How to get the leader node of a WSO2 ESB Cluster
This post describes how to check whether the current node is the leader node of an ESB cluster. An example use-case of using this property is to write a cluster aware scheduled task in WSO2 ESB.
While writing a proxy service or a sequence, there might be an instance where you want only one ESB node to do a particular task. In that case, we can use the leader node in the cluster or try to get hold of a cluster-wide lock (using a DB based solution or Hazelcast). This blog post explains how to achieve this using Hazelcast leader node.
Hazelcast Leader node
WSO2 carbon server which WSO2 ESB is built upon uses Hazelcast as the cluster coordination implementation. In Hazelcast, there is a mechanism to reliably get the leader node of a cluster. Only one node in a cluster at a given time becomes the leader node. We can use this property of Hazelcast within WSO2 ESB using either a script mediator or a class mediator to get the leader node.
Retrieving leader with a Class mediator
Following is a Java code snippet to get the leader node of a Hazelcast cluster. We can use this within a class mediator to get the leader node of the ESB cluster.
You can follow this guide to write your own class mediator.
Retrieving leader with a Script Mediator
We can use the same Java code with some modifications within the script mediator as well. This option is much easier to implement and deploy in an ESB cluster.
Use case
One of the use cases of this property is to create a cluster aware scheduled task in WSO2 ESB.
Up to ESB 4.8.1 scheduled tasks cannot be controlled to be scheduled only in a single node at a time. The only option is to use pinned servers, but this has a single point of failure.
Even though the scheduled tasks deployed in the cluster injects a message to each proxy or sequence in the cluster, we can make only one proxy service to mediate the message by evaluating the leader node property as mentioned above.
Hope this is useful to someone.
Cheers!
Written by Asitha Nanayakkara