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svn commit: r1900823 [18/22] - in /helix/site-content: ./ 0.9.9-docs/ 0.9.9-docs/css/ 0.9.9-docs/images/ 0.9.9-docs/js/ 0.9.9-docs/recipes/ 0.9.9-docs/releasenotes/ 1.0.2-docs/ 1.0.2-docs/css/ 1.0.2-docs/images/ 1.0.2-docs/js/ 1.0.2-docs/recipes/ 1.0.2...
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--- helix/site-content/1.0.5-docs/tutorial_user_content_store.html (added)
+++ helix/site-content/1.0.5-docs/tutorial_user_content_store.html Thu May 12 05:29:03 2022
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+ <li>Tutorial - User Defined Content Store for Tasks</li>
+ <li class="publishDate version-date pull-right">Last Published: 2022-05-11</li>
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+
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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.
+-->
+<section>
+ <div class="page-header">
+ <h2 id="Helix_Tutorial:_User_Defined_Content_Store_for_Tasks"><a href="./Tutorial.html">Helix Tutorial</a>: User Defined Content Store for Tasks</h2>
+ </div>
+ <p>The purpose of user defined content store is to provide an easy use feature for some task dedicated meta temporary store. In this chapter, we'll learn how to implement and use content store in the user defined tasks.</p>
+ <section>
+ <h3 id="Content_Store_Implementation">Content Store Implementation</h3>
+ <p>Extends abstract class UserContentStore.</p>
+ <div class="source">
+ <pre><code>private static class ContentStoreTask extends UserContentStore implements Task {
+ @Override public TaskResult run() {
+ ...
+ }
+ @Override public void cancel() {
+ ...
+ }
+}
+</code></pre>
+ </div>
+ <p>The default methods support 3 types of scopes:</p>
+ <ol style="list-style-type: decimal">
+ <li>WORKFLOW: Define the content store in workflow level</li>
+ <li>JOB: Define the content store in job level</li>
+ <li>TASK: Define the content store in task level</li>
+ </ol>
+ </section>
+ <section>
+ <h3 id="Content_Store_Usage">Content Store Usage</h3>
+ <p>Access content store in Task.run() method.</p>
+ <div class="source">
+ <pre><code> private static class ContentStoreTask extends UserContentStore implements Task {
+ @Override public TaskResult run() {
+ // put values into the store
+ putUserContent("ContentTest", "Value1", Scope.JOB);
+ putUserContent("ContentTest", "Value2", Scope.WORKFLOW);
+ putUserContent("ContentTest", "Value3", Scope.TASK);
+
+ // get the values with the same key in the different scopes
+ if (!getUserContent("ContentTest", Scope.JOB).equals("Value1") ||
+ !getUserContent("ContentTest", Scope.WORKFLOW).equals("Value2") ||
+ !getUserContent("ContentTest", Scope.TASK).equals("Value3")) {
+ return new TaskResult(TaskResult.Status.FAILED, null);
+ }
+
+ return new TaskResult(TaskResult.Status.COMPLETED, null);
+ }
+ }
+</code></pre>
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Added: helix/site-content/1.0.5-docs/tutorial_user_def_rebalancer.html
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--- helix/site-content/1.0.5-docs/tutorial_user_def_rebalancer.html (added)
+++ helix/site-content/1.0.5-docs/tutorial_user_def_rebalancer.html Thu May 12 05:29:03 2022
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+ <li class="publishDate version-date pull-right">Last Published: 2022-05-11</li>
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+
+ <div class="main-body">
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+ <div class="body-content">
+<!--
+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
+
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+
+Unless required by applicable law or agreed to in writing,
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+<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><code>void init(HelixManager manager);
+
+IdealState computeNewIdealState(String resourceName, IdealState currentIdealState,
+ final CurrentStateOutput currentStateOutput, final ClusterDataCache clusterData);
+</code></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>
+ <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><code>IdealState idealState = helixAdmin.getResourceIdealState(clusterName, resourceName);
+idealState.setRebalanceMode(RebalanceMode.USER_DEFINED);
+idealState.setRebalancerClassName(className);
+helixAdmin.setResourceIdealState(clusterName, resourceName, idealState);
+</code></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>
+ </section>
+ <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><code>@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;
+}
+</code></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><code>{
+ "lock_0": ["Participant_A"],
+ "lock_1": ["Participant_A"],
+ "lock_2": ["Participant_A"],
+ "lock_3": ["Participant_A"],
+ "lock_4": ["Participant_A"],
+ "lock_5": ["Participant_A"],
+}
+</code></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><code>{
+ "lock_0": ["Participant_A"],
+ "lock_1": ["Participant_B"],
+ "lock_2": ["Participant_A"],
+ "lock_3": ["Participant_B"],
+ "lock_4": ["Participant_A"],
+ "lock_5": ["Participant_B"],
+}
+</code></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><code>{
+ "lock_0": ["Participant_A"],
+ "lock_1": ["Participant_B"],
+ "lock_2": ["Participant_C"],
+ "lock_3": ["Participant_A"],
+ "lock_4": ["Participant_B"],
+ "lock_5": ["Participant_C"],
+}
+</code></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><code>{
+ "lock_0": ["Participant_A"],
+ "lock_1": ["Participant_C"],
+ "lock_2": ["Participant_A"],
+ "lock_3": ["Participant_C"],
+ "lock_4": ["Participant_A"],
+ "lock_5": ["Participant_C"],
+}
+</code></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><code>{
+ "lock_0": ["Participant_A"],
+ "lock_1": ["Participant_B"],
+ "lock_2": ["Participant_C"],
+ "lock_3": ["Participant_A"],
+ "lock_4": ["Participant_B"],
+ "lock_5": ["Participant_C"],
+}
+</code></pre>
+ </div>
+ <p>The rebalancer was invoked once again and the resulting IdealState preference lists reflect the steady state.</p>
+ </section>
+ <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. In 0.7.0, Helix will persist the result of running the rebalancer.</li>
+ </ul>
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Added: helix/site-content/1.0.5-docs/tutorial_yaml.html
URL: http://svn.apache.org/viewvc/helix/site-content/1.0.5-docs/tutorial_yaml.html?rev=1900823&view=auto
==============================================================================
--- helix/site-content/1.0.5-docs/tutorial_yaml.html (added)
+++ helix/site-content/1.0.5-docs/tutorial_yaml.html Thu May 12 05:29:03 2022
@@ -0,0 +1,331 @@
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+ <li>Tutorial - YAML Cluster Setup</li>
+ <li class="publishDate version-date pull-right">Last Published: 2022-05-11</li>
+ </ul>
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+
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+ <div class="body-content">
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+under the License.
+-->
+<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><code>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
+</code></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><code>helix/helix-core/target/helix-core/pkg/bin/YAMLClusterSetup.sh localhost:2199 lock-manager-config.yaml
+</code></pre>
+ </div>
+ <p>or with code:</p>
+ <div class="source">
+ <pre><code>YAMLClusterSetup setup = new YAMLClusterSetup(zkAddress);
+InputStream input =
+ Thread.currentThread().getContextClassLoader()
+ .getResourceAsStream("lock-manager-config.yaml");
+YAMLClusterSetup.YAMLClusterConfig config = setup.setupCluster(input);
+</code></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>
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\ No newline at end of file
Modified: helix/site-content/Architecture.html
URL: http://svn.apache.org/viewvc/helix/site-content/Architecture.html?rev=1900823&r1=1900822&r2=1900823&view=diff
==============================================================================
--- helix/site-content/Architecture.html (original)
+++ helix/site-content/Architecture.html Thu May 12 05:29:03 2022
@@ -1,13 +1,8 @@
<!DOCTYPE html>
-
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@@ -147,18 +121,17 @@
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- <a href="./" id="bannerLeft"><img src="images/helix-logo.jpg" alt='"''"' /></a>
- <p class="lead">A cluster management framework for partitioned and replicated distributed resources</p>
+ <a href="" id="bannerLeft"><img src="" alt='"''"' /></a>
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+ <li><a href="" title="Apache Helix">Apache Helix</a></li>
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<li>Architecture</li>
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@@ -169,257 +142,7 @@
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-<section>
- <div class="page-header">
- <h2 id="Architecture">Architecture</h2>
- </div>
- <p>Helix aims to provide the following abilities to a distributed system:</p>
- <ul>
- <li>Automatic management of a cluster hosting partitioned, replicated resources</li>
- <li>Soft and hard failure detection and handling</li>
