== Clustering

To enable clustering you need to have

* at least two appNG Nodes
* a load balancer that is able to distribute requests between several nodes, for example http://www.haproxy.org/[HAProxy^]
* set the {manager-guide}#platform-properties[platform property^]  `messagingEnabled` to `true`
* configure the details for the chosen message exchange

There are currently three techniques for appNG to exchange messages between nodes in a cluster:

* https://en.wikipedia.org/wiki/Multicast[Multicast^]
* https://hazelcast.org[Hazelcast^]
* https://redis.io[Redis^]
* https://www.rabbitmq.com[RabbitMQ^]

=== Using Multicast
The following platform-properties need to be configured when using multicast:

* set `messagingReceiver` to `org.appng.core.controller.messaging.MulticastReceiver` (this is the default)
* set `messagingGroupAddress` to an appropriate value (default: `224.2.2.4`)
* set `messagingGroupPort` to an appropriate value (default: `4000`)

=== Using Hazelcast
With https://hazelcast.org[Hazelcast^], a https://docs.hazelcast.org/docs/4.0/manual/html-single/index.html#reliable-topic[Reliable Topic^] is used to publish and subscribe cluster events.

The following platform-properties need to be configured:

* set `messagingReceiver` to `org.appng.core.controller.messaging.HazelCastReceiver`
* `hazelcastTopicName`: The name of the topic (default: `appng-messaging`)

=== Using RabbitMQ
With https://www.rabbitmq.com[RabbitMQ^], appNG uses a queue based publish/subscribe mechanism for cluster communication.
The following platform-properties need to be configured:

* set `messagingReceiver` to `org.appng.core.controller.messaging.RabbitMQReceiver`
* `rabbitMQAdresses`: A comma separated list of <host>:<port> for RabbitMQ server(s) (default: `localhost:5672`): 
* `rabbitMQUser`: Username (default: `guest`)
* `rabbitMQPassword`: Password (default: `guest`)
* `rabbitMQExchange`: Name of the exchange where the receiver binds its messaging queue on. +
Be aware that this name must be different among different clusters using the same RabbitMQ server (default: `appng-messaging`).
* `rabbitMQAutoDeleteQueue`: If the queue to create should be marked as auto-delete (default: `true`).
* `rabbitMQDurableQueue`: If the queue to create should be marked as durable (default: `false`).
* `rabbitMQExclusiveQueue`: If the queue to create should be marked as exclusive (default: `true`).

=== Using Redis
With https://redis.io[Redis^], cluster communication is based on a publish/subscribe mechanism that works on a messaging channel.
The following platform-properties need to be configured:

* set `messagingReceiver` to `org.appng.core.controller.messaging.JedisReceiver`
* `redisMessagingHost`: Host of the Redis server (default: `localhost`).
* `redisMessagingPort`: Port of the Redis server (default: `6379`).
* `redisMessagingPassword`: Password of the Redis server (no default).
* `redisMessagingTimeout`: Timeout is optional. If not defined, Redis default is used (no default).
* `redisMessagingChannel`: Channel where all cluster nodes should publish and subscribe. +
Be aware that this name must be different among different clusters using the same Redis server (default: `appng-messaging`).