Modified: helix/site-content/0.9.9-docs/Tutorial.html URL: http://svn.apache.org/viewvc/helix/site-content/0.9.9-docs/Tutorial.html?rev=1900184&r1=1900183&r2=1900184&view=diff ============================================================================== --- helix/site-content/0.9.9-docs/Tutorial.html (original) +++ helix/site-content/0.9.9-docs/Tutorial.html Sat Apr 23 04:22:15 2022 @@ -1,200 +1,353 @@ - <!DOCTYPE html> + + <!-- - Generated by Apache Maven Doxia at 2022-04-23 - Rendered using Maven Reflow Skin 1.0.0 (http://andriusvelykis.github.com/reflow-maven-skin) + | Generated by Apache Maven Doxia Site Renderer 1.11.1 from src/site/markdown/Tutorial.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 – Tutorial</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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All Rights Reserved.</p> - <p><a href="http://github.com/andriusvelykis/reflow-maven-skin"; title="Reflow Maven skin">Reflow Maven skin</a> by <a href="http://andrius.velykis.lt"; target="_blank" title="Andrius Velykis">Andrius Velykis</a>.</p> - </div> - </div> - </div> - - <!-- Le javascript - ================================================== --> - <!-- Placed at the end of the document so the pages load faster --> - - <!-- Fallback jQuery loading from Google CDN: - http://stackoverflow.com/questions/1014203/best-way-to-use-googles-hosted-jquery-but-fall-back-to-my-hosted-library-on-go --> - <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js";></script> - <script type="text/javascript"> - if (typeof jQuery == 'undefined') - { - document.write(unescape("%3Cscript src='./js/jquery-1.8.3.min.js' type='text/javascript'%3E%3C/script%3E")); - } - </script> - - <script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/js/bootstrap.min.js";></script> - <script src="./js/lightbox.js"></script> - <script src="./js/jquery.smooth-scroll.min.js"></script> - <!-- back button support for smooth scroll --> - <script src="./js/jquery.ba-bbq.min.js"></script> - - <script src="./js/reflow-skin.js"></script> - - </body> +<h1>Helix Tutorial</h1> +<p>In this tutorial, we will cover the roles of a Helix-managed cluster, and show the code you need to write to integrate with it. In many cases, there is a simple default behavior that is often appropriate, but you can also customize the behavior.</p> +<p>Convention: we first cover the <i>basic</i> approach, which is the easiest to implement. Then, we'll describe <i>advanced</i> options, which give you more control over the system behavior, but require you to write more code.</p><section><section> +<h3><a name="Prerequisites"></a>Prerequisites</h3> +<ol style="list-style-type: decimal"> + +<li>Read <a href="../Concepts.html">Concepts/Terminology</a> and <a href="../Architecture.html">Architecture</a></li> +<li>Read the <a href="./Quickstart.html">Quickstart guide</a> to learn how Helix models and manages a cluster</li> +<li>Install Helix source. See: <a href="./Quickstart.html">Quickstart</a> for the steps.</li> +</ol></section><section> +<h3><a name="Tutorial_Outline"></a>Tutorial Outline</h3> +<ol style="list-style-type: decimal"> + +<li><a href="./tutorial_participant.html">Participant</a></li> +<li><a href="./tutorial_spectator.html">Spectator</a></li> +<li><a href="./tutorial_controller.html">Controller</a></li> +<li><a href="./tutorial_rebalance.html">Rebalancing Algorithms</a></li> +<li><a href="./tutorial_user_def_rebalancer.html">User-Defined Rebalancing</a></li> +<li><a href="./tutorial_state.html">State Machines</a></li> +<li><a href="./tutorial_messaging.html">Messaging</a></li> +<li><a href="./tutorial_health.html">Customized health check</a></li> +<li><a href="./tutorial_throttling.html">Throttling</a></li> +<li><a href="./tutorial_propstore.html">Application Property Store</a></li> +<li><a href="./tutorial_admin.html">Admin Interface</a></li> +<li><a href="./tutorial_yaml.html">YAML Cluster Setup</a></li> +<li><a href="./tutorial_agent.html">Helix