- <li>Automatic load balancing via smart placement of resources on servers (nodes) based on server capacity and resource profile (size of partitions, access patterns, etc)</li>
- <li>Centralized config management and self discovery, eliminating the need to modify config on each node</li>
- <li>Fault tolerance and optimized rebalancing during cluster expansion</li>
- <li>Management of the entire operational lifecycle of a node. Add, start, stop, enable, and disable without downtime</li>
- <li>Monitoring of cluster health and alerting on SLA violations</li>
- <li>A service discovery mechanism to route requests</li>
- </ul>
- <p>To build such a system, we need a mechanism to coordinate between different nodes and other components in the system. This mechanism can be achieved with software that reacts to any change in the cluster and comes up with a set of tasks needed to bring the cluster to a stable state. The set of tasks will be assigned to one or more nodes in the cluster. Helix serves this purpose of managing the various components in the cluster.</p>
- <p><img src="images/system.png" alt="Helix Design"></p>
-</section>
-<section>
- <h2 id="Distributed_System_Components">Distributed System Components</h2>
- <p>In general any distributed system cluster will have the following components and properties:</p>
- <ul>
- <li>A set of nodes also referred to as <b>instances</b></li>
- <li>A set of <b>resources</b> which can be databases, lucene indexes or tasks</li>
- <li>Subdivisions of each resource into one or more <b>partitions</b></li>
- <li>Copies of each resource called <b>replicas</b></li>
- <li>The <b>state</b> of each replica, e.g. Master, Slave, Leader, Standby, Online, Offline, etc.</li>
- </ul>
-</section>
-<section>
- <h2 id="Roles">Roles</h2>
- <p><img src="images/HELIX-components.png" alt="Helix Design"></p>
- <p>Not all nodes in a distributed system will perform similar functionalities. For example, a few nodes might be serving requests and a few nodes might be sending requests, and some other nodes might be controlling the nodes in the cluster. Thus, Helix categorizes nodes by their specific roles in the system.</p>
- <p>Helix divides nodes into 3 logical components based on their responsibilities:</p>
- <ol style="list-style-type: decimal">
- <li><b>Participant</b>: The nodes that actually host the distributed resources</li>
- <li><b>Spectator</b>: The nodes that simply observe the current state of each Participant and routes requests accordingly. Routers, for example, need to know the instance on which a partition is hosted and its state in order to route the request to the appropriate endpoint</li>
- <li><b>Controller</b>: The node that observes and controls the Participant nodes. It is responsible for coordinating all transitions in the cluster and ensuring that state constraints are satisfied while maintaining cluster stability</li>
- </ol>
- <p>These are simply logical components and can be deployed according to system requirements. For example, the Controller:</p>
- <ol style="list-style-type: decimal">
- <li>can be deployed as a separate service</li>
- <li>can be deployed along with a Participant but only one Controller will be active at any given time.</li>
- </ol>
- <p>Both have pros and cons, which will be discussed later and one can chose the mode of deployment as per system needs.</p>
-</section>
-<section>
- <h2 id="Cluster_State_Metadata_Store">Cluster State Metadata Store</h2>
- <p>We need a distributed store to maintain the state of the cluster and a notification system to notify if there is any change in the cluster state. Helix uses <a class="externalLink" href="http://zookeeper.apache.org">Apache ZooKeeper</a> to achieve this functionality.</p>
- <p>Zookeeper provides:</p>
- <ul>
- <li>A way to represent PERSISTENT state which remains until its deleted</li>
- <li>A way to represent TRANSIENT/EPHEMERAL state which vanishes when the process that created the state dies</li>
- <li>A notification mechanism when there is a change in PERSISTENT and EPHEMERAL state</li>
- </ul>
- <p>The namespace provided by ZooKeeper is much like that of a standard file system. A name is a sequence of path elements separated by a slash (/). Every node (ZNode) in ZooKeeper's namespace is identified by a path.</p>
- <p>More info on Zookeeper can be found at <a class="externalLink" href="http://zookeeper.apache.org">http://zookeeper.apache.org</a></p>
-</section>
-<section>
- <h2 id="State_Machine_and_Constraints">State Machine and Constraints</h2>
