Added: helix/site-content/0.6.5-docs/tutorial_spectator.html URL: http://svn.apache.org/viewvc/helix/site-content/0.6.5-docs/tutorial_spectator.html?rev=1669051&view=auto ============================================================================== --- helix/site-content/0.6.5-docs/tutorial_spectator.html (added) +++ helix/site-content/0.6.5-docs/tutorial_spectator.html Wed Mar 25 05:14:56 2015 @@ -0,0 +1,294 @@ + +<!DOCTYPE html> +<!-- + Generated by Apache Maven Doxia at 2015-03-24 + Rendered using Maven Reflow Skin 1.0.0 (http://andriusvelykis.github.com/reflow-maven-skin) +--> +<html xml:lang="en" lang="en"> + + <head> + <meta charset="UTF-8" /> + <title>Apache Helix - Tutorial - Spectator</title> + <meta name="viewport" content="width=device-width, initial-scale=1.0" /> + <meta name="description" content="" /> + <meta http-equiv="content-language" content="en" /> + + <link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/css/bootstrap.min.css"; rel="stylesheet" /> + <link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/css/bootstrap-responsive.min.css"; rel="stylesheet" /> + <link href="./css/docs.css" rel="stylesheet" /> + <link href="./css/reflow-skin.css" rel="stylesheet" /> + + + <link href="./css/lightbox.css" rel="stylesheet" /> + + <link href="./css/site.css" rel="stylesheet" /> + <link href="./css/print.css" rel="stylesheet" media="print" /> + + <!-- Le HTML5 shim, for IE6-8 support of HTML5 elements --> + <!--[if lt IE 9]> + <script src="http://html5shim.googlecode.com/svn/trunk/html5.js";></script> + <![endif]--> + +<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. --> +<p></p> +<div class="section"> + <div class="page-header"> + <h2 id="Helix_Tutorial:_Spectator"><a href="./Tutorial.html">Helix Tutorial</a>: Spectator</h2> + </div> + <p>Next, we'll learn how to implement a <b>spectator</b>. 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"> + <h3 id="Start_a_Connection">Start a Connection</h3> + <p>Same as for a participant, The Helix manager is the common component that connects each system component with the cluster.</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, the only thing that needs to be registered is the listener. When the ExternalView changes, the listener is notified.</p> + <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>manager = HelixManagerFactory.getZKHelixManager(clusterName, + instanceName, + InstanceType.SPECTATOR, + zkConnectString); +manager.connect(); +RoutingTableProvider routingTableProvider = new RoutingTableProvider(); +manager.addExternalViewChangeListener(routingTableProvider); +</pre> + </div> + </div> + <div class="section"> + <h3 id="Spectator_Code">Spectator Code</h3> + <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>// 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> + </div> +</div> + </div> + </div> + </div> + </div> + + </div><!-- /container --> + + <!-- Footer + ================================================== --> + <footer class="well"> + <div class="container"> + <div class="row"> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Get Helix</li> + <li> + <a href="download.html" title="Download">Download </a> + </li> + <li> + <a href="Building.html" title="Building">Building </a> + </li> + <li> + <a href="releasenotes/release-0.6.5.html" title="Release Notes">Release Notes </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Hands-On</li> + <li> + <a href="Quickstart.html" title="Quick Start">Quick Start </a> + </li> + <li> + <a href="Tutorial.html" title="Tutorial">Tutorial </a> + </li> + <li> + <a href="../javadocs/0.6.5" title="Javadocs">Javadocs </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Recipes</li> + <li> + <a href="recipes/lock_manager.html" title="Distributed lock manager">Distributed lock manager </a> + </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> + </li> + </ul> + </div> + </div> + </div> + </footer> + + <div class="container subfooter"> + <div class="row"> + <div class="span12"> + <p class="pull-right"><a href="#">Back to top</a></p> + <p class="copyright">Copyright ©2015 <a href="http://www.apache.org/";>The Apache Software Foundation</a>. 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Added: helix/site-content/0.6.5-docs/tutorial_state.html URL: http://svn.apache.org/viewvc/helix/site-content/0.6.5-docs/tutorial_state.html?rev=1669051&view=auto ============================================================================== --- helix/site-content/0.6.5-docs/tutorial_state.html (added) +++ helix/site-content/0.6.5-docs/tutorial_state.html Wed Mar 25 05:14:56 2015 @@ -0,0 +1,355 @@ + +<!DOCTYPE html> +<!-- + Generated by Apache Maven Doxia at 2015-03-24 + Rendered using Maven Reflow Skin 1.0.0 (http://andriusvelykis.github.com/reflow-maven-skin) +--> +<html xml:lang="en" lang="en"> + + <head> + <meta charset="UTF-8" /> + <title>Apache Helix - Tutorial - State Machine Configuration</title> + <meta name="viewport" content="width=device-width, initial-scale=1.0" /> + <meta name="description" content="" /> + <meta http-equiv="content-language" content="en" /> + + <link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/css/bootstrap.min.css"; rel="stylesheet" /> + <link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/css/bootstrap-responsive.min.css"; rel="stylesheet" /> + <link href="./css/docs.css" rel="stylesheet" /> + <link href="./css/reflow-skin.css" rel="stylesheet" /> + + + <link href="./css/lightbox.css" rel="stylesheet" /> + + <link href="./css/site.css" rel="stylesheet" /> + <link href="./css/print.css" rel="stylesheet" media="print" /> + + <!-- Le HTML5 shim, for IE6-8 support of HTML5 elements --> + <!--[if lt IE 9]> + <script src="http://html5shim.googlecode.com/svn/trunk/html5.js";></script> + <![endif]--> + +<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. --> +<p></p> +<div class="section"> + <div class="page-header"> + <h2 id="Helix_Tutorial:_State_Machine_Configuration"><a href="./Tutorial.html">Helix Tutorial</a>: State Machine Configuration</h2> + </div> + <p>In this chapter, we'll learn about the state models provided by Helix, and how to create your own custom state model.</p> + <div class="section"> + <h3 id="State_Models">State Models</h3> + <p>Helix comes with 3 default state models that are commonly used. It is possible to have multiple state models in a cluster. Every resource that is added should be configured to use a state model that govern its <i>ideal state</i>.</p> + <div class="section"> + <h4 id="MASTER-SLAVE">MASTER-SLAVE</h4> + <ul> + <li>3 states: OFFLINE, SLAVE, MASTER</li> + <li>Maximum number of masters: 1</li> + <li>Slaves are based on the replication factor. The replication factor can be specified while adding the resource.</li> + </ul> + </div> + <div class="section"> + <h4 id="ONLINE-OFFLINE">ONLINE-OFFLINE</h4> + <ul> + <li>Has 2 states: OFFLINE and ONLINE. This simple state model is a good starting point for most applications.</li> + </ul> + </div> + <div class="section"> + <h4 id="LEADER-STANDBY">LEADER-STANDBY</h4> + <ul> + <li>1 Leader and multiple stand-bys. The idea is that exactly one leader accomplishes a designated task, the stand-bys are ready to take over if the leader fails.</li> + </ul> + </div> + </div> + <div class="section"> + <h3 id="Constraints">Constraints</h3> + <p>In addition to the state machine configuration, one can specify the constraints of states and transitions.</p> + <p>For example, one can say:</p> + <ul> + <li> <p>MASTER:1 <br />Maximum number of replicas in MASTER state at any time is 1</p></li> + <li> <p>OFFLINE-SLAVE:5 <br />Maximum number of OFFLINE-SLAVE transitions that can happen concurrently in the system is 5 in this example.</p></li> + </ul> + <div class="section"> + <h4 id="Dynamic_State_Constraints">Dynamic State Constraints</h4> + <p>We also support two dynamic upper bounds for the number of replicas in each state:</p> + <ul> + <li>N: The number of replicas in the state is at most the number of live participants in the cluster</li> + <li>R: The number of replicas in the state is at most the specified replica count for the partition</li> + </ul> + </div> + <div class="section"> + <h4 id="State_Priority">State Priority</h4> + <p>Helix uses a greedy approach to satisfy the state constraints. For example, if the state machine configuration says it needs 1 MASTER and 2 SLAVES, but only 1 node is active, Helix must promote it to MASTER. This behavior is achieved by providing the state priority list as [MASTER, SLAVE].