The interface offered to the application programmer by the transaction processing system consists of the following procedure calls. Whenever an application invokes one of these operations, it is sent a response message by the system.

• A requestor can CALL an event by specifying its event code and providing the input parameters needed for the event to execute. When the call is made, the system forwards it to a servers that implement its related transactions. The system response message contains the unique event ID that denotes the running event or an error code if the request failed.
• A requestor can WAIT on the result of a particular event, specified by its event ID. That is, the requestor can tell the system that it wants to be sent the outcome of the event. The system response is a message that contains the outcome of the event.
• A server tells the system that it is ready to receive a new transaction by using the GET operation. The system response is a copy of a request for an event that includes one of the servers transactions.
• The END operation is used by the server to return the outcome of a transaction it is executing. The system response is the complete message of the two-phase commit discussed above.

A GET_RESPONSE function is provided to be used by applications to obtain any system responses intended for them. Calling this function blocks the process until a message is available.