _partition + "..."); + _thread.interrupt(); + _thread.join(2000); + _thread = null; + LOG.debug("Stopping " + _consumerId + " for " + _partition + " done"); + } +} +</code></pre></div></section></section> + </main> + </div> + </div> + <hr/> + <footer> + <div class="container-fluid"> + <div class="row-fluid"> +<div class="row span16"><div>Apache Helix, Apache, the Apache feather logo, and the Apache Helix project logos are trademarks of The Apache Software Foundation. + All other marks mentioned may be trademarks or registered trademarks of their respective owners.</div> + <a href="https://helix.apache.org/1.0.3-docs/privacy-policy.html";>Privacy Policy</a> + </div> + </div> + </div> + </footer> +<script> + if(anchors) { + anchors.add(); + } +</script> + </body> +</html> \ No newline at end of file Added: helix/site-content/1.0.3-docs/recipes/rsync_replicated_file_store.html URL: http://svn.apache.org/viewvc/helix/site-content/1.0.3-docs/recipes/rsync_replicated_file_store.html?rev=1900184&view=auto ============================================================================== --- helix/site-content/1.0.3-docs/recipes/rsync_replicated_file_store.html (added) +++ helix/site-content/1.0.3-docs/recipes/rsync_replicated_file_store.html Sat Apr 23 04:22:15 2022 @@ -0,0 +1,284 @@ +<!DOCTYPE html> + + +<!-- + | Generated by Apache Maven Doxia Site Renderer 1.11.1 from src/site/markdown/recipes/rsync_replicated_file_store.md at 2022-04-23 + | Rendered using Apache Maven Fluido Skin 1.11.0-SNAPSHOT +--> +<html xmlns="http://www.w3.org/1999/xhtml"; lang="en"> + <head> + <meta charset="UTF-8" /> + <meta name="viewport" content="width=device-width, initial-scale=1" /> + <meta name="generator" content="Apache Maven Doxia Site Renderer 1.11.1" /> + <title>Apache Helix – Near-Realtime Rsync Replicated File System</title> + <link rel="stylesheet" href="../css/apache-maven-fluido-1.11.0-SNAPSHOT.min.css" /> + <link rel="stylesheet" href="../css/site.css" /> + <link rel="stylesheet" href="../css/print.css" media="print" /> + <script src="../js/apache-maven-fluido-1.11.0-SNAPSHOT.min.js"></script> +<script type="text/javascript"> + + var _gaq = _gaq || []; + _gaq.push(['_setAccount', 'UA-3211522-12']); + _gaq.push(['_trackPageview']); + + (function() { + var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; + ga.src = ('https:' == document.location.protocol ? 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See the License for the +specific language governing permissions and limitations +under the License. +--> +<section> +<h2><a name="Near-Realtime_Rsync_Replicated_File_System"></a>Near-Realtime Rsync Replicated File System</h2><section> +<h3><a name="Quick_Demo"></a>Quick Demo</h3> +<ul> + +<li>This demo starts 3 instances with id's as <code>localhost_12001, localhost_12002, localhost_12003</code></li> +<li>Each instance stores its files under <code>/tmp/<id>/filestore</code></li> +<li><code>localhost_12001</code> is designated as the master, and <code>localhost_12002</code> and <code>localhost_12003</code> are the slaves</li> +<li>Files written to the master are replicated to the slaves automatically. In this demo, a.txt and b.txt are written to <code>/tmp/localhost_12001/filestore</code> and they get replicated to other folders.</li> +<li>When the master is stopped, <code>localhost_12002</code> is promoted to master.</li> +<li>The other slave <code>localhost_12003</code> stops replicating from <code>localhost_12001</code> and starts replicating from new master <code>localhost_12002</code></li> +<li>Files written to new master <code>localhost_12002</code> are replicated to <code>localhost_12003</code></li> +<li>In the end state of this quick demo, <code>localhost_12002</code> is the master and <code>localhost_12003</code> is the slave. Manually create files under <code>/tmp/localhost_12002/filestore</code> and see that appear in <code>/tmp/localhost_12003/filestore</code></li> +<li>Ignore the interrupted exceptions on the console :-)</li> +</ul> + +<div class="source"><pre class="prettyprint"><code>git clone https://git-wip-us.apache.org/repos/asf/helix.git +cd helix +git checkout tags/helix-1.0.3 +cd recipes/rsync-replicated-file-system/ +mvn clean install package -DskipTests +cd target/rsync-replicated-file-system-pkg/bin +chmod +x * +./quickdemo + +</code></pre></div></section><section> +<h3><a name="Overview"></a>Overview</h3> +<p>There are many applications that require storage for storing large number of relatively small data files. Examples include media stores to store small videos, images, mail attachments etc. Each of these objects is typically kilobytes, often no larger than a few megabytes. An additional distinguishing feature of these use cases is that files are typically only added or deleted, rarely updated. When there are updates, they do not have any concurrency requirements.