</p> + </div> + <div class="section"> + <h4 id="State_Transition_Priority">State Transition Priority</h4> + <p>Helix tries to fire as many transitions as possible in parallel to reach the stable state without violating constraints. By default, Helix simply sorts the transitions alphabetically and fires as many as it can without violating the constraints. You can control this by overriding the priority order.</p> + </div> + </div> + <div class="section"> + <h3 id="Special_States">Special States</h3> + <p>There are a few Helix-defined states that are important to be aware of.</p> + <div class="section"> + <h4 id="DROPPED">DROPPED</h4> + <p>The DROPPED state is used to signify a replica that was served by a given participant, but is no longer served. This allows Helix and its participants to effectively clean up. There are two requirements that every new state model should follow with respect to the DROPPED state:</p> + <ul> + <li>The DROPPED state must be defined</li> + <li>There must be a path to DROPPED for every state in the model</li> + </ul> + </div> + <div class="section"> + <h4 id="ERROR">ERROR</h4> + <p>The ERROR state is used whenever the participant serving a partition encountered an error and cannot continue to serve the partition. HelixAdmin has "reset" functionality to allow for participants to recover from the ERROR state.</p> + </div> + </div> + <div class="section"> + <h3 id="Annotated_Example">Annotated Example</h3> + <p>Below is a complete definition of a Master-Slave state model. Notice the fields marked REQUIRED; these are essential for any state model definition.</p> + <div class="source"> + <pre>StateModelDefinition stateModel = new StateModelDefinition.Builder("MasterSlave") + // OFFLINE is the state that the system starts in (initial state is REQUIRED) + .initialState("OFFLINE") + + // Lowest number here indicates highest priority, no value indicates lowest priority + .addState("MASTER", 1) + .addState("SLAVE", 2) + .addState("OFFLINE") + + // Note the special inclusion of the DROPPED state (REQUIRED) + .addState(HelixDefinedState.DROPPED.toString()) + + // No more than one master allowed + .upperBound("MASTER", 1) + + // R indicates an upper bound of number of replicas for each partition + .dynamicUpperBound("SLAVE", "R") + + // Add some high-priority transitions + .addTransition("SLAVE", "MASTER", 1) + .addTransition("OFFLINE", "SLAVE", 2) + + // Using the same priority value indicates that these transitions can fire in any order + .addTransition("MASTER", "SLAVE", 3) + .addTransition("SLAVE", "OFFLINE", 3) + + // Not specifying a value defaults to lowest priority + // Notice the inclusion of the OFFLINE to DROPPED transition + // Since every state has a path to OFFLINE, they each now have a path to DROPPED (REQUIRED) + .addTransition("OFFLINE", HelixDefinedState.DROPPED.toString()) + + // Create the StateModelDefinition instance + .build(); + + // Use the isValid() function to make sure the StateModelDefinition will work without issues + Assert.assertTrue(stateModel.isValid()); +</pre> + </div> + </div> +</div> + </div> + </div> + </div> + </div> + + </div><!-- /container --> + + <!-- Footer + ================================================== --> + <footer class="well"> + <div class="container"> + <div class="row"> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Get Helix</li> + <li> + <a href="download.html" title="Download">Download </a> + </li> + <li> + <a href="Building.html" title="Building">Building </a> + </li> + <li> + <a href="releasenotes/release-0.6.5.html" title="Release Notes">Release Notes </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Hands-On</li> + <li> + <a href="Quickstart.html" title="Quick Start">Quick Start </a> + </li> + <li> + <a href="Tutorial.html" title="Tutorial">Tutorial </a> + </li> + <li> + <a href="../javadocs/0.6.5" title="Javadocs">Javadocs </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Recipes</li> + <li> + <a href="recipes/lock_manager.html" title="Distributed lock manager">Distributed lock manager </a> + </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> + </li> + </ul> + </div> + </div> + </div> + </footer> + + <div class="container subfooter"> + <div class="row"> + <div class="span12"> + <p class="pull-right"><a href="#">Back to top</a></p> + <p class="copyright">Copyright ©2015 <a href="http://www.apache.org/";>The Apache Software Foundation</a>. 