Both message interfaces and imported interfaces can be designed to send a request or to respond to a request. You may or may not receive a direct response message to this request. For message interfaces, you define this using the characteristics synchronous or asynchronous and outbound and inbound.

These characteristics determine the direction of an interface:

• An outbound interface sends a request message that is only used to make a communication partner aware of data sent, without waiting for a response message. In the case of the latter, we also refer to publishing interfaces.

• An inbound interface receives a request message that you reply to with a direct response message or whose data you can process in the system without a response.

A distinction is made between synchronous and asynchronous communication for both imported interfaces and message interfaces. You define the mode of communication when you define an interface:

• In synchronous communication, an response is expected from the receiver system on arrival of a query sent by the sender system. Once this message has been sent, no further messages can be sent until the answer to the query has arrived at the sender system.
• However, in asynchronous communication the response is not immediate. A sending process can send multiple messages to a receiver in a bundle and then continue executing the process.

The following graphic shows how the mode and direction of interfaces influence message exchange between interfaces: