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Posted to commits@helix.apache.org by ki...@apache.org on 2013/05/21 02:45:05 UTC
svn commit: r1484629 [20/23] - in /incubator/helix/site-content: ./
apidocs/reference/org/apache/helix/agent/
apidocs/reference/org/apache/helix/manager/zk/
apidocs/reference/org/apache/helix/recipes/rabbitmq/ helix-admin-webapp/
helix-agent/ helix-cor...
Added: incubator/helix/site-content/tutorial_health.html
URL: http://svn.apache.org/viewvc/incubator/helix/site-content/tutorial_health.html?rev=1484629&view=auto
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--- incubator/helix/site-content/tutorial_health.html (added)
+++ incubator/helix/site-content/tutorial_health.html Tue May 21 00:45:01 2013
@@ -0,0 +1,232 @@
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+ <!-- -
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements. See the NOTICE file
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+to you under the Apache License, Version 2.0 (the
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+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. --><h1>Helix Tutorial: Customizing Health Checks</h1><p>In this chapter, we'll learn how to customize the health check, based on metrics of your distributed system. </p><div class="section"><div class="section"><h3>Health Checks<a name="Health_Checks"></a></h3><p>Note: <i>this in currently in development mode, not yet ready for production.</i></p><p>Helix provides the ability for each node in the system to report health metrics on a periodic basis. </p><p>Helix supports multiple ways to aggregate these metrics:</p>
+<ul>
+ <li>SUM</li>
+ <li>AVG</li>
+ <li>EXPONENTIAL DECAY</li>
+ <li>WINDOW</li>
+</ul><p>Helix persists the aggregated value only.</p><p>Applications can define a threshold on the aggregate values according to the SLAs, and when the SLA is violated Helix will fire an alert. Currently Helix only fires an alert, but in a future release we plan to use these metrics to either mark the node dead or load balance the partitions. This feature will be valuable for distributed systems that support multi-tenancy and have a large variation in work load patterns. In addition, this can be used to detect skewed partitions (hotspots) and rebalance the cluster.</p></div></div>
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Added: incubator/helix/site-content/tutorial_messaging.html
URL: http://svn.apache.org/viewvc/incubator/helix/site-content/tutorial_messaging.html?rev=1484629&view=auto
==============================================================================
--- incubator/helix/site-content/tutorial_messaging.html (added)
+++ incubator/helix/site-content/tutorial_messaging.html Tue May 21 00:45:01 2013
@@ -0,0 +1,256 @@
+<!DOCTYPE html>
+<!--
+ | Generated by Apache Maven Doxia at May 20, 2013
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+ Apache Helix</a>
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+ <li id="publishDate" class="pull-right">Last Published: 2013-05-20</li>
+
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+ </div>
+
+
+
+ <div id="bodyColumn" >
+
+ <!-- -
+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. --><h1>Helix Tutorial: Messaging</h1><p>In this chapter, we'll learn about messaging, a convenient feature in Helix for sending messages between nodes of a cluster. This is an interesting feature which is quite useful in practice. It is common that nodes in a distributed system require a mechanism to interact with each other. </p><div class="section"><div class="section"><h3>Example: Bootstrapping a Replica<a name="Example:_Bootstrapping_a_Replica"></a></h3><p>Consider a search system where the index replica starts up and it does not have an index. A typical solution is to get the index from a common location, or to copy the index from another replica.</p><p>Helix provides a messaging api for intra-cluster communication between nodes in the system. Helix provides a mechanism to specify the message recipient in terms of resource, partition, and state rather than specifying hostnames. Helix ensures that the message is delivered to all of the required recipie
nts. In this particular use case, the instance can specify the recipient criteria as all replicas of the desired partition to bootstrap. Since Helix is aware of the global state of the system, it can send the message to appropriate nodes. Once the nodes respond, Helix provides the bootstrapping replica with all the responses.</p><p>This is a very generic api and can also be used to schedule various periodic tasks in the cluster, such as data backups, log cleanup, etc. System Admins can also perform ad-hoc tasks, such as on-demand backups or a system command (such as rm -rf ;) across all nodes of the cluster</p>
+<div class="source"><pre class="prettyprint"> ClusterMessagingService messagingService = manager.getMessagingService();
+
+ // Construct the Message
+ Message requestBackupUriRequest = new Message(
+ MessageType.USER_DEFINE_MSG, UUID.randomUUID().toString());
+ requestBackupUriRequest
+ .setMsgSubType(BootstrapProcess.REQUEST_BOOTSTRAP_URL);
+ requestBackupUriRequest.setMsgState(MessageState.NEW);
+
+ // Set the Recipient criteria: all nodes that satisfy the criteria will receive the message
+ Criteria recipientCriteria = new Criteria();
+ recipientCriteria.setInstanceName("%");
+ recipientCriteria.setRecipientInstanceType(InstanceType.PARTICIPANT);
+ recipientCriteria.setResource("MyDB");
+ recipientCriteria.setPartition("");
+
+ // Should be processed only by process(es) that are active at the time of sending the message
+ // This means if the recipient is restarted after message is sent, it will not be processe.