Agent (for non-JVM systems)</a></li> +<li><a href="./tutorial_task_framework.html">Task Framework</a> +<ol style="list-style-type: decimal"> + +<li><a href="./tutorial_user_content_store.html">Task with User Defined Content Store</a></li> +<li><a href="./tutorial_task_throttling.html">Task Throttling</a></li> +<li><a href="./quota_scheduling.html">Quota-based Scheduling</a></li> +</ol> +</li> +<li><a href="./tutorial_rest_service.html">Helix REST Service 2.0</a></li> +<li><a href="./tutorial_ui.html">Helix UI Setup</a></li> +</ol></section><section> +<h3><a name="Preliminaries"></a>Preliminaries</h3> +<p>First, we need to set up the system. Let's walk through the steps in building a distributed system using Helix.</p><section> +<h4><a name="Start_ZooKeeper"></a>Start ZooKeeper</h4> +<p>This starts a zookeeper in standalone mode. For production deployment, see <a class="externalLink" href="http://zookeeper.apache.org";>Apache ZooKeeper</a> for instructions.</p> + +<div class="source"><pre class="prettyprint"><code>./start-standalone-zookeeper.sh 2199 & +</code></pre></div></section><section> +<h4><a name="Create_a_Cluster"></a>Create a Cluster</h4> +<p>Creating a cluster will define the cluster in appropriate znodes on ZooKeeper.</p> +<p>Using the Java API:</p> + +<div class="source"><pre class="prettyprint"><code>// Create setup tool instance +// Note: ZK_ADDRESS is the host:port of Zookeeper +String ZK_ADDRESS = "localhost:2199"; +admin = new ZKHelixAdmin(ZK_ADDRESS); + +String CLUSTER_NAME = "helix-demo"; +//Create cluster namespace in zookeeper +admin.addCluster(CLUSTER_NAME); +</code></pre></div> +<p>OR</p> +<p>Using the command-line interface:</p> + +<div class="source"><pre class="prettyprint"><code>./helix-admin.sh --zkSvr localhost:2199 --addCluster helix-demo +</code></pre></div></section><section> +<h4><a name="Configure_the_Nodes_of_the_Cluster"></a>Configure the Nodes of the Cluster</h4> +<p>First we'll add new nodes to the cluster, then configure the nodes in the cluster. Each node in the cluster must be uniquely identifiable. +The most commonly used convention is hostname:port.</p> + +<div class="source"><pre class="prettyprint"><code>String CLUSTER_NAME = "helix-demo"; +int NUM_NODES = 2; +String hosts[] = new String[]{"localhost","localhost"}; +String ports[] = new String[]{"7000","7001"}; +for (int i = 0; i < NUM_NODES; i++) +{ + InstanceConfig instanceConfig = new InstanceConfig(hosts[i]+ "_" + ports[i]); + instanceConfig.setHostName(hosts[i]); + instanceConfig.setPort(ports[i]); + instanceConfig.setInstanceEnabled(true); + + //Add additional system specific configuration if needed. These can be accessed during the node start up. + instanceConfig.getRecord().setSimpleField("key", "value"); + admin.addInstance(CLUSTER_NAME, instanceConfig); +} +</code></pre></div></section><section> +<h4><a name="Configure_the_Resource"></a>Configure the Resource</h4> +<p>A <b>resource</b> represents the actual task performed by the nodes. It can be a database, index, topic, queue or any other processing entity. +A resource can be divided into many sub-parts known as <b>partitions</b>.</p><section> +<h5><a name="Define_the_State_Model_and_Constraints"></a>Define the State Model and Constraints</h5> +<p>For scalability and fault tolerance, each partition can have one or more replicas. +The <b>state model</b> allows one to declare the system behavior by first enumerating the various STATES, and the TRANSITIONS between them. +A simple model is ONLINE-OFFLINE where ONLINE means the task is active and OFFLINE means it's not active. +You can also specify how many replicas must be in each state, these are known as <b>constraints</b>. +For example, in a search system, one might need more than one node serving the same index to handle the load.