Apache Tomcat 9 Configuration Reference (9.0.83) - The Cluster Membership object
Author: Filip Hanik

## The Cluster Membership object

### Table of Contents

- Introduction
- Default Implementation
- Attributes
  1. Common Attributes
  2. Multicast Attributes
  3. Static Membership Attributes
- Nested Components
  1. StaticMember Attributes
  2. Setting

### Introduction

The membership component in the Apache Tribes Channel is responsible for dynamic discovery of other members(nodes) in the cluster. There are currently two different membership service, the `org.apache.catalina.tribes.membership.McastService` and the `org.apache.catalina.tribes.membership.StaticMembershipService`. The `McastService` builds a multicast based membership service that sends UDP packets to multicast IP addresses. The `StaticMembershipService` builds a unicast based membership service that sends TCP packets to predefined member address.

### Default Implementation

The default implementation of the cluster group notification is built on top of multicast heartbeats sent using UDP packets to a multicast IP address. Cluster members are grouped together by using the same multicast address/port combination. Each member sends out a heartbeat with a given interval (`frequency`), and this heartbeat is used for dynamic discovery. In a similar fashion, if a heartbeat has not been received in a timeframe specified by `dropTime` ms. a member is considered suspect and the channel and any membership listener will be notified.

### Attributes

#### Common Attributes

Attribute | Description
--- | ---
`className` | The implementation of the membership component. Two implementations available, `org.apache.catalina.tribes.membership.McastService` and `org.apache.catalina.tribes.membership.StaticMembershipService`.

#### Multicast Attributes

Attribute | Description
--- | ---
`className` | The value is `org.apache.catalina.tribes.membership.McastService`. This implementation uses multicast heartbeats for member discovery.
`address` | The multicast address that the membership will broadcast its presence and listen for other heartbeats on. The default value is `228.0.0.4` Make sure your network is enabled for multicast traffic. The multicast address, in conjunction with the `port` is what creates a cluster group. To divide up your farm into several different group, or to split up QA from production, change the `port` or the `address` Previously known as mcastAddr.
`port` | The multicast port, the default value is `45564` The multicast port, in conjunction with the `address` is what creates a cluster group. To divide up your farm into several different group, or to split up QA from production, change the `port` or the `address`
`frequency` | The frequency in milliseconds in which heartbeats are sent out. The default value is `500` ms. In most cases the default value is sufficient. Changing this value, simply changes the interval in between heartbeats.
`dropTime` | The membership component will time out members and notify the Channel if a member fails to send a heartbeat within a give time. The default value is `3000` ms. This means, that if a heartbeat is not received from a member in that timeframe, the membership component will notify the cluster of this. On a high latency network you may wish to increase this value, to protect against false positives. Apache Tribes also provides a `TcpFailureDetector` that will verify a timeout using a TCP connection when a heartbeat timeout has occurred. This protects against false positives.
`bind` | Use this attribute if you wish to bind your multicast traffic to a specific network interface. By default, or when this attribute is unset, it tries to bind to `0.0.0.0` and sometimes on multihomed hosts this becomes a problem.
`ttl` | The time-to-live setting for the multicast heartbeats. This setting should be a value between 0 and 255. The default value is VM implementation specific.
`domain` | Apache Tribes has the ability to logically group members into domains, by using this domain attribute. The `org.apache.catalina.tribes.Member.getDomain()` method returns the value specified here.
`soTimeout` | The sending and receiving of heartbeats is done on a single thread, hence to avoid blocking this thread forever, you can control the `SO_TIMEOUT` value on this socket. If a value smaller or equal to 0 is presented, the code will default this value to frequency
`recoveryEnabled` | In case of a network failure, Java multicast socket don't transparently fail over, instead the socket will continuously throw IOException upon each receive request. When recoveryEnabled is set to true, this will close the multicast socket and open a new socket with the same properties as defined above. The default is `true`.
`recoveryCounter` | When `recoveryEnabled==true` this value indicates how many times an error has to occur before recovery is attempted. The default is `10`.
`recoverySleepTime` | When `recoveryEnabled==true` this value indicates how long time (in milliseconds) the system will sleep in between recovery attempts, until it recovers successfully. The default is `5000` (5 seconds).
`localLoopbackDisabled` | Membership uses multicast, it will call `java.net.MulticastSocket.setLoopbackMode(localLoopbackDisabled)`. When `localLoopbackDisabled==true` multicast messages will not reach other nodes on the same local machine. The default is `false`.