</p> +<p>These are much simpler requirements than what general purpose distributed file system have to satisfy; these would include concurrent access to files, random access for reads and updates, posix compliance, and others. To satisfy those requirements, general DFSs are also pretty complex that are expensive to build and maintain.</p> +<p>A different implementation of a distributed file system includes HDFS which is inspired by Google's GFS. This is one of the most widely used distributed file system that forms the main data storage platform for Hadoop. HDFS is primary aimed at processing very large data sets and distributes files across a cluster of commodity servers by splitting up files in fixed size chunks. HDFS is not particularly well suited for storing a very large number of relatively tiny files.</p></section><section> +<h3><a name="File_Store"></a>File Store</h3> +<p>It's possible to build a vastly simpler system for the class of applications that have simpler requirements as we have pointed out.</p> +<ul> + +<li>Large number of files but each file is relatively small</li> +<li>Access is limited to create, delete and get entire files</li> +<li>No updates to files that are already created (or it's feasible to delete the old file and create a new one)</li> +</ul> +<p>We call this system a Partitioned File Store (PFS) to distinguish it from other distributed file systems. This system needs to provide the following features:</p> +<ul> + +<li>CRD access to large number of small files</li> +<li>Scalability: Files should be distributed across a large number of commodity servers based on the storage requirement</li> +<li>Fault-tolerance: Each file should be replicated on multiple servers so that individual server failures do not reduce availability</li> +<li>Elasticity: It should be possible to add capacity to the cluster easily</li> +</ul> +<p>Apache Helix is a generic cluster management framework that makes it very easy to provide scalability, fault-tolerance and elasticity features. +rsync can be easily used as a replication channel between servers so that each file gets replicated on multiple servers.</p></section><section> +<h3><a name="Design"></a>Design</h3><section> +<h4><a name="High_Level"></a>High Level</h4> +<ul> + +<li>Partition the file system based on the file name</li> +<li>At any time a single writer can write, we call this a master</li> +<li>For redundancy, we need to have additional replicas called slave. Slaves can optionally serve reads</li> +<li>Slave replicates data from the master</li> +<li>When a master fails, a slave gets promoted to master</li> +</ul></section><section> +<h4><a name="Transaction_Log"></a>Transaction Log</h4> +<p>Every write on the master will result in creation/deletion of one or more files. In order to maintain timeline consistency slaves need to apply the changes in the same order +To facilitate this, the master logs each transaction in a file and each transaction is associated with an 64 bit ID in which the 32 LSB represents a sequence number and MSB represents the generation number +The sequence number gets incremented on every transaction and the generation is incremented when a new master is elected</p></section><section> +<h4><a name="Replication"></a>Replication</h4> +<p>Replication is required for slaves to keep up with changes on the master. Every time the slave applies a change it checkpoints the last applied transaction ID. +During restarts, this allows the slave to pull changes from the last checkpointed ID. Similar to master, the slave logs each transaction to the transaction logs but instead of generating new transaction ID, it uses the same ID generated by the master.</p></section><section> +<h4><a name="Failover"></a>Failover</h4> +<p>When a master fails, a new slave will be promoted to master. If the previous master node is reachable, then the new master will flush all the +changes from previous the master before taking up mastership. The new master will record the end transaction ID of the current generation and then start a new generation +with sequence starting from 1. After this the master will begin accepting writes.</p> +<p><img src="../images/PFS-Generic.png" alt="Partitioned File Store" /></p></section></section><section> +<h3><a name="Rsync-based_Solution"></a>Rsync-based Solution</h3> +<p><img src="../images/RSYNC_BASED_PFS.png" alt="Rsync based File Store" /></p> +<p>This application demonstrates a file store that uses rsync as the replication mechanism. One can envision a similar system where instead of using rsync, one +can implement a custom solution to notify the slave of the changes and also provide an api to pull the change files.