</p> +<p>The allowed states:</p> +<ul> + +<li>LEADER</li> +<li>STANDBY</li> +<li>OFFLINE</li> +</ul> +<p>The allowed transitions:</p> +<ul> + +<li>OFFLINE to STANDBY</li> +<li>STANDBY to OFFLINE</li> +<li>STANDBY to LEADER</li> +<li>LEADER to STANDBY</li> +</ul> +<p>The constraints:</p> +<ul> + +<li>no more than 1 LEADER per partition</li> +<li>the rest of the replicas should be standbys</li> +</ul> +<p>The following snippet shows how to declare the state model and constraints for the LEADER-STANDBY model.</p> + +<div class="source"><pre class="prettyprint"><code>String STATE_MODEL_NAME = "LeaderStandby"; +StateModelDefinition.Builder builder = new StateModelDefinition.Builder(STATE_MODEL_NAME); +// Define your own states: those are opaque strings to Helix +// Only the topology of the state machine (initial state, transitions, priorities, final DROPPED state) is meaningful to Helix +String LEADER = "LEADER"; +String STANDBY = "STANDBY"; +String OFFLINE = "OFFLINE"; + +// Add states and their rank to indicate priority. A lower rank corresponds to a higher priority +builder.addState(LEADER, 1); +builder.addState(STANDBY, 2); +builder.addState(OFFLINE); +// Note the special inclusion of the DROPPED state (REQUIRED) +builder.addState(HelixDefinedState.DROPPED.name()); + +// Set the initial state when the node starts +builder.initialState(OFFLINE); + +// Add transitions between the states. +builder.addTransition(OFFLINE, STANDBY); +builder.addTransition(STANDBY, OFFLINE); +builder.addTransition(STANDBY, LEADER); +builder.addTransition(LEADER, STANDBY); + +// There must be a path to DROPPED from each state (REQUIRED) +builder.addTransition(OFFLINE, HelixDefinedState.DROPPED.name()); + +// set constraints on states + +// static constraint: upper bound of 1 LEADER +builder.upperBound(LEADER, 1); + +// dynamic constraint: R means it should be derived based on the replication factor for the cluster +// this allows a different replication factor for each resource without +// having to define a new state model + +builder.dynamicUpperBound(STANDBY, "R"); + +StateModelDefinition myStateModel = builder.build(); +admin.addStateModelDef(CLUSTER_NAME, STATE_MODEL_NAME, myStateModel); +</code></pre></div></section><section> +<h5><a name="Assigning_Partitions_to_Nodes"></a>Assigning Partitions to Nodes</h5> +<p>The final goal of Helix is to ensure that the constraints on the state model are satisfied. +Helix does this by assigning a <b>state</b> to a partition (such as LEADER, STANDBY), and placing it on a particular node.</p> +<p>There are 3 assignment modes Helix can operate in:</p> +<ul> + +<li>FULL_AUTO: Helix decides the placement and state of a partition.</li> +<li>SEMI_AUTO: Application decides the placement but Helix decides the state of a partition.</li> +<li>CUSTOMIZED: Application controls the placement and state of a partition.</li> +</ul> +<p>For more information on the assignment modes, see the <a href="./tutorial_rebalance.html">Rebalancing Algorithms</a> section of this tutorial.</p> + +<div class="source"><pre class="prettyprint"><code>String RESOURCE_NAME = "MyDB"; +int NUM_PARTITIONS = 6; +STATE_MODEL_NAME = "LeaderStandby"; +String MODE = "SEMI_AUTO"; +int NUM_REPLICAS = 2; + +admin.addResource(CLUSTER_NAME, RESOURCE_NAME, NUM_PARTITIONS, STATE_MODEL_NAME, MODE); +admin.rebalance(CLUSTER_NAME, RESOURCE_NAME, NUM_REPLICAS); +</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/0.9.9-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 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. --> -<html xml:lang="en" lang="en"> +<h1>Auto-exit of Maintenance Mode</h1><section> +<h2><a name="Introduction"></a>Introduction</h2> +<p>Auto-exit of maintenance mode is a feature addition to Helix. It concerns a feature that allows clusters to switch out of maintenance mode automatically to save operational costs and to improve availability.