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Only a centralized cluster manager with global knowledge (i.e. Helix) is capable of coordinating this decision.</p> + <div class="section"> + <h3 id="Throttling">Throttling</h3> + <p>Since all state changes in the system are triggered through transitions, Helix can control the number of transitions that can happen in parallel. Some of the transitions may be lightweight, but some might involve moving data, which is quite expensive from a network and IOPS perspective.</p> + <p>Helix allows applications to set a threshold on transitions. The threshold can be set at multiple scopes:</p> + <ul> + <li>MessageType e.g STATE_TRANSITION</li> + <li>TransitionType e.g SLAVE-MASTER</li> + <li>Resource e.g database</li> + <li>Node i.e per-node maximum transitions in parallel</li> + </ul> + </div> +</div> + </div> + </div> + </div> + </div> + + </div><!-- /container --> + + <!-- Footer + ================================================== --> + <footer class="well"> + <div class="container"> + <div class="row"> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Get Helix</li> + <li> + <a href="download.html" title="Download">Download </a> + </li> + <li> + <a href="Building.html" title="Building">Building </a> + </li> + <li> + <a href="releasenotes/release-0.6.5.html" title="Release Notes">Release Notes </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Hands-On</li> + <li> + <a href="Quickstart.html" title="Quick Start">Quick Start </a> + </li> + <li> + <a href="Tutorial.html" title="Tutorial">Tutorial </a> + </li> + <li> + <a href="../javadocs/0.6.5" title="Javadocs">Javadocs </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Recipes</li> + <li> + <a href="recipes/lock_manager.html" title="Distributed lock manager">Distributed lock manager </a> + </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> + </li> + </ul> + </div> + </div> + </div> + </footer> + + <div class="container subfooter"> + <div class="row"> + <div class="span12"> + <p class="pull-right"><a href="#">Back to top</a></p> + <p class="copyright">Copyright ©2015 <a href="http://www.apache.org/";>The Apache Software Foundation</a>. 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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. --> +<p></p> +<div class="section"> + <div class="page-header"> + <h2 id="Helix_Tutorial:_User-Defined_Rebalancing"><a href="./Tutorial.html">Helix Tutorial</a>: User-Defined Rebalancing</h2> + </div> + <p>Even though Helix can compute both the location and the state of replicas internally using a default fully-automatic rebalancer, specific applications may require rebalancing strategies that optimize for different requirements. Thus, Helix allows applications to plug in arbitrary rebalancer algorithms that implement a provided interface. One of the main design goals of Helix is to provide maximum flexibility to any distributed application. Thus, it allows applications to fully implement the rebalancer, which is the core constraint solver in the system, if the application developer so chooses.</p> + <p>Whenever the state of the cluster changes, as is the case when participants join or leave the cluster, Helix automatically calls the rebalancer to compute a new mapping of all the replicas in the resource. When using a pluggable rebalancer, the only required step is to register it with Helix. Subsequently, no additional bootstrapping steps are necessary. Helix uses reflection to look up and load the class dynamically at runtime. As a result, it is also technically possible to change the rebalancing strategy used at any time.