Configuring Cluster Coordination

Multiple WSO2 SP nodes can be configured to work together by configuring a cluster coordination strategy that is used in various deployments such as the Minimum HA Deployment and Fully Distributed Deployment. At present, cluster coordination is supported via an RDBMS instance using and RDBMS coordination strategy. Support for cluster coordination via a Zookeeper instance will be supported in the near future.

At any given time, there is a leader in an SP cluster that is arbitrarily selected among the members of the cluster. The RDBMS coordination strategy that is used for cluster coordination works on the concept of heartbeats where the members of the cluster periodically send heartbeat signals via the datasource to the leader of the cluster. If the leader node does not detect a pre configured consecutive number of heartbeats from a specific node, the relevant node is removed from the cluster. Simillarly, if the leader node fails to update its heartbeat, the cluster re-elects a new leader.

Prerequisites

In order to configure a cluster, the following prerequisites must be completed:

A minimum of two binary packs of WSO2 SP must be available.
A working RDBMS instance must be available to be shared among the nodes of the cluster.

Currently, we only support MySQL. Support for other databases will be added soon.

Configuring the Cluster with the RDBMS coordination strategy

To configure a cluster for several nodes, the cluster.config section of the <SP_HOME>/conf/<worker|manager>/deployment.yaml should be configured for all the nodes as follows:

Parameter	Purpose	Sample Values
`enabled`	Set this value to `true` to enable cluster coordination for the node.	`true/false`
`groupId`	The group ID is used to identify the cluster to which the node belongs. Nodes that belong to the same cluster must be configured with the same group ID.	`group-1`
`coordinationStrategyClass`	The clustering class to be used.	`org.wso2.carbon.cluster.coordinator.rdbms.RDBMSCoordinationStrategy`
`strategyConfig > datasource`	The shared datasource to be used in the cluster. The datasource specified must be properly configured in the `deployment.yaml` file. For detailed instructions to configure a datasource, see Configuring Datasources.	`WSO2_CARBON_DB`
`strategyConfig > heartbeatInterval`	This value defines the time interval in milliseconds between heartbeat pulses sent by nodes to indicate that they are still alive within the cluster.	`1000`
`strategyConfig > heartbeatMaxRetry`	The number of times the heartbeat pulse can be unavailable until a node is identified as unresponsive. If a node fails to send its heartbeat pulse to the leader of the cluster after a number of retries equal to the number specified here, that node is removed from the cluster.	`2`
`strategyConfig > eventPollingInterval`	The time interval in millseconds at which a node listens to identify the changes happening within the cluster. The changes may include a new node joining the cluster, a node being removed from the cluster and the coordinator changed event.	`1000`

Following is a sample segment of the configurations needed for RDBMS coordination in the deployment.yaml

Sample deployment.yaml segment

cluster.config:
  enabled: true
  groupId:  <GROUP ID>
  coordinationStrategyClass: org.wso2.carbon.cluster.coordinator.rdbms.RDBMSCoordinationStrategy
  strategyConfig:
    datasource: <DATASOURCE NAME>
    heartbeatInterval: 1000
    heartbeatMaxRetry: 2
    eventPollingInterval: 1000