Apache Tomcat 8 Configuration Reference (8.0.37) - The Cluster Channel object
Author: Filip Hanik

## The Cluster Channel object

### Table of Contents

- Introduction
- Nested Components
- Attributes
  1. Common Attributes
  2. org.apache.catalina.tribes.group.GroupChannel Attributes

### Introduction

The cluster channel is the main component of a small framework we've nicknamed Apache Tribes.
The channel manages a set of sub components and together they create a group communication framework.
This framework is then used internally by the components that need to send messages between different Tomcat instances.
A few examples of these components would be the SimpleTcpCluster that does the messaging for the DeltaManager, or the BackupManager that uses a different replication strategy. The ReplicatedContext object does also use the channel object to communicate context attribute changes.

### Nested Components

Channel/Membership:
The Membership component is responsible for auto discovering new nodes in the cluster and also to provide for notifications for any nodes that have not responded with a heartbeat. The default implementation uses multicast.
In the membership component you configure how your nodes, aka. members, are to be discovered and/or divided up. You can always find out more about Apache Tribes

Channel/Sender:
The Sender component manages all outbound connections and data messages that are sent over the network from one node to another. This component allows messages to be sent in parallel. The default implementation uses TCP client sockets, and socket tuning for outgoing messages are configured here.
You can always find out more about Apache Tribes

Channel/Sender/Transport:
The Transport component is the bottom IO layer for the sender component. The default implementation uses non-blocking TCP client sockets.
You can always find out more about Apache Tribes

Channel/Receiver:
The receiver component listens for messages from other nodes. Here you will configure the cluster thread pool, as it will dispatch incoming messages to a thread pool for faster processing. The default implementation uses non-blocking TCP server sockets.
You can always find out more about Apache Tribes

Channel/Interceptor:
The channel will send messages through an interceptor stack. Because of this, you have the ability to customize the way messages are sent and received, and even how membership is handled.
You can always find out more about Apache Tribes

### Attributes

#### Common Attributes

Attribute | Description
--- | ---
`className` | The default value here is `org.apache.catalina.tribes.group.GroupChannel` and is currently the only implementation available.

#### org.apache.catalina.tribes.group.GroupChannel Attributes

Attribute | Description
--- | ---
`heartbeat` | Flag whether the channel manages its own heartbeat. If set to true, the channel start a local thread for the heart beat. If set this flag to false, you must set SimpleTcpCluster#heartbeatBackgroundEnabled to true. default value is true.
`heartbeatSleeptime` | If heartbeat == true, specifies the interval of heartbeat thread in milliseconds. The default is 5000 (5 seconds).
`optionCheck` | If set to true, the GroupChannel will check the option flags that each interceptor is using. Reports an error if two interceptor share the same flag. The default is false.

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