</p> + <p>The Rebalancer interface is as follows:</p> + <div class="source"> + <pre>void init(HelixManager manager); + +IdealState computeNewIdealState(String resourceName, IdealState currentIdealState, + final CurrentStateOutput currentStateOutput, final ClusterDataCache clusterData); +</pre> + </div> + <p>The first parameter is the resource to rebalance, the second is pre-existing ideal mappings, the third is a snapshot of the actual placements and state assignments, and the fourth is a full cache of all of the cluster data available to Helix. Internally, Helix implements the same interface for its own rebalancing routines, so a user-defined rebalancer will be cognizant of the same information about the cluster as an internal implementation. Helix strives to provide applications the ability to implement algorithms that may require a large portion of the entire state of the cluster to make the best placement and state assignment decisions possible.</p> + <p>An IdealState is a full representation of the location of each replica of each partition of a given resource. This is a simple representation of the placement that the algorithm believes is the best possible. If the placement meets all defined constraints, this is what will become the actual state of the distributed system.</p> + <div class="section"> + <h3 id="Specifying_a_Rebalancer">Specifying a Rebalancer</h3> + <p>For implementations that set up the cluster through existing code, the following HelixAdmin calls will update the Rebalancer class:</p> + <div class="source"> + <pre>IdealState idealState = helixAdmin.getResourceIdealState(clusterName, resourceName); +idealState.setRebalanceMode(RebalanceMode.USER_DEFINED); +idealState.setRebalancerClassName(className); +helixAdmin.setResourceIdealState(clusterName, resourceName, idealState); +</pre> + </div> + <p>There are two key fields to set to specify that a pluggable rebalancer should be used. First, the rebalance mode should be set to USER_DEFINED, and second the rebalancer class name should be set to a class that implements Rebalancer and is within the scope of the project. The class name is a fully-qualified class name consisting of its package and its name. Without specification of the USER_DEFINED mode, the user-defined rebalancer class will not be used even if specified. Furthermore, Helix will not attempt to rebalance the resources through its standard routines if its mode is USER_DEFINED, regardless of whether or not a rebalancer class is registered.</p> + </div> + <div class="section"> + <h3 id="Example">Example</h3> + <p>In the next release (0.7.0), we will provide a full recipe of a user-defined rebalancer in action.</p> + <p>Consider the case where partitions are locks in a lock manager and 6 locks are to be distributed evenly to a set of participants, and only one participant can hold each lock. We can define a rebalancing algorithm that simply takes the modulus of the lock number and the number of participants to evenly distribute the locks across participants. Helix allows capping the number of partitions a participant can accept, but since locks are lightweight, we do not need to define a restriction in this case. The following is a succinct implementation of this algorithm.</p> + <div class="source"> + <pre>@Override +IdealState computeNewIdealState(String resourceName, IdealState currentIdealState, + final CurrentStateOutput currentStateOutput, final ClusterDataCache clusterData) { + // Get the list of live participants in the cluster + List<String> liveParticipants = new ArrayList<String>(clusterData.getLiveInstances().keySet()); + + // Count the number of participants allowed to lock each lock (in this example, this is 1) + int lockHolders = Integer.parseInt(currentIdealState.getReplicas()); + + // Fairly assign the lock state to the participants using a simple mod-based sequential + // assignment. For instance, if each lock can be held by 3 participants, lock 0 would be held + // by participants (0, 1, 2), lock 1 would be held by (1, 2, 3), and so on, wrapping around the + // number of participants as necessary. + int i = 0; + for (String partition : currentIdealState.getPartitionSet()) { + List<String> preferenceList = new ArrayList<String>(); + for (int j = i; j < i + lockHolders; j++) { + int participantIndex = j % liveParticipants.size(); + String participant = liveParticipants.get(participantIndex); + // enforce that a participant can only have one instance of a given lock + if (!preferenceList.contains(participant)) { + preferenceList.add(participant); + } + } + currentIdealState.setPreferenceList(partition, preferenceList); + i++; + } + return assignment; +} +</pre> + </div> + <p>Here are the IdealState preference lists emitted by the user-defined rebalancer for a 3-participant system whenever there is a change to the set of participants.