- <p>Even though the concepts of Resources, Partitions, and Replicas are common to most distributed systems, one thing that differentiates one distributed system from another is the way each partition is assigned a state and the constraints on each state.</p>
- <p>For example:</p>
- <ol style="list-style-type: decimal">
- <li>If a system is serving read-only data then all of a partition's replicas are equivalent and they can either be ONLINE or OFFLINE.</li>
- <li>If a system takes <i>both</i> reads and writes but must ensure that writes go through only one partition, the states will be MASTER, SLAVE, and OFFLINE. Writes go through the MASTER and replicate to the SLAVEs. Optionally, reads can go through SLAVEs.</li>
- </ol>
- <p>Apart from defining the state for each partition, the transition path between states can be application specific. For example, in order to become MASTER it might be a requirement to first become a SLAVE. This ensures that if the SLAVE does not have the data as part of OFFLINE-SLAVE transition it can bootstrap data from other nodes in the system.</p>
- <p>Helix provides a way to configure an application-specific state machine along with constraints on each state. Along with constraints on STATE, Helix also provides a way to specify constraints on transitions. (More on this later.)</p>
- <div class="source">
- <pre><code> OFFLINE | SLAVE | MASTER
- _____________________________
- | | | |
-OFFLINE | N/A | SLAVE | SLAVE |
- |__________|________|_________|
- | | | |
-SLAVE | OFFLINE | N/A | MASTER |
- |__________|________|_________|
- | | | |
-MASTER | SLAVE | SLAVE | N/A |
- |__________|________|_________|
-
-</code></pre>
- </div>
- <p><img src="images/statemachine.png" alt="Helix Design"></p>
-</section>
-<section>
- <h2 id="Concepts">Concepts</h2>
- <p>The following terminologies are used in Helix to model resources following a state machine.</p>
- <ul>
- <li><b>IdealState</b>: The state in which we need the cluster to be in if all nodes are up and running. In other words, all state constraints are satisfied.</li>
- <li><b>CurrentState</b>: The actual current state of each node in the cluster</li>
- <li><b>ExternalView</b>: The combined view of the CurrentState of all nodes.</li>
- </ul>
- <p>The goal of Helix is always to make the CurrentState (and by extension, the ExternalView) of the system same as the IdealState. Some scenarios where this may not be true are:</p>
- <ul>
- <li>Some or all nodes are down</li>
- <li>One or more nodes fail</li>
- <li>New nodes are added and the partitions need to be reassigned</li>
- </ul>
- <section>
- <h3 id="IdealState">IdealState</h3>
- <p>Helix lets the application define the IdealState for each resource. It consists of:</p>
- <ul>
- <li>A list of partitions, e.g. 64</li>
- <li>Number of replicas for each partition, e.g. 3</li>
- <li>The assigned node and state for each replica</li>
- </ul>
- <p>Example:</p>
- <ul>
- <li>Partition-1, replica-1: Master, Node-1</li>
- <li>Partition-1, replica-2: Slave, Node-2</li>
- <li>Partition-1, replica-3: Slave, Node-3</li>
- <li>â¦..</li>
- <li>â¦..</li>
- <li>Partition-p, replica-r: Slave, Node-n</li>
- </ul>
- <p>Helix comes with various algorithms to automatically assign the partitions to nodes. The default algorithm minimizes the number of shuffles that happen when new nodes are added to the system.</p>
- </section>
- <section>
- <h3 id="CurrentState">CurrentState</h3>
- <p>Every participant in the cluster hosts one or more partitions of a resource. Each of the partitions has a state associated with it.</p>
- <p>Example Node-1</p>
- <ul>
- <li>Partition-1, Master</li>
- <li>Partition-2, Slave</li>
- <li>â¦.</li>
- <li>â¦.</li>
- <li>Partition-p, Slave</li>
- </ul>
- </section>
- <section>
- <h3 id="ExternalView">ExternalView</h3>
- <p>External clients needs to know the state of each partition in the cluster and the Node hosting that partition. Helix provides one view of the system to Spectators as the ExternalView. The ExternalView is simply an aggregate of all node CurrentStates.</p>
- <ul>
- <li>Partition-1, replica-1, Master, Node-1</li>
- <li>Partition-1, replica-2, Slave, Node-2</li>
- <li>Partition-1, replica-3, Slave, Node-3</li>
- <li>â¦..</li>
- <li>â¦..</li>
- <li>Partition-p, replica-3, Slave, Node-n</li>
- </ul>
- </section>
-</section>
-<section>
- <h2 id="Process_Workflow">Process Workflow</h2>
- <p>Mode of operation in a cluster</p>
- <p>A node process can be one of the following:</p>
- <ul>
- <li>Participant: The process registers itself in the cluster and acts on the messages received in its queue and updates the current state. Example: a storage node in a distributed database</li>