+ recipientCriteria.setSessionSpecific(true);
+
+ // wait for 30 seconds
+ int timeout = 30000;
+
+ // the handler that will be invoked when any recipient responds to the message.
+ BootstrapReplyHandler responseHandler = new BootstrapReplyHandler();
+
+ // this will return only after all recipients respond or after timeout
+ int sentMessageCount = messagingService.sendAndWait(recipientCriteria,
+ requestBackupUriRequest, responseHandler, timeout);
+</pre></div><p>See HelixManager.DefaultMessagingService in <a class="externalLink" href="http://helix.incubator.apache.org/apidocs/reference/org/apache/helix/messaging/DefaultMessagingService.html">Javadocs</a> for more info.</p></div></div>
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+ <div class="row span12">Copyright © 2013
+ <a href="http://www.apache.org/">The Apache Software Foundation</a>.
+ All Rights Reserved.
+
+ </div>
+
+ <?xml version="1.0" encoding="UTF-8"?>
+<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="http://helix.incubator.apache.org/privacy-policy.html">Privacy Policy</a>
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Added: incubator/helix/site-content/tutorial_participant.html
URL: http://svn.apache.org/viewvc/incubator/helix/site-content/tutorial_participant.html?rev=1484629&view=auto
==============================================================================
--- incubator/helix/site-content/tutorial_participant.html (added)
+++ incubator/helix/site-content/tutorial_participant.html Tue May 21 00:45:01 2013
@@ -0,0 +1,289 @@
+<!DOCTYPE html>
+<!--
+ | Generated by Apache Maven Doxia at May 20, 2013
+ | Rendered using Apache Maven Fluido Skin 1.3.0
+-->
+<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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+ <ul class="dropdown-menu">
+
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+</li>
+
+ <li> <a href="recipes/rabbitmq_consumer_group.html" title="Rabbit MQ consumer group">Rabbit MQ consumer group</a>
+</li>
+
+ <li> <a href="recipes/rsync_replicated_file_store.html" title="Rsync replicated file store">Rsync replicated file store</a>
+</li>
+
+ <li> <a href="recipes/service_discovery.html" title="Service Discovery">Service Discovery</a>
+</li>
+
+ <li> <a href="recipes/task_dag_execution.html" title="Distributed task DAG Execution">Distributed task DAG Execution</a>
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+
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+
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+ </div>
+
+
+
+ <div id="bodyColumn" >
+
+ <!-- -
+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. --><h1>Helix Tutorial: Participant</h1><p>In this chapter, we'll learn how to implement a PARTICIPANT, which is a primary functional component of a distributed system.</p><div class="section"><div class="section"><h3>Start the Helix agent<a name="Start_the_Helix_agent"></a></h3><p>The Helix agent is a common component that connects each system component with the controller.</p><p>It requires the following parameters:</p>
+<ul>
+ <li>clusterName: A logical name to represent the group of nodes</li>
+ <li>instanceName: A logical name of the process creating the manager instance. Generally this is host:port.</li>
+ <li>instanceType: Type of the process. This can be one of the following types, in this case, use PARTICIPANT
+ <ul>
+ <li>CONTROLLER: Process that controls the cluster, any number of controllers can be started but only one will be active at any given time.</li>
+ <li>PARTICIPANT: Process that performs the actual task in the distributed system.</li>
+ <li>SPECTATOR: Process that observes the changes in the cluster.</li>
+ <li>ADMIN: To carry out system admin actions.</li>
+ </ul></li>
+ <li>zkConnectString: Connection string to Zookeeper. This is of the form host1:port1,host2:port2,host3:port3.</li>
+</ul><p>After the Helix manager instance is created, only thing that needs to be registered is the state model factory. The methods of the State Model will be called when controller sends transitions to the Participant. In this example, well use the OnlineOffline factory. Other options include:</p>
+<ul>
+ <li>MasterSlaveStateModelFactory</li>
+ <li><i>NEED TO COMPLETE THIS LIST</i></li>
+</ul>
+<div class="source"><pre class="prettyprint"> manager = HelixManagerFactory.getZKHelixManager(clusterName,
+ instanceName,
+ InstanceType.PARTICIPANT,
+ zkConnectString);
+ StateMachineEngine stateMach = manager.getStateMachineEngine();
+
+ //create a stateModelFactory that returns a statemodel object for each partition.
