Apache Tomcat 9 Configuration Reference (9.0.119) - The Cluster object

## The Cluster object

### Table of Contents

- Introduction
- Security
- Engine vs Host placement
- Context Attribute Replication
- Nested Components
- Attributes
  1. SimpleTcpCluster Attributes

### Introduction

The tomcat cluster implementation provides session replication, context attribute replication and cluster wide WAR file deployment. While the `Cluster` configuration is fairly complex, the default configuration will work for most people out of the box.

The Tomcat Cluster implementation is very extensible, and hence we have exposed a myriad of options, making the configuration seem like a lot, but don't lose faith, instead you have a tremendous control over what is going on.

### Security

The cluster implementation is written on the basis that a secure, trusted network is used for all of the cluster related network traffic. It is not safe to run a cluster on a insecure, untrusted network.

There are many options for providing a secure, trusted network for use by a Tomcat cluster. These include:

- private LAN
- a Virtual Private Network (VPN)
- IPSEC

The EncryptInterceptor provides confidentiality and integrity protection but it does not protect against all risks associated with running a Tomcat cluster on an untrusted network, particularly DoS attacks.

### Engine vs Host placement

You can place the `<Cluster>` element inside either the `<Engine>` container or the `<Host>` container.
Placing it in the engine, means that you will support clustering in all virtual hosts of Tomcat, and share the messaging component. When you place the `<Cluster>` inside the `<Engine>` element, the cluster will append the host name of each session manager to the managers name so that two contexts with the same name but sitting inside two different hosts will be distinguishable.

### Context Attribute Replication

To configure context attribute replication, simply do this by swapping out the context implementation used for your application context.

    <Context className="org.apache.catalina.ha.context.ReplicatedContext"/>

This context extends the Tomcat `StandardContext` so all the options from the base implementation are valid.

### Nested Components

Manager:
The session manager element identifies what kind of session manager is used in this cluster implementation. This manager configuration is identical to the one you would use in a regular `<Context>` configuration.
The default value is the `org.apache.catalina.ha.session.DeltaManager` that is closely coupled with the `SimpleTcpCluster` implementation. Other managers like the `org.apache.catalina.ha.session.BackupManager` are/could be loosely coupled and don't rely on the `SimpleTcpCluster` for its data replication.

Channel:
The Channel and its sub components are all part of the IO layer for the cluster group, and is a module in it's own that we have nick named "Tribes"
Any configuring and tuning of the network layer, the messaging and the membership logic will be done in the channel and its nested components. You can always find out more about Apache Tribes

Valve:
The Tomcat Cluster implementation uses `Tomcat Valves` to track when requests enter and exit the servlet container. It uses these valves to be able to make intelligent decisions on when to replicate data, which is always at the end of a request.

Deployer:
The Deployer component is the Tomcat Farm Deployer. It allows you to deploy and undeploy applications cluster wide.

ClusterListener:
ClusterListener's are used to track messages sent and received using the `SimpleTcpCluster`. If you wish to track messages, you can add a listener here, or you can add a valve to the channel object.

### Attributes

#### SimpleTcpCluster Attributes

Attribute | Description
--- | ---
`className` | The main cluster class, currently only one is available, `org.apache.catalina.ha.tcp.SimpleTcpCluster`
`channelSendOptions` | The Tribes channel send options. This option is used for all messages sent through the SimpleTcpCluster. The value may be specified either as an integer (add values to use multiple flags) or using a comma-separate list of the human-readable option names. Human-readable names are translated to their integer values on startup. The different values offer a tradeoff between throughput on the sending node and the reliability of replication should the sending or receiving node(s) fail. The supported values are: `byte_message`, `byte` or `1` The message is a pure byte message and no marshaling or unmarshalling will be performed `use_ack` or `2` The message is sent and an ACK is received when the message has been received by the recipient. If no ack is received, the message is not considered successful. `synchronized_ack`, `sync` or `4` Has no effect unless `use_ack` is set. The message is sent and an ACK is received when the message has been received and processed by the recipient. If no ack is received, the message is not considered successful. `asynchronous`, `async` or `8` The message will be placed on a queue and sent by a separate thread. It is possible for update messages for a session to be processed by the receiving node in a different order to the order in which they were sent. `secure` or `16` Not implemented. Has no effect if used. `udp` or `32` The message will be sent using UDP instead of TCP. `multicast` or `64` Not implemented. Messages will not be sent if used. The default value is `8` (`async`).
`channelStartOptions` | Sets the start and stop flags for the <Channel> object used by the cluster. The default is `Channel.DEFAULT` which starts all the channel services, such as sender, receiver, membership sender and membership receiver. The following flags are available today:
    Channel.DEFAULT = Channel.SND_RX_SEQ (1) |
                      Channel.SND_TX_SEQ (2) |
                      Channel.MBR_RX_SEQ (4) |
                      Channel.MBR_TX_SEQ (8);
When using the static membership service `org.apache.catalina.tribes.membership.StaticMembershipService` you must ensure that this attribute is configured to use the default value.
`heartbeatBackgroundEnabled` | Flag whether invoke channel heartbeat at container background thread. Default value is false. Enable this flag don't forget to disable the channel heartbeat thread.
`notifyLifecycleListenerOnFailure` | Flag whether notify LifecycleListeners if all ClusterListener couldn't accept channel message. Default value is false.