</p></section><section> +<h2><a name="Background"></a>Background</h2><section> +<h3><a name="Maintenance_Mode"></a>Maintenance Mode</h3> +<p>The maintenance mode in Helix refers to a Helix cluster state where the Helix Controller will not trigger any type of rebalance. In short, if a cluster is in maintenance, the Controller will not bootstrap any new partitions. However, this does not mean that there won't be any (upward) state transitions. For example, partitions missing a top-state (e.g. <b>MASTER/LEADER</b>) replica will get a state transition in order to fill in the void for a top-state replica.</p><section> +<h4><a name="Possible_Scenarios"></a>Possible Scenarios</h4> +<p>There are two possible conditions under which a cluster will go into maintenance mode automatically.</p> +<ol style="list-style-type: decimal"> + +<li>There are more offline/disabled instances than ClusterConfig's <b>MAX_OFFLINE_INSTANCES_ALLOWED</b>.</li> +<li>There exist instances that have more partitions than ClusterConfig's <b>MAX_PARTITIONS_PER_INSTANCE</b>.</li> +</ol> +<p>Difficulty in Manually Exiting Maintenance Mode</p> +<p>Although maintenance mode has been designed to prevent a large-scale reshuffling of replicas when there are intermittent, transient Participant connection issues. When in maintenance, no new partitions will be bootstrapped, which will block many native operations such as creation of stores/DBs. Currently, in order to unblock, an operator must manually disable maintenance mode via Helix REST API. Since it's difficult to predict when temporary disconnects will happen, it has been a challenge to address them in a prompt manner.</p></section></section></section><section> +<h2><a name="Problem_Statement"></a>Problem Statement</h2> +<p>Currently, once clusters enter maintenance mode automatically, exiting must be done manually. This is an added human effort. We want a feature to automate this to reduce downtime and increase availability.</p></section><section> +<h2><a name="Architecture.2FImplementation"></a>Architecture/Implementation</h2><section> +<h3><a name="Maintenance_Recovery_Threshold"></a>Maintenance Recovery Threshold</h3> +<p>Recall that there are two cases in which a cluster may enter maintenance mode automatically:</p> +<blockquote> + +<ol style="list-style-type: decimal"> + +<li>There are more offline/disabled instances than ClusterConfig's MAX_OFFLINE_INSTANCES_ALLOWED.</li> +<li>There exist instances that have more partitions than ClusterConfig's MAX_PARTITIONS_PER_INSTANCE.</li> +</ol> +</blockquote> +<p>It is important to note that we are mainly concerned with Case 1. Case 2 is meant to be a sanity check against Helix's CRUSH-ed rebalance algorithm, and this has little to do with the original motivation behind cluster maintenance mode. So, although we will <i>still</i> check against Case 2 when determining whether it is safe to exit maintenance mode, we will only address Case 1 in this section.</p> +<p>With that said, it is not hard to imagine a cluster in production experiencing a flip-flop behavior around the exact value of <b>ClusterConfig's MAX_OFFLINE_INSTANCES_ALLOWED</b> field. Such behavior is undesirable and may defeat the purpose of using this feature. To mitigate it in design is to have a separate, <i>more strict</i> threshold for recovering out of maintenance mode. We therefore propose to add a new field, <b>MAINTENANCE_RECOVERY_THRESHOLD</b>. <b>MAINTENANCE_RECOVERY_THRESHOLD</b> will be defined by the user and hold a more conservative value than <b>MAX_OFFLINE_INSTANCES_ALLOWED</b> to allow for some wiggle room in deciding what value is enough to call a cluster “<i>sufficiently recovered</i>”. Additionally, we will also give users the flexibility to <i>“opt-out”</i> of the auto-exit feature by treating a <b>MAINTENANCE_RECOVERY_THRESHOLD</b> value of -1 as a disable signal.