</p> + <ul> + <li>Participant_A joins</li> + </ul> + <div class="source"> + <pre>{ + "lock_0": ["Participant_A"], + "lock_1": ["Participant_A"], + "lock_2": ["Participant_A"], + "lock_3": ["Participant_A"], + "lock_4": ["Participant_A"], + "lock_5": ["Participant_A"], +} +</pre> + </div> + <p>A preference list is a mapping for each resource of partition to the participants serving each replica. The state model is a simple LOCKED/RELEASED model, so participant A holds all lock partitions in the LOCKED state.</p> + <ul> + <li>Participant_B joins</li> + </ul> + <div class="source"> + <pre>{ + "lock_0": ["Participant_A"], + "lock_1": ["Participant_B"], + "lock_2": ["Participant_A"], + "lock_3": ["Participant_B"], + "lock_4": ["Participant_A"], + "lock_5": ["Participant_B"], +} +</pre> + </div> + <p>Now that there are two participants, the simple mod-based function assigns every other lock to the second participant. On any system change, the rebalancer is invoked so that the application can define how to redistribute its resources.</p> + <ul> + <li>Participant_C joins (steady state)</li> + </ul> + <div class="source"> + <pre>{ + "lock_0": ["Participant_A"], + "lock_1": ["Participant_B"], + "lock_2": ["Participant_C"], + "lock_3": ["Participant_A"], + "lock_4": ["Participant_B"], + "lock_5": ["Participant_C"], +} +</pre> + </div> + <p>This is the steady state of the system. Notice that four of the six locks now have a different owner. That is because of the naïve modulus-based assignmemt approach used by the user-defined rebalancer. However, the interface is flexible enough to allow you to employ consistent hashing or any other scheme if minimal movement is a system requirement.</p> + <ul> + <li>Participant_B fails</li> + </ul> + <div class="source"> + <pre>{ + "lock_0": ["Participant_A"], + "lock_1": ["Participant_C"], + "lock_2": ["Participant_A"], + "lock_3": ["Participant_C"], + "lock_4": ["Participant_A"], + "lock_5": ["Participant_C"], +} +</pre> + </div> + <p>On any node failure, as in the case of node addition, the rebalancer is invoked automatically so that it can generate a new mapping as a response to the change. Helix ensures that the Rebalancer has the opportunity to reassign locks as required by the application.</p> + <ul> + <li>Participant_B (or the replacement for the original Participant_B) rejoins</li> + </ul> + <div class="source"> + <pre>{ + "lock_0": ["Participant_A"], + "lock_1": ["Participant_B"], + "lock_2": ["Participant_C"], + "lock_3": ["Participant_A"], + "lock_4": ["Participant_B"], + "lock_5": ["Participant_C"], +} +</pre> + </div> + <p>The rebalancer was invoked once again and the resulting IdealState preference lists reflect the steady state.</p> + </div> + <div class="section"> + <h3 id="Caveats">Caveats</h3> + <ul> + <li>The rebalancer class must be available at runtime, or else Helix will not attempt to rebalance at all</li> + <li>The Helix controller will only take into account the preference lists in the new IdealState for this release. In 0.7.0, Helix rebalancers will be able to compute the full resource assignment, including the states.</li> + <li>Helix does not currently persist the new IdealState computed by the user-defined rebalancer. However, the Helix property store is available for saving any computed state. 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See the License for the +specific language governing permissions and limitations +under the License. --> +<p></p> +<div class="section"> + <div class="page-header"> + <h2 id="Helix_Tutorial:_YAML_Cluster_Setup"><a href="./Tutorial.html">Helix Tutorial</a>: YAML Cluster Setup</h2> + </div> + <p>As an alternative to using Helix Admin to set up the cluster, its resources, constraints, and the state model, Helix supports bootstrapping a cluster configuration based on a YAML file. Below is an annotated example of such a file for a simple distributed lock manager where a lock can only be LOCKED or RELEASED, and each lock only allows a single participant to hold it in the LOCKED state.