Communication between IT systems occurs through interfaces. Therefore, an interface is the basic element of system integration. Through an interface, an IT system (the sender) sends information to another IT system (the receiver). A particular IT system can be both a sender and a receiver.
IT systems that are connected via interfaces exchange messages. A message is sent by the sender IT system, with the expectation that receiving the message—immediately or later—initiates an activity in the receiver IT system. For the receiver IT system, each message received constitutes an event to which it responds.
For instance, if an invoice is sent in an electronic format, this event is an invoice receipt for the receiver of the invoice. After the receipt of the invoice, the receiver has a certain time frame to pay the bill. The receiver IT system has to confirm the receipt of the invoice and possibly activate another IT system, for example, an accounting...