</p><section> +<h4><a name="Concepts"></a>Concepts</h4> +<ul> + +<li>file_store_dir: Root directory for the actual data files</li> +<li>change_log_dir: The transaction logs are generated under this folder</li> +<li>check_point_dir: The slave stores the check points ( last processed transaction) here</li> +</ul></section><section> +<h4><a name="Master"></a>Master</h4> +<ul> + +<li>File server: This component supports file uploads and downloads and writes the files to <code>file_store_dir</code>. This is not included in this application. The idea is that most applications have different ways of implementing this component and have some associated business logic. It is not hard to come up with such a component if needed.</li> +<li>File store watcher: This component watches the <code>file_store_dir</code> directory on the local file system for any changes and notifies the registered listeners of the changes</li> +<li>Change log generator: This registers as a listener of the file store watcher and on each notification logs the changes into a file under <code>change_log_dir</code></li> +</ul></section><section> +<h4><a name="Slave"></a>Slave</h4> +<ul> + +<li>File server: This component on the slave will only support reads</li> +<li>Cluster state observer: Slave observes the cluster state and is able to know who is the current master</li> +<li>Replicator: This has two subcomponents +<ul> + +<li>Periodic rsync of change log: This is a background process that periodically rsyncs the <code>change_log_dir</code> of the master to its local directory</li> +<li>Change Log Watcher: This watches the <code>change_log_dir</code> for changes and notifies the registered listeners of the change</li> +<li>On demand rsync invoker: This is registered as a listener to change log watcher and on every change invokes rsync to sync only the changed file</li> +</ul> +</li> +</ul></section><section> +<h4><a name="Coordination"></a>Coordination</h4> +<p>The coordination between nodes is done by Helix. Helix does the partition management and assigns the partition to multiple nodes based on the replication factor. It elects one the nodes as master and designates others as slaves. +It provides notifications to each node in the form of state transitions (Offline to Slave, Slave to Master). It also provides notifications when there is change is cluster state. +This allows the slave to stop replicating from current master and start replicating from new master.</p> +<p>In this application, we have only one partition but its very easy to extend it to support multiple partitions. By partitioning the file store, one can add new nodes and Helix will automatically +re-distribute partitions among the nodes. To summarize, Helix provides partition management, fault tolerance and facilitates automated cluster expansion.</p></section></section></section> + </main> + </div> + </div> + <hr/> + <footer> + <div class="container-fluid"> + <div class="row-fluid"> +<div class="row span16"><div>Apache Helix, Apache, the Apache feather logo, and the Apache Helix project logos are trademarks of The Apache Software Foundation. + All other marks mentioned may be trademarks or registered trademarks of their respective owners.</div> + <a href="https://helix.apache.org/1.0.3-docs/privacy-policy.html";>Privacy Policy</a> + </div> + </div> + </div> + </footer> +<script> + if(anchors) { + anchors.add(); + } +</script> + </body> +</html> \ No newline at end of file Added: helix/site-content/1.0.3-docs/recipes/service_discovery.html URL: http://svn.apache.org/viewvc/helix/site-content/1.0.3-docs/recipes/service_discovery.html?rev=1900184&view=auto ============================================================================== --- helix/site-content/1.0.3-docs/recipes/service_discovery.html (added) +++ helix/site-content/1.0.3-docs/recipes/service_discovery.html Sat Apr 23 04:22:15 2022 @@ -0,0 +1,329 @@ +<!DOCTYPE html> + + +<!-- + | Generated by Apache Maven Doxia Site Renderer 1.11.1 from src/site/markdown/recipes/service_discovery.md at 2022-04-23 + | Rendered using Apache Maven Fluido Skin 1.11.0-SNAPSHOT +--> +<html xmlns="http://www.w3.org/1999/xhtml"; lang="en"> + <head> + <meta charset="UTF-8" /> + <meta name="viewport" content="width=device-width, initial-scale=1" /> + <meta name="generator" content="Apache Maven Doxia Site Renderer 1.11.1" /> + <title>Apache Helix – Service Discovery</title> + <link rel="stylesheet" href="../css/apache-maven-fluido-1.11.0-SNAPSHOT.min.css" /> + <link rel="stylesheet" href="../css/site.css" /> + <link rel="stylesheet" href="../css/print.css" media="print" /> + <script src="../js/apache-maven-fluido-1.11.0-SNAPSHOT.min.js"></script> +<script type="text/javascript"> + + var _gaq = _gaq || []; + _gaq.push(['_setAccount', 'UA-3211522-12']); + _gaq.push(['_trackPageview']); + + (function() { + var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; + ga.src = ('https:' == document.location.protocol ? 