#### Static Membership Attributes

When using the static membership service you must ensure that the `channelStartOptions` attribute of the `Cluster` element is set to the default value of `15`.

Attribute | Description
--- | ---
`className` | The value is `org.apache.catalina.tribes.membership.StaticMembershipService`.
`connectTimeout` | Timeout for attempting a TCP connection to address of predefined static member. Default is `500` ms.
`expirationTime` | If members have failed to update their alive time within the given time, this membership will notify the memberDisappeared event to cluster. Default is `5000` ms.
`rpcTimeout` | Timeout for messages that used for member notification to/from othee nodes. Default is `3000` ms.
`useThread` | If set to true, this membership service will start a local thread for sending a ping message. if set to `false`, channel heartbeat will send a ping message. Default is `false`.
`pingInterval` | If `useThread` == `true`, defines the interval of sending a ping message. Default is `1000` ms.

### Nested Components

Static Membership Service allows nesting of a <LocalMember> and <Member> element.

#### StaticMember Attributes

LocalMember:
Static member that is the local member of the static cluster group.

Note: In Tomcat 9.0.17 and later, The setting of local member is not required. It is possible to set up a list of all cluster members including local member instead of setting this components.

Attribute | Description
--- | ---
`className` | Only one implementation available:`org.apache.catalina.tribes.membership.StaticMember`
`port` | There is no need to set. The value of this attribute inherits from the cluster receiver setting.
`securePort` | There is no need to set. The value of this attribute inherits from the cluster receiver setting.
`host` | There is no need to set. The value of this attribute inherits from the cluster receiver setting.
`domain` | The logical cluster domain for that this static member listens for cluster messages. Two different type of values are possible: 1. Regular string values like "staging-domain" or "tomcat-cluster" will be converted into bytes using ISO-8859-1 encoding. 2. byte array in string form, for example {216,123,12,3}
`uniqueId` | A universally uniqueId for this static member. The values must be 16 bytes in the following form: 1. byte array in string form, for example {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}

Member:
Static member that add to the static cluster group.

Attribute | Description
--- | ---
`className` | Only one implementation available:`org.apache.catalina.tribes.membership.StaticMember`
`port` | The port that this static member listens to for cluster messages
`securePort` | The secure port this static member listens to for encrypted cluster messages default value is `-1`, this value means the member is not listening on a secure port
`host` | The host (or network interface) that this static member listens for cluster messages. Three different type of values are possible: 1. IP address in the form of "216.123.1.23" 2. Hostnames like "tomcat01.mydomain.com" or "tomcat01" as long as they resolve correctly 3. byte array in string form, for example {216,123,12,3}
`domain` | The logical cluster domain for that this static member listens for cluster messages. Two different type of values are possible: 1. Regular string values like "staging-domain" or "tomcat-cluster" will be converted into bytes using ISO-8859-1 encoding. 2. byte array in string form, for example {216,123,12,3}
`uniqueId` | A universally uniqueId for this static member. The values must be 16 bytes in the following form: 1. byte array in string form, for example {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}

#### Setting

Before Tomcat 9.0.16

         <Membership className="org.apache.catalina.tribes.membership.StaticMembershipService">
           <LocalMember className="org.apache.catalina.tribes.membership.StaticMember"
                      uniqueId="{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}"/>
           <Member className="org.apache.catalina.tribes.membership.StaticMember"
                      port="4004"
                      host="tomcat02.mydomain.com"
                      uniqueId="{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,1}"/>
         </Membership>

Tomcat9.0.17 and later

         <Membership className="org.apache.catalina.tribes.membership.StaticMembershipService">
           <Member className="org.apache.catalina.tribes.membership.StaticMember"
                      port="4004"
                      host="tomcat01.mydomain.com"
                      uniqueId="{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}"/>
           <Member className="org.apache.catalina.tribes.membership.StaticMember"
                      port="4004"
                      host="tomcat02.mydomain.com"
                      uniqueId="{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,1}"/>
         </Membership>