</p> + <div class="source"> + <pre>clusterName: lock-manager-custom-rebalancer # unique name for the cluster (required) +resources: + - name: lock-group # unique resource name (required) + rebalancer: # required + mode: USER_DEFINED # required - USER_DEFINED means we will provide our own rebalancer + class: org.apache.helix.userdefinedrebalancer.LockManagerRebalancer # required for USER_DEFINED + partitions: + count: 12 # number of partitions for the resource (default is 1) + replicas: 1 # number of replicas per partition (default is 1) + stateModel: + name: lock-unlock # model name (required) + states: [LOCKED, RELEASED, DROPPED] # the list of possible states (required if model not built-in) + transitions: # the list of possible transitions (required if model not built-in) + - name: Unlock + from: LOCKED + to: RELEASED + - name: Lock + from: RELEASED + to: LOCKED + - name: DropLock + from: LOCKED + to: DROPPED + - name: DropUnlock + from: RELEASED + to: DROPPED + - name: Undrop + from: DROPPED + to: RELEASED + initialState: RELEASED # (required if model not built-in) + constraints: + state: + counts: # maximum number of replicas of a partition that can be in each state (required if model not built-in) + - name: LOCKED + count: "1" + - name: RELEASED + count: "-1" + - name: DROPPED + count: "-1" + priorityList: [LOCKED, RELEASED, DROPPED] # states in order of priority (all priorities equal if not specified) + transition: # transitions priority to enforce order that transitions occur + priorityList: [Unlock, Lock, Undrop, DropUnlock, DropLock] # all priorities equal if not specified +participants: # list of nodes that can serve replicas (optional if dynamic joining is active, required otherwise) + - name: localhost_12001 + host: localhost + port: 12001 + - name: localhost_12002 + host: localhost + port: 12002 + - name: localhost_12003 + host: localhost + port: 12003 +</pre> + </div> + <p>Using a file like the one above, the cluster can be set up either with the command line:</p> + <div class="source"> + <pre>helix/helix-core/target/helix-core/pkg/bin/YAMLClusterSetup.sh localhost:2199 lock-manager-config.yaml +</pre> + </div> + <p>or with code:</p> + <div class="source"> + <pre>YAMLClusterSetup setup = new YAMLClusterSetup(zkAddress); +InputStream input = + Thread.currentThread().getContextClassLoader() + .getResourceAsStream("lock-manager-config.yaml"); +YAMLClusterSetup.YAMLClusterConfig config = setup.setupCluster(input); +</pre> + </div> + <p>Some notes:</p> + <ul> + <li> <p>A rebalancer class is only required for the USER_DEFINED mode. It is ignored otherwise.</p></li> + <li> <p>Built-in state models, like OnlineOffline, LeaderStandby, and MasterSlave, or state models that have already been added only require a name for stateModel. If partition and/or replica counts are not provided, a value of 1 is assumed.</p></li> + </ul> +</div> + </div> + </div> + </div> + </div> + + </div><!-- /container --> + + <!-- Footer + ================================================== --> + <footer class="well"> + <div class="container"> + <div class="row"> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Get Helix</li> + <li> + <a href="download.html" title="Download">Download </a> + </li> + <li> + <a href="Building.html" title="Building">Building </a> + </li> + <li> + <a href="releasenotes/release-0.6.5.html" title="Release Notes">Release Notes </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Hands-On</li> + <li> + <a href="Quickstart.html" title="Quick Start">Quick Start </a> + </li> + <li> + <a href="Tutorial.html" title="Tutorial">Tutorial </a> + </li> + <li> + <a href="../javadocs/0.6.5" title="Javadocs">Javadocs </a> + </li> + </ul> + </div> + <div class="span3 bottom-nav"> + <ul class="nav nav-list"> + <li class="nav-header">Recipes</li> + <li> + <a href="recipes/lock_manager.html" title="Distributed lock manager">Distributed lock manager </a> + </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> + </li> + </ul> + </div> + </div> + </div> + </footer> + + <div class="container subfooter"> + <div class="row"> + <div class="span12"> + <p class="pull-right"><a href="#">Back to top</a></p> + <p class="copyright">Copyright ©2015 <a href="http://www.apache.org/";>The Apache Software Foundation</a>. 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