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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. +--> +<section> +<h2><a name="Service_Discovery"></a>Service Discovery</h2> +<p>One of the common usage of ZooKeeper is to enable service discovery. +The basic idea is that when a server starts up it advertises its configuration/metadata such as its hostname and port on ZooKeeper. +This allows clients to dynamically discover the servers that are currently active. One can think of this like a service registry to which a server registers when it starts and +is automatically deregistered when it shutdowns or crashes. In many cases it serves as an alternative to VIPs.</p> +<p>The core idea behind this is to use ZooKeeper ephemeral nodes. The ephemeral nodes are created when the server registers and all its metadata is put into a ZNode. +When the server shutdowns, ZooKeeper automatically removes this ZNode.</p> +<p>There are two ways the clients can dynamically discover the active servers:</p><section> +<h3><a name="ZooKeeper_Watch"></a>ZooKeeper Watch</h3> +<p>Clients can set a child watch under specific path on ZooKeeper. +When a new service is registered/deregistered, ZooKeeper notifies the client via a watch event and the client can read the list of services. Even though this looks trivial, +there are lot of things one needs to keep in mind like ensuring that you first set the watch back on ZooKeeper before reading data.</p></section><section> +<h3><a name="Poll"></a>Poll</h3> +<p>Another approach is for the client to periodically read the ZooKeeper path and get the list of services.</p> +<p>Both approaches have pros and cons, for example setting a watch might trigger herd effect if there are large number of clients. This is problematic, especially when servers are starting up. +But the advantage to setting watches is that clients are immediately notified of a change which is not true in case of polling. +In some cases, having both watches and polls makes sense; watch allows one to get notifications as soon as possible while poll provides a safety net if a watch event is missed because of code bug or ZooKeeper fails to notify.</p></section><section> +<h3><a name="Other_Developer_Considerations"></a>Other Developer Considerations</h3> +<ul> + +<li>What happens when the ZooKeeper session expires? All the watches and ephemeral nodes previously added or created by this server are lost. One needs to add the watches again, recreate the ephemeral nodes, and so on.</li> +<li>Due to network issues or Java GC pauses session expiry might happen again and again; this phenomenon is known as flapping. It's important for the server to detect this and deregister itself.</li> +</ul></section><section> +<h3><a name="Other_Operational_Considerations"></a>Other Operational Considerations</h3> +<ul> + +<li>What if the node is behaving badly? One might kill the server, but it will lose the ability to debug. It would be nice to have the ability to mark a server as disabled and clients know that a node is disabled and will not contact that node.</li> +</ul></section><section> +<h3><a name="Configuration_Ownership"></a>Configuration Ownership</h3> +<p>This is an important aspect that is often ignored in the initial stages of your development. Typically, the service discovery pattern means that servers start up with some configuration which it simply puts into ZooKeeper. While this works well in the beginning, configuration management becomes very difficult since the servers themselves are statically configured. Any change in server configuration implies restarting the server. Ideally, it will be nice to have the ability to change configuration dynamically without having to restart a server.</p> +<p>Ideally you want a hybrid solution, a node starts with minimal configuration and gets the rest of configuration from ZooKeeper.</p></section><section> +<h3><a name="Using_Helix_for_Service_Discovery"></a>Using Helix for Service Discovery</h3> +<p>Even though Helix has a higher-level abstraction in terms of state machines, constraints and objectives, service discovery is one of things has been a prevalent use case from the start. +The controller uses the exact mechanism we described above to discover when new servers join the cluster. We create these ZNodes under /CLUSTERNAME/LIVEINSTANCES. +Since at any time there is only one controller, we use a ZK watch to track the liveness of a server.