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# Helix Tutorial

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.

Convention: we first cover the _basic_ approach, which is the easiest to implement.  Then, we'll describe _advanced_ options, which give you more control over the system behavior, but require you to write more code.


### Prerequisites

1. Read [Concepts/Terminology](./Concepts.html) and [Architecture](./Architecture.html)
2. Read the [Quickstart guide](./Quickstart.html) to learn how Helix models and manages a cluster
3. Install Helix source.  See: [Quickstart](./Quickstart.html) for the steps.

### Tutorial Outline

1. [Participant](./tutorial_participant.html)
2. [Spectator](./tutorial_spectator.html)
3. [Controller](./tutorial_controller.html)
4. [Rebalancing Algorithms](./tutorial_rebalance.html)
5. [State Machines](./tutorial_state.html)
6. [Messaging](./tutorial_messaging.html)
7. [Customized health check](./tutorial_health.html)
8. [Throttling](./tutorial_throttling.html)
9. [Application Property Store](./tutorial_propstore.html)
10. [Admin Interface](./tutorial_admin.html)

### Preliminaries

First, we need to set up the system.  Let\'s walk through the steps in building a distributed system using Helix.

### Start Zookeeper

This starts a zookeeper in standalone mode. For production deployment, see [Apache Zookeeper](http://zookeeper.apache.org) for instructions.

```
    ./start-standalone-zookeeper.sh 2199 &
```

### Create a cluster

Creating a cluster will define the cluster in appropriate znodes on zookeeper.   

Using the java API:

```
    // 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);
```

OR

Using the command-line interface:

```
    ./helix-admin.sh --zkSvr localhost:2199 --addCluster helix-demo 
```


### Configure the nodes of the cluster

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.

```
    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);
      
    }
```

### Configure the resource

A _resource_ 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 _partitions_.


#### Define the _state model_ and _constraints_

For scalability and fault tolerance, each partition can have one or more replicas. 
The _state model_ 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 _constraints_.
For example, in a search system, one might need more than one node serving the same index to handle the load.

The allowed states: 

* MASTER
* SLAVE
* OFFLINE

The allowed transitions: 

* OFFLINE to SLAVE
* SLAVE to OFFLINE
* SLAVE to MASTER
* MASTER to SLAVE

The constraints:

* no more than 1 MASTER per partition
* the rest of the replicas should be slaves

The following snippet shows how to declare the _state model_ and _constraints_ for the MASTER-SLAVE model.

```

    StateModelDefinition.Builder builder = new StateModelDefinition.Builder(STATE_MODEL_NAME);

    // Add states and their rank to indicate priority. A lower rank corresponds to a higher priority
    builder.addState(MASTER, 1);
    builder.addState(SLAVE, 2);
    builder.addState(OFFLINE);

    // Set the initial state when the node starts
    builder.initialState(OFFLINE);

    // Add transitions between the states.
    builder.addTransition(OFFLINE, SLAVE);
    builder.addTransition(SLAVE, OFFLINE);
    builder.addTransition(SLAVE, MASTER);
    builder.addTransition(MASTER, SLAVE);

    // set constraints on states.

    // static constraint: upper bound of 1 MASTER
    builder.upperBound(MASTER, 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(SLAVE, "R");

    StateModelDefinition statemodelDefinition = builder.build();
    admin.addStateModelDef(CLUSTER_NAME, STATE_MODEL_NAME, myStateModel);
```

#### Assigning partitions to nodes

The final goal of Helix is to ensure that the constraints on the state model are satisfied. 
Helix does this by assigning a STATE to a partition (such as MASTER, SLAVE), and placing it on a particular node.

There are 3 assignment modes Helix can operate on

* AUTO_REBALANCE: Helix decides the placement and state of a partition.
* AUTO: Application decides the placement but Helix decides the state of a partition.
* CUSTOM: Application controls the placement and state of a partition.

For more info on the assignment modes, see [Rebalancing Algorithms](./tutorial_rebalance.html) of the tutorial.

```
    String RESOURCE_NAME = "MyDB";
    int NUM_PARTITIONS = 6;
    STATE_MODEL_NAME = "MasterSlave";
    String MODE = "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);
```