+ stateModelFactory = new OnlineOfflineStateModelFactory();
+ stateMach.registerStateModelFactory(stateModelType, stateModelFactory);
+ manager.connect();
+</pre></div><p>Helix doesn't know what it means to change from OFFLIN-->ONLINE or ONLINE-->OFFLINE. The following code snippet shows where you insert your system logic for these two state transitions.</p>
+<div class="source"><pre class="prettyprint">public class OnlineOfflineStateModelFactory extends
+ StateModelFactory<StateModel> {
+ @Override
+ public StateModel createNewStateModel(String stateUnitKey) {
+ OnlineOfflineStateModel stateModel = new OnlineOfflineStateModel();
+ return stateModel;
+ }
+ @StateModelInfo(states = "{'OFFLINE','ONLINE'}", initialState = "OFFINE")
+ public static class OnlineOfflineStateModel extends StateModel {
+
+ @Transition(from = "OFFLINE", to = "ONLINE")
+ public void onBecomeOnlineFromOffline(Message message,
+ NotificationContext context) {
+
+ System.out.println("OnlineOfflineStateModel.onBecomeOnlineFromOffline()");
+
+ ////////////////////////////////////////////////////////////////////////////////////////////////
+ // Application logic to handle transition //
+ // For example, you might start a service, run initialization, etc //
+ ////////////////////////////////////////////////////////////////////////////////////////////////
+ }
+
+ @Transition(from = "ONLINE", to = "OFFLINE")
+ public void onBecomeOfflineFromOnline(Message message,
+ NotificationContext context) {
+
+ System.out.println("OnlineOfflineStateModel.onBecomeOfflineFromOnline()");
+
+ ////////////////////////////////////////////////////////////////////////////////////////////////
+ // Application logic to handle transition //
+ // For example, you might shutdown a service, log this event, or change monitoring settings //
+ ////////////////////////////////////////////////////////////////////////////////////////////////
+ }
+ }
+}
+</pre></div></div></div>
+ </div>
+ </div>
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+
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+ <div class="container">
+ <div class="row span12">Copyright © 2013
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Added: incubator/helix/site-content/tutorial_propstore.html
URL: http://svn.apache.org/viewvc/incubator/helix/site-content/tutorial_propstore.html?rev=1484629&view=auto
==============================================================================
--- incubator/helix/site-content/tutorial_propstore.html (added)
+++ incubator/helix/site-content/tutorial_propstore.html Tue May 21 00:45:01 2013
@@ -0,0 +1,226 @@
+<!DOCTYPE html>
+<!--
+ | Generated by Apache Maven Doxia at May 20, 2013
+ | Rendered using Apache Maven Fluido Skin 1.3.0
+-->
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+ <img src="http://incubator.apache.org/images/egg-logo.png" />
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+
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+ <ul class="breadcrumb">
+
+
+ <li id="projectVersion">Version: 0.6.2-incubating-SNAPSHOT</li>
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+ Apache Helix</a>
+ </li>
+ <li class="divider ">/</li>
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+
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+ <li id="publishDate" class="pull-right">Last Published: 2013-05-20</li>
+
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+
+
+
+ <div id="bodyColumn" >
+
+ <!-- -
+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. --><h1>Helix Tutorial: Application Property Store</h1><p>In this chapter, we'll learn how to use the application property store.</p><div class="section"><div class="section"><h3>Property Store<a name="Property_Store"></a></h3><p>It is common that an application needs support for distributed, shared data structures. Helix uses Zookeeper to store the application data and hence provides notifications when the data changes.</p><p>While you could use Zookeeper directly, Helix supports caching the data and a write-through cache. This is far more efficient than reading from Zookeeper for every access.</p><p>See <a class="externalLink" href="http://helix.incubator.apache.org/apidocs/reference/org/apache/helix/store/package-summary.html">HelixManager.getHelixPropertyStore</a> for details.</p></div></div>
+ </div>
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+
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+
+ <footer>
+ <div class="container">
+ <div class="row span12">Copyright © 2013
+ <a href="http://www.apache.org/">The Apache Software Foundation</a>.
