When you create a WebSphere ESB stand-alone profile or a deployment manager profile, you get an enterprise service bus based on two service integration buses. These buses, with names starting SCA, are for you to deploy SCA modules.
The SCA.SYSTEM bus provides a scope within which resources, such as queue destinations, are configured for mediation modules and interaction endpoints. The bus enables message routing between endpoints with specific quality of interaction service and can temporarily persist messages if required. You can configure a variety of quality of service from secure, assured delivery (where messages are guaranteed not to get lost and are transported securely) to best-effort (where messages might get lost in case of a system failure). Endpoint implementers choose their desired quality of service by declaring the annotations on SCA module exports and imports. If a quality of service is not specified, WebSphere ESB applies its defaults.
In a stand-alone profile, there is one bus member that provides the one messaging engine in a bus, a topology that is adequate for some applications.
A single messaging engine might not be adequate if the number of client connections becomes excessive, if the rate of message throughput cannot be sustained by the one messaging engine, or if the size of messages has a detrimental affect on the message buffers used by the messaging engine.
To add more than one messaging engine to a service integration bus, you need to use a profile for a managed node in a deployment manager cell.