</p> +<p>This recipe simply demonstrates how one can re-use that part for implementing service discovery. This demonstrates multiple modes of service discovery:</p> +<ul> + +<li>POLL: The client reads from zookeeper at regular intervals 30 seconds. Use this if you have 100's of clients</li> +<li>WATCH: The client sets up watcher and gets notified of the changes. Use this if you have 10's of clients</li> +<li>NONE: This does neither of the above, but reads directly from zookeeper when ever needed</li> +</ul> +<p>Helix provides these additional features compared to other implementations available elsewhere:</p> +<ul> + +<li>It has the concept of disabling a node which means that a badly behaving node can be disabled using the Helix admin API</li> +<li>It automatically detects if a node connects/disconnects from zookeeper repeatedly and disables the node</li> +<li>Configuration management +<ul> + +<li>Allows one to set configuration via the admin API at various granulaties like cluster, instance, resource, partition</li> +<li>Configurations can be dynamically changed</li> +<li>The server is notified when configurations change</li> +</ul> +</li> +</ul></section><section> +<h3><a name="Checkout_and_Build"></a>Checkout and Build</h3> + +<div class="source"><pre class="prettyprint"><code>git clone https://git-wip-us.apache.org/repos/asf/helix.git +cd helix +git checkout tags/helix-1.0.3 +mvn clean install package -DskipTests +cd recipes/service-discovery/target/service-discovery-pkg/bin +chmod +x * +</code></pre></div></section><section> +<h3><a name="Start_ZooKeeper"></a>Start ZooKeeper</h3> + +<div class="source"><pre class="prettyprint"><code>./start-standalone-zookeeper 2199 +</code></pre></div></section><section> +<h3><a name="Run_the_Demo"></a>Run the Demo</h3> + +<div class="source"><pre class="prettyprint"><code>./service-discovery-demo.sh +</code></pre></div></section><section> +<h3><a name="Output"></a>Output</h3> + +<div class="source"><pre class="prettyprint"><code>START:Service discovery demo mode:WATCH + Registering service + host.x.y.z_12000 + host.x.y.z_12001 + host.x.y.z_12002 + host.x.y.z_12003 + host.x.y.z_12004 + SERVICES AVAILABLE + SERVICENAME HOST PORT + myServiceName host.x.y.z 12000 + myServiceName host.x.y.z 12001 + myServiceName host.x.y.z 12002 + myServiceName host.x.y.z 12003 + myServiceName host.x.y.z 12004 + Deregistering service: + host.x.y.z_12002 + SERVICES AVAILABLE + SERVICENAME HOST PORT + myServiceName host.x.y.z 12000 + myServiceName host.x.y.z 12001 + myServiceName host.x.y.z 12003 + myServiceName host.x.y.z 12004 + Registering service:host.x.y.z_12002 +END:Service discovery demo mode:WATCH +============================================= +START:Service discovery demo mode:POLL + Registering service + host.x.y.z_12000 + host.x.y.z_12001 + host.x.y.z_12002 + host.x.y.z_12003 + host.x.y.z_12004 + SERVICES AVAILABLE + SERVICENAME HOST PORT + myServiceName host.x.y.z 12000 + myServiceName host.x.y.z 12001 + myServiceName host.x.y.z 12002 + myServiceName host.x.y.z 12003 + myServiceName host.x.y.z 12004 + Deregistering service: + host.x.y.z_12002 + Sleeping for poll interval:30000 + SERVICES AVAILABLE + SERVICENAME HOST PORT + myServiceName host.x.y.z 12000 + myServiceName host.x.y.z 12001 + myServiceName host.x.y.z 12003 + myServiceName host.x.y.z 12004 + Registering service:host.x.y.z_12002 +END:Service discovery demo mode:POLL +============================================= +START:Service discovery demo mode:NONE + Registering service + host.x.y.z_12000 + host.x.y.z_12001 + host.x.y.z_12002 + host.x.y.z_12003 + host.x.y.z_12004 + SERVICES AVAILABLE + SERVICENAME HOST PORT + myServiceName host.x.y.z 12000 + myServiceName host.x.y.z 12001 + myServiceName host.x.y.z 12002 + myServiceName host.x.y.z 12003 + myServiceName host.x.y.z 12004 + Deregistering service: + host.x.y.z_12000 + SERVICES AVAILABLE + SERVICENAME HOST PORT + myServiceName host.x.y.z 12001 + myServiceName host.x.y.z 12002 + myServiceName host.x.y.z 12003 + myServiceName host.x.y.z 12004 + Registering service:host.x.y.z_12000 +END:Service discovery demo mode:NONE +============================================= +</code></pre></div></section></section> + </main> + </div> + </div> + <hr/> + <footer> + <div class="container-fluid"> + <div class="row-fluid"> +<div class="row span16"><div>Apache Helix, Apache, the Apache feather logo, and the Apache Helix project logos are trademarks of The Apache Software Foundation. + All other marks mentioned may be trademarks or registered trademarks of their respective owners.</div> + <a href="https://helix.apache.org/1.0.3-docs/privacy-policy.html";>Privacy Policy</a> + </div> + </div> + </div> + </footer> +<script> + if(anchors) { + anchors.add(); + } +</script> + </body> +</html> \ No newline at end of file