- <li>Spectator: The process is simply interested in the changes in the ExternalView.</li>
- <li>Controller: This process actively controls the cluster by reacting to changes in cluster state and sending state transition messages to Participants.</li>
- </ul>
- <section>
- <h3 id="Participant_Node_Process">Participant Node Process</h3>
- <ul>
- <li>When the Participant starts up, it registers itself under <b>LiveInstances</b></li>
- <li>After registering, it waits for new <b>messages</b> in the message queue</li>
- <li>When it receives a message, it will perform the required task as indicated in the message</li>
- <li>After the task is completed, depending on the task outcome it updates the CurrentState</li>
- </ul>
- </section>
- <section>
- <h3 id="Controller_Process">Controller Process</h3>
- <ul>
- <li>Watches IdealState</li>
- <li>Notified when a Participant goes down, comes up, is added, or is removed. Watches the ephemeral LiveInstance ZNode and the CurrentState of each Participant in the cluster</li>
- <li>Triggers appropriate state transitions by sending messages to Participants</li>
- </ul>
- </section>
- <section>
- <h3 id="Spectator_Process">Spectator Process</h3>
- <ul>
- <li>When the process starts, it asks the Helix agent to be notified of changes in ExternalView</li>
- <li>Whenever it receives a notification, it reads the ExternalView and performs required duties</li>
- </ul>
- </section>
- <section>
- <h3 id="Interaction_between_the_Controller.2C_Participant_and_Spectator">Interaction between the Controller, Participant and Spectator</h3>
- <p>The following picture shows how Controllers, Participants and Spectators interact with each other.</p>
- <p><img src="images/helix-architecture.png" alt="Helix Architecture"></p>
- </section>
-</section>
-<section>
- <h2 id="Core_Controller_Algorithm">Core Controller Algorithm</h2>
- <ul>
- <li>Get the IdealState and the CurrentState of active storage nodes from ZooKeeper</li>
- <li>Compute the delta between IdealState and CurrentState for each partition replica across all Participant nodes</li>
- <li>For each partition compute tasks based on the State Machine Table. It's possible to configure priority on the state Transition. For example, in case of MasterSlave:
- <ul>
- <li>Attempt mastership transfer if possible without violating constraints</li>
- <li>Partition addition</li>
- <li>Partition drop</li>
- </ul> </li>
- <li>Add the transition tasks in parallel if possible to the respective queue for each storage node (if the tasks added are mutually independent)</li>
- <li>If a transition task is dependent on another task being completed, do not add that task</li>
- <li>After any task is completed by a Participant, Controllers gets notified of the change and the algorithm is re-run until the CurrentState matches the IdealState.</li>
- </ul>
-</section>
-<section>
- <h2 id="Helix_ZNode_Layout">Helix ZNode Layout</h2>
- <p>Helix organizes ZNodes under the cluster name in multiple levels.</p>
- <p>The top level (under the cluster name) ZNodes are all Helix-defined and in upper case:</p>
- <ul>
- <li>PROPERTYSTORE: application property store</li>
- <li>STATEMODELDEFES: state model definitions</li>
- <li>INSTANCES: instance runtime information including current state and messages</li>
- <li>CONFIGS: configurations</li>
- <li>IDEALSTATES: ideal states</li>
- <li>EXTERNALVIEW: external views</li>
- <li>LIVEINSTANCES: live instances</li>
- <li>CONTROLLER: cluster controller runtime information</li>
- </ul>
- <p>Under INSTANCES, there are runtime ZNodes for each instance. An instance organizes ZNodes as follows:</p>
- <ul>
- <li>CURRENTSTATES
- <ul>
- <li>sessionId</li>
- <li>resourceName</li>
- </ul> </li>
- <li>ERRORS</li>
- <li>STATUSUPDATES</li>
- <li>MESSAGES</li>
- <li>HEALTHREPORT</li>
- </ul>
- <p>Under CONFIGS, there are different scopes of configurations:</p>
- <ul>
- <li>RESOURCE: contains resource scope configurations</li>
- <li>CLUSTER: contains cluster scope configurations</li>
- <li>PARTICIPANT: contains participant scope configurations</li>
- </ul>
- <p>The following image shows an example of the Helix ZNode layout for a cluster named âtest-clusterâ:</p>
- <p><img src="images/helix-znode-layout.png" alt="Helix znode layout"></p>
-</section>
+$bodyWithHeader
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@@ -432,102 +155,106 @@ MASTER | SLAVE | SLAVE | N/A |
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@@ -539,22 +266,18 @@ MASTER | SLAVE | SLAVE | N/A |
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