</p> +<p>In short,</p> +<ul> + +<li>If NumOfflineDisabledInstances < NUM_OFFLINE_INSTANCES_FOR_AUTO_EXIT, exit maintenance mode</li> +<li>NUM_OFFLINE_INSTANCES_FOR_AUTO_EXIT <= MAX_OFFLINE_INSTANCES_ALLOWED</li> +<li>NUM_OFFLINE_INSTANCES_FOR_AUTO_EXIT < 0 or not set → Opt-out of auto-exiting of maintenance mode</li> +</ul></section><section> +<h3><a name="Additional_Fields_in_MaintenanceSignal"></a>Additional Fields in MaintenanceSignal</h3> +<p>Currently, Helix stores the maintenance signal in /{CLUSTER_NAME}/CONTROLLER/MAINTENANCE. A sample maintenance signal ZNode will look like the following:</p> + +<div class="source"><pre class="prettyprint"><code class="language-json">{ + "id" : "maintenance", + "simpleFields" : { + "REASON" : "Offline Instances count 5 greater than allowed count 4. Stop rebalance and put the cluster CLUSTER_TestClusterInMaintenanceModeWhenReachingOfflineInstancesLimit into maintenance mode.", + "TRIGGERED_BY" : "CONTROLLER", + "TIMESTAMP" : "12312838831***", + // The following are sample user-defined fields from the REST call payload // + "id" : "hulee", + "jira" : "HELIX-123", + "cluster" : "testCluster" + }, + "listFields" : { + }, + "mapFields" : { + } +} +</code></pre></div> +<p>A simpleField will be newly added (notice the “<b>TRIGGERED_BY</b>” field) to denote whether the current maintenance signal has been generated automatically by the Controller. There will initially be <b>two</b> possible entries for this field:</p> +<ul> + +<li><b>CONTROLLER</b></li> +<li><b>USER</b></li> +</ul> +<p>Also, the TIMESTAMP field will be added so that the value could be retrieved via REST.</p> +<p>In addition, we will allow addition of custom fields in simpleFields. See the example above. Users will be able to add these fields by using the REST endpoints that will be provided when <b>enabling</b> the maintenance mode. Note that when disabling maintenance mode, these fields will no longer be available because disabling maintenance mode will remove the maintenance signal altogether.</p></section><section> +<h3><a name="Recording_Maintenance_History"></a>Recording Maintenance History</h3> +<p>Currently, Helix maintains the controller history in the /{clusterName}/CONTROLLER/HISTORY ZNode. We plan to add a history for maintenance mode. This way, users will be able to access the history in ZooKeeper directly, or utilize the REST endpoint for viewing. The format will be in a human-readable format like the following (this is how Helix maintains the Controller leader history):</p> + +<div class="source"><pre class="prettyprint"><code class="language-java">long currentTime = System.currentTimeMillis(); +DateFormat df = new SimpleDateFormat("yyyy-MM-dd-HH:mm:ss"); +df.setTimeZone(TimeZone.getTimeZone("UTC")); +String dateTime = df.format(new Date(currentTime)); +</code></pre></div> +<p>Note that Helix only records <i>10 most recent</i> changes in the HISTORY ZNode. We will follow a similar protocol for persisting maintenance mode-related history. Note that <b>all changes around maintenance mode will be recorded</b> (both entering and exiting of the mode).</p></section><section> +<h3><a name="Backward_Compatibility"></a>Backward Compatibility</h3> +<p>Old versions of Helix Controllers will not refer to the newly-added fields, in which case they will simply lack the auto-exit functionality.</p></section><section> +<h3><a name="Asynchronous_Processing"></a>Asynchronous Processing</h3> +<p>Note that in order for a cluster to exit maintenance mode, the Controller must check against <i>all conditions</i> that are outlined above. That is, it needs to make sure <b>1) the total count of offline/disabled instances is less than the threshold and that 2) there are no instances with more partitions than the threshold</b>. Checking against Condition 2 requires traversing all resources' states, which could be costly in clusters of large scale. From this, we potentially risk adding a significant latency to the pipeline, which is undesirable because it could cause problems regarding availability (for example, longer pipeline runs may exacerbate the rate at which ZooKeeper Event Callbacks accumulate. Helix currently deals with a large number of ZK Event Callbacks by re-booting the Controller, which could nick Helix's as well as applications' availability numbers).