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<head>
  <title>Tutorial - Controller</title>
</head>

## [Helix Tutorial](./Tutorial.html): Controller

Next, let\'s implement the controller.  This is the brain of the cluster.  Helix makes sure there is exactly one active controller running the cluster.

### Start the Helix Controller

It requires the following parameters:

* clusterId: A logical ID to represent the group of nodes
* controllerId: A logical ID of the process creating the controller instance. Generally this is host:port.
* zkConnectString: Connection string to Zookeeper. This is of the form host1:port1,host2:port2,host3:port3.

```
HelixConnection connection = new ZKHelixConnection(zkConnectString);
HelixController controller = connection.createController(clusterId, controllerId);
```

### Controller Code

The Controller needs to know about all changes in the cluster. Helix takes care of this with the default implementation.
If you need additional functionality, see GenericHelixController and ZKHelixController for how to configure the pipeline.

```
HelixConnection connection = new ZKHelixConnection(zkConnectString);
HelixController controller = connection.createController(clusterId, controllerId);
controller.start();
```
The snippet above shows how the controller is started. You can also start the controller using command line interface.

```
cd helix/helix-core/target/helix-core-pkg/bin
./run-helix-controller.sh --zkSvr <Zookeeper ServerAddress (Required)>  --cluster <Cluster name (Required)>
```

### Controller Deployment Modes

Helix provides multiple options to deploy the controller.

#### STANDALONE

The Controller can be started as a separate process to manage a cluster. This is the recommended approach. However, since one controller can be a single point of failure, multiple controller processes are required for reliability.  Even if multiple controllers are running, only one will be actively managing the cluster at any time and is decided by a leader-election process. If the leader fails, another leader will take over managing the cluster.

Even though we recommend this method of deployment, it has the drawback of having to manage an additional service for each cluster. See Controller As a Service option.

#### EMBEDDED

If setting up a separate controller process is not viable, then it is possible to embed the controller as a library in each of the participants.

#### CONTROLLER AS A SERVICE

One of the cool features we added in Helix is to use a set of controllers to manage a large number of clusters.

For example if you have X clusters to be managed, instead of deploying X*3 (3 controllers for fault tolerance) controllers for each cluster, one can deploy just 3 controllers.  Each controller can manage X/3 clusters.  If any controller fails, the remaining two will manage X/2 clusters.