+ All Rights Reserved.
+
+ </div>
+
+ <?xml version="1.0" encoding="UTF-8"?>
+<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="http://helix.incubator.apache.org/privacy-policy.html">Privacy Policy</a>
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Added: incubator/helix/site-content/tutorial_rebalance.html
URL: http://svn.apache.org/viewvc/incubator/helix/site-content/tutorial_rebalance.html?rev=1484629&view=auto
==============================================================================
--- incubator/helix/site-content/tutorial_rebalance.html (added)
+++ incubator/helix/site-content/tutorial_rebalance.html Tue May 21 00:45:01 2013
@@ -0,0 +1,325 @@
+<!DOCTYPE html>
+<!--
+ | Generated by Apache Maven Doxia at May 20, 2013
+ | Rendered using Apache Maven Fluido Skin 1.3.0
+-->
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+</li>
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+</li>
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+
+ <!-- -
+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. --><h1>Helix Tutorial: Rebalancing Algorithms</h1><p>The placement of partitions in a distributed system is essential for the reliability and scalability of the system. For example, when a node fails, it is important that the partitions hosted on that node are reallocated evenly among the remaining nodes. Consistent hashing is one such algorithm that can satisfy this guarantee. Helix provides a variant of consistent hashing based on the RUSH algorithm.</p><p>This means given a number of partitions, replicas and number of nodes, Helix does the automatic assignment of partition to nodes such that:</p>
+<ul>
+ <li>Each node has the same number of partitions</li>
+ <li>Replicas of the same partition do not stay on the same node</li>
+ <li>When a node fails, the partitions will be equally distributed among the remaining nodes</li>
+ <li>When new nodes are added, the number of partitions moved will be minimized along with satisfying the above criteria</li>
+</ul><p>Helix employs a rebalancing algorithm to compute the <i>ideal state</i> of the system. When the <i>current state</i> differs from the <i>ideal state</i>, Helix uses it as the target state of the system and computes the appropriate transitions needed to bring it to the <i>ideal state</i>.</p><p>Helix makes it easy to perform this operation, while giving you control over the algorithm. In this section, we'll see how to implement the desired behavior.</p><p>Helix has three options for rebalancing, in increasing order of customization by the system builder:</p>
+<ul>
+ <li>AUTO_REBALANCE</li>
+ <li>AUTO</li>
+ <li>CUSTOM</li>
+</ul>
+<div class="source"><pre class="prettyprint"> |AUTO REBALANCE| AUTO | CUSTOM |
+ -----------------------------------------
+ LOCATION | HELIX | APP | APP |
+ -----------------------------------------
+ STATE | HELIX | HELIX | APP |
+ -----------------------------------------
+</pre></div><div class="section"><div class="section"><h3>AUTO_REBALANCE<a name="AUTO_REBALANCE"></a></h3><p>When the idealstate mode is set to AUTO_REBALANCE, Helix controls both the location of the replica along with the state. This option is useful for applications where creation of a replica is not expensive. </p><p>For example, consider this system that uses a MasterSlave state model, with 3 partitions and 2 replicas in the ideal state.</p>
+<div class="source"><pre class="prettyprint">{
+ "id" : "MyResource",
+ "simpleFields" : {
+ "IDEAL_STATE_MODE" : "AUTO_REBALANCE",
+ "NUM_PARTITIONS" : "3",
+ "REPLICAS" : "2",
+ "STATE_MODEL_DEF_REF" : "MasterSlave",
+ }
+ "listFields" : {
+ "MyResource_0" : [],
+ "MyResource_1" : [],
+ "MyResource_2" : []
+ },
+ "mapFields" : {
+ }
+}
+</pre></div><p>If there are 3 nodes in the cluster, then Helix will balance the masters and slaves equally. The ideal state is therefore:</p>
+<div class="source"><pre class="prettyprint">{
+ "id" : "MyResource",
+ "simpleFields" : {
+ "NUM_PARTITIONS" : "3",
+ "REPLICAS" : "2",
+ "STATE_MODEL_DEF_REF" : "MasterSlave",
+ },
+ "mapFields" : {
+ "MyResource_0" : {
+ "N1" : "MASTER",
