This guide co-authored by Eric Hubert, Ruwan Linton, Asanka Abeysinghe and Afkham Azeez, briefly outlines the need for clustering in an enterprise application deployment, summarizes some clustering basics, and describes how to configure clustering support available in the Apache Synapse light-weight ESB, as well as the commercially supported WSO2 ESB based on Apache Synapse. It also gives some insights into implementing clustering, targeted at users wishing to deepen their knowledge and help them get started on implementing stateful custom mediators.