Providing High Availability

High Availability (HA) is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period. In a production Stratos deployment it is recommended to include HA features for all the software products used to make the PaaS cloud available to its highest possible level. The following section describes how each WSO2 Private PaaS product could be configured to provide HA:

1. Stratos Core (Stratos Manager, Auto-scaler and Cloud Controller)
   Stratos core products use Carbon registry for managing their states. Carbon Registry is built into each Stratos core product and could be configured with a Relational Database Management System (RDBMS) for data persistence. Therefore, HA could be enabled for Stratos core products by enabling HA for the connected RDBMS and running multiple instances of each product in Master-Slave mode using Linux-HA. Currently, none of Stratos core products are supported to be run in Master-Master mode. The default RDBMS used for Carbon Registry is MySQL. For more information, see setting up a MySQL cluster for HA.
2. Message Broker (ActiveMQ)
   ActiveMQ supports HA in three different ways:
1. Shared File System Master Slave
2. JDBC Master Slave
3. Replicated Level DB Store
3. Complex Event Processor (CEP)
   There are two methods for enabling HA in CEP:
   1. Distributed Cache Mode Deployment
   2. Persistence Mode Deployment
4. Business Activity Monitor (BAM)
   There are two methods for enabling HA in BAM:
   1. Partially Distributed
   2. Fully Distributed
5. Stratos Load Balancer
   Stratos Load Balancer uses Apache Synapse for implementing session affinity and currently Synapse does not support distributed session management. Therefore, Stratos Load Balancer could be run in a distributed environment, but session affinity will not be supported.