+ "N2" : "SLAVE",
+ },
+ "MyResource_1" : {
+ "N2" : "MASTER",
+ "N3" : "SLAVE",
+ },
+ "MyResource_2" : {
+ "N3" : "MASTER",
+ "N1" : "SLAVE",
+ }
+ }
+}
+</pre></div><p>Another typical example is evenly distributing a group of tasks among the currently healthy processes. For example, if there are 60 tasks and 4 nodes, Helix assigns 15 tasks to each node. When one node fails, Helix redistributes its 15 tasks to the remaining 3 nodes, resulting in a balanced 20 tasks per node. Similarly, if a node is added, Helix re-allocates 3 tasks from each of the 4 nodes to the 5th node, resulting in a balanced distribution of 12 tasks per node.. </p><div class="section"><h4>AUTO<a name="AUTO"></a></h4><p>When the application needs to control the placement of the replicas, use the AUTO idealstate mode.</p><p>Example: In the ideal state below, the partition 'MyResource_0' is constrained to be placed only on node1 or node2. The choice of <i>state</i> is still controlled by Helix. That means MyResource_0.MASTER could be on node1 and MyResource_0.SLAVE on node2, or vice-versa but neither would be placed on node3.</p>
+<div class="source"><pre class="prettyprint">{
+ "id" : "MyResource",
+ "simpleFields" : {
+ "IDEAL_STATE_MODE" : "AUTO",
+ "NUM_PARTITIONS" : "3",
+ "REPLICAS" : "2",
+ "STATE_MODEL_DEF_REF" : "MasterSlave",
+ }
+ "listFields" : {
+ "MyResource_0" : [node1, node2],
+ "MyResource_1" : [node2, node3],
+ "MyResource_2" : [node3, node1]
+ },
+ "mapFields" : {
+ }
+}
+</pre></div><p>The MasterSlave state model requires that a partition has exactly one MASTER at all times, and the other replicas should be SLAVEs. In this simple example with 2 replicas per partition, there would be one MASTER and one SLAVE. Upon failover, a SLAVE has to assume mastership, and a new SLAVE will be generated.</p><p>In this mode when node1 fails, unlike in AUTO-REBALANCE mode the partition is <i>not</i> moved from node1 to node3. Instead, Helix will decide to change the state of MyResource_0 on node2 from SLAVE to MASTER, based on the system constraints. </p></div><div class="section"><h4>CUSTOM<a name="CUSTOM"></a></h4><p>Finally, Helix offers a third mode called CUSTOM, in which the application controls the placement <i>and</i> state of each replica. The application needs to implement a callback interface that Helix invokes when the cluster state changes. Within this callback, the application can recompute the idealstate. Helix will then issue appropriate tra
nsitions such that <i>Idealstate</i> and <i>Currentstate</i> converges.</p><p>Here's an example, again with 3 partitions, 2 replicas per partition, and the MasterSlave state model:</p>
+<div class="source"><pre class="prettyprint">{
+ "id" : "MyResource",
+ "simpleFields" : {
+ "IDEAL_STATE_MODE" : "CUSTOM",
+ "NUM_PARTITIONS" : "3",
+ "REPLICAS" : "2",
+ "STATE_MODEL_DEF_REF" : "MasterSlave",
+ },
+ "mapFields" : {
+ "MyResource_0" : {
+ "N1" : "MASTER",
+ "N2" : "SLAVE",
+ },
+ "MyResource_1" : {
+ "N2" : "MASTER",
+ "N3" : "SLAVE",
+ },
+ "MyResource_2" : {
+ "N3" : "MASTER",
+ "N1" : "SLAVE",
+ }
+ }
+}
+</pre></div><p>Suppose the current state of the system is MyResource_0 -> {N1:MASTER, N2:SLAVE} and the application changes the ideal state to MyResource_0 -> {N1:SLAVE,N2:MASTER}. While the application decides which node is MASTER and which is SLAVE, Helix will not blindly issue MASTER>SLAVE to N1 and SLAVE>MASTER to N2 in parallel, since that might result in a transient state where both N1 and N2 are masters, which violates the MasterSlave constraint that there is exactly one MASTER at a time. Helix will first issue MASTER>SLAVE to N1 and after it is completed, it will issue SLAVE>MASTER to N2. </p></div></div></div>
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+ <!-- -
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements. See the NOTICE file
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+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
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+
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+