</p> +<p>For this reason, we will make the checks happen asynchronously. Additional tweak may be required to ensure that the maintenance signal is cached appropriately through each run of the pipeline to avoid the race condition.</p></section><section> +<h3><a name="Periodic_Check_on_Cluster_Recovery_Status"></a>Periodic Check on Cluster Recovery Status</h3> +<p>Auto-exit is inherently triggered as part of Helix's rebalance pipeline; that is, if there is no rebalance triggered, there wouldn't be any auto-exits triggered either. In theory, this is a non-issue because whether a cluster goes into or recovers out of maintenance mode is determined by changes around LiveInstances, and a LiveInstance change is an event that triggers Helix's rebalance pipeline.</p> +<p>Regardless, there has been anecdotal reports where such events seemed to have been “swallowed” or “disappeared” - if that is true for any reason (mostly a ZK callback queue issue), one way to mitigate it is to enable periodically-triggered rebalances. Note that this is actually a feature <i>already supported</i> by Helix.</p></section></section><section> +<h2><a name="How_to_Use_the_Auto-Exit_Feature"></a>How to Use the Auto-Exit Feature</h2><section> +<h3><a name="Why_Use_Auto-Exit"></a>Why Use Auto-Exit</h3> +<p>Helix is used to manage resources in distributed clusters; therefore, it inevitably gets to have hundreds of instances. With so many clusters and traffic to the ZooKeeper, Helix's metadata store, there are cases in which some Participants in the cluster experience transient connection failure, which may cause Helix to respond to each little change that happens to the cluster. For users of stateful systems, this may be undesirable, so they opt to set a config for entering maintenance mode automatically. Maintenance mode is a temporary mode that the cluster can enter in order to ensure that there are no bootstrapping state transitions on instances.</p> +<p>However, no bootstrapping state transitions could mean that some operations such as addition of resources would be halted, which causes periods of unavailability. It was the cluster operator's responsibility to determine whether the given cluster has sufficiently recovered enough to exit maintenance mode. The auto-exit feature removes such overhead.</p></section><section> +<h3><a name="Guide"></a>Guide</h3> +<p>First, we encourage all users of this feature to understand Helix's workflow below:</p> +<p><img src="./images/auto-exit-maintenance.jpg" alt="Intro" /></p> +<p>In order to use this feature, you'd need to set the following config parameters in your cluster's ClusterConfig.</p><section> +<h4><a name="Auto-enter_maintenance_mode"></a>Auto-enter maintenance mode</h4> +<p><b>MAX_OFFLINE_INSTANCES_ALLOWED</b>: the number of offline and disabled instances allowed before the cluster automatically enters maintenance mode.</p> +<p><b>MAX_PARTITIONS_PER_INSTANCE</b>: the number of partitions on any given instance, where, if any instance in the cluster happens to have more partitions than this number, the cluster automatically enters maintenance mode</p></section><section> +<h4><a name="Auto-exit_maintenance_mode"></a>Auto-exit maintenance mode</h4> +<p><b>NUM_OFFLINE_INSTANCES_FOR_AUTO_EXIT</b>: set this value to allow your cluster to auto-exit when the number of offline and disabled instances are at this value. Note that this value must be less than MAX_OFFLINE_INSTANCES_ALLOWED (Read the design above on why). Note that the appropriate value for this is dependent on the characteristics of the cluster. In general, start with 1, meaning that the cluster will only auto-exit maintenance mode when it is down to 1 offline or disable instance, and increase the value as you increase tolerance.</p> +<p>Note that the cluster will auto-exit only if it has <i>automatically</i> entered maintenance mode previously.</p></section></section></section><section> +<h2><a name="FAQ"></a>FAQ</h2><section><section> +<h4><a name="How_is_this_related_to_DelayedAutoRebalancer.3F"></a>How is this related to <b>DelayedAutoRebalancer</b>?