A foreign bus is a property of a service integration bus, and represents a bus in another cell (or within the same cell) or a WebSphere MQ network, with which the service integration bus can exchange messages.
Messages are routed to a foreign bus either directly through a link between the two buses, or indirectly through one or more intermediate buses. Regardless of where buses exist, of one is foreign to the other, it requires a foreign bus link. For example, if a cell contains more than one bus, each of the buses within it are regarded as foreign to one another. Before you can create and configure a link, you must create a foreign bus.
In Figure 1, buses 1 and 2 are linked directly, while buses 1 and 3 are linked indirectly through bus 2.
You can define an explicit destination on a foreign bus, to which an application can send messages. You can also configure default properties to be used for messages sent to destinations on a foreign bus when there is no explicit foreign destination definition, and the application does not explicitly provide values for the properties. An application cannot receive messages from a foreign destination; it can only consume messages from a destination on the bus to which it is connected.
An application subscribing to a local topic space can receive messages published to a topic on a foreign bus. To allow publish/subscribe messaging between buses, topic space names on a local bus must be mapped to topic space names on a foreign bus.
A topic space mapping allows subscribers on the local topic space to receive messages published in the foreign topic space. For publications to flow from the local topic space into the foreign bus, an equivalent topic space mapping is required by the foreign bus.
Topic space mapping is administered through the routing properties for a foreign bus. Topic space names for the local bus are mapped to topic space names defined on the foreign bus. It will be common for the two names to match. Note that mapping two topic spaces implies that the topics within them are "the same".