+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. --><h1>Helix Tutorial: Spectator</h1><p>Next, we'll learn how to implement a SPECTATOR. Typically, a spectator needs to react to changes within the distributed system. Examples: a client that needs to know where to send a request, a topic consumer in a consumer group. The spectator is automatically informed of changes in the <i>external state</i> of the cluster, but it does not have to add any code to keep track of other components in the system.</p><div class="section"><div class="section"><h3>Start the Helix agent<a name="Start_the_Helix_agent"></a></h3><p>Same as for a PARTICIPANT, The Helix agent is the common component that connects each system component with the controller.</p><p>It requires the following parameters:</p>
+<ul>
+ <li>clusterName: A logical name to represent the group of nodes</li>
+ <li>instanceName: A logical name of the process creating the manager instance. Generally this is host:port.</li>
+ <li>instanceType: Type of the process. This can be one of the following types, in this case, use SPECTATOR:
+ <ul>
+ <li>CONTROLLER: Process that controls the cluster, any number of controllers can be started but only one will be active at any given time.</li>
+ <li>PARTICIPANT: Process that performs the actual task in the distributed system.</li>
+ <li>SPECTATOR: Process that observes the changes in the cluster.</li>
+ <li>ADMIN: To carry out system admin actions.</li>
+ </ul></li>
+ <li>zkConnectString: Connection string to Zookeeper. This is of the form host1:port1,host2:port2,host3:port3.</li>
+</ul><p>After the Helix manager instance is created, only thing that needs to be registered is the state model factory. The methods of the State Model will be called when controller sends transitions to the Participant. In this example, well use the OnlineOffline factory. Other options:</p><p><i>NEED TO ADD OTHER OPTIONS HERE</i></p></div><div class="section"><h3>Spectator Code<a name="Spectator_Code"></a></h3><p>A spectator observes the cluster and is notified when the state of the system changes. Helix consolidates the state of entire cluster in one Znode called ExternalView. Helix provides a default implementation RoutingTableProvider that caches the cluster state and updates it when there is a change in the cluster.</p>
+<div class="source"><pre class="prettyprint">manager = HelixManagerFactory.getZKHelixManager(clusterName,
+ instanceName,
+ InstanceType.PARTICIPANT,
+ zkConnectString);
+manager.connect();
+RoutingTableProvider routingTableProvider = new RoutingTableProvider();
+manager.addExternalViewChangeListener(routingTableProvider);
+</pre></div><p>In the following code snippet, the application sends the request to a valid instance by interrogating the external view. Suppose the desired resource for this request is in the partition myDB_1.</p>
+<div class="source"><pre class="prettyprint">## instances = routingTableProvider.getInstances(, "PARTITION_NAME", "PARTITION_STATE");
+instances = routingTableProvider.getInstances("myDB", "myDB_1", "ONLINE");
+
+////////////////////////////////////////////////////////////////////////////////////////////////
+// Application-specific code to send a request to one of the instances //
+////////////////////////////////////////////////////////////////////////////////////////////////
+
+theInstance = instances.get(0); // should choose an instance and throw an exception if none are available
+result = theInstance.sendRequest(yourApplicationRequest, responseObject);
+
+</pre></div><p>When the external view changes, the application needs to react by sending requests to a different instance. </p><p><i>SHOULD WE ADD AN EXAMPLE OF THIS? IT WOULD MAKE THINGS MORE COMPLICATED, BECAUSE THE APP SHOULD HAVE A LOCAL COPY OF THE ROUTING TABLE AND UPDATE IT ONLY WHEN THE EXTERNAL VIEW CHANGES, THAT'S HOW ID IMPLEMENT IT.</i></p></div></div>
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+ All Rights Reserved.
+
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+<div class="row span16">
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