</h4> +<ul> + +<li>If you are using DelayedAutoRebalancer for your clusters, this auto-exit feature would still work as expected. Fundamentally, DelayedAutoRebalancer and maintenance mode are mutually independent. That is, under maintenance mode, Helix uses a different rebalancing strategy. Once the cluster auto-exits maintenance mode, it will go back to whichever rebalancing mode it was on previously.</li> +<li>Delayed Partition Movement is a feature in Helix's FULL-AUTO rebalancer.</li> +<li>The rebalance delay will work at the instance level and entering/exiting maintenance mode will not affect the time at which the delay-counting started.</li> +</ul></section><section> +<h4><a name="How_do_I_know_what_the_appropriate_value_is_for_MAINTENANCE_RECOVERY_THRESHOLD.3F"></a>How do I know what the appropriate value is for <b><b>MAINTENANCE_RECOVERY_THRESHOLD</b></b>?</h4> +<ul> - <head> - <meta charset="UTF-8" /> - <title>Apache Helix – Auto-exit of Maintenance Mode</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"; 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All Rights Reserved.</p> - <p><a href="http://github.com/andriusvelykis/reflow-maven-skin"; title="Reflow Maven skin">Reflow Maven skin</a> by <a href="http://andrius.velykis.lt"; target="_blank" title="Andrius Velykis">Andrius Velykis</a>.</p> - </div> - </div> - </div> - - <!-- Le javascript - ================================================== --> - <!-- Placed at the end of the document so the pages load faster --> - - <!-- Fallback jQuery loading from Google CDN: - http://stackoverflow.com/questions/1014203/best-way-to-use-googles-hosted-jquery-but-fall-back-to-my-hosted-library-on-go --> - <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js";></script> - <script type="text/javascript"> - if (typeof jQuery == 'undefined') - { - document.write(unescape("%3Cscript src='./js/jquery-1.8.3.min.js' type='text/javascript'%3E%3C/script%3E")); - } - </script> - - <script src="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.2/js/bootstrap.min.js";></script> - <script src="./js/lightbox.js"></script> - <script src="./js/jquery.smooth-scroll.min.js"></script> - <!-- back button support for smooth scroll --> - <script src="./js/jquery.ba-bbq.min.js"></script> - - <script src="./js/reflow-skin.js"></script> - - </body> +<li>As a framework, we cannot provide the right value for this threshold. It should depend on the nature of the application and the risk tolerance thereof. Familiarize yourself with the rules outlined above and start with a low value (for example, 0) and increase your tolerance.</li> +</ul></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/0.9.9-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 Modified: helix/site-content/0.9.9-docs/css/print.css URL: http://svn.apache.org/viewvc/helix/site-content/0.9.9-docs/css/print.css?rev=1900184&r1=1900183&r2=1900184&view=diff ============================================================================== --- helix/site-content/0.9.9-docs/css/print.css (original) +++ helix/site-content/0.9.9-docs/css/print.css Sat Apr 23 04:22:15 2022 @@ -1,80 +1,21 @@ -.navbar, -.breadcrumb, -.toc-separator -#toc-bar, -#toc-sidebar, -footer, -.subfooter { - display: none !important; -} - -body { - padding-top: 0px !important; -} - -/* CSS below taken from HTML5 Boilerplate */ -* { - background: transparent !important; - color: #000 !important; /* Black prints faster: h5bp.com/s */ - box-shadow:none !important; - text-shadow: none !important; -} - -a, -a:visited { - text-decoration: underline; -} - -a[href]:after { - content: " (" attr(href) ")"; -} - -abbr[title]:after { - content: " (" attr(title) ")"; -} - /* - * Don't show links for images, or javascript/internal links, or header links + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. 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