The first version of RFC is synchronous RFC (sRFC). This type of RFC executes the function call based on synchronous communication, which means that the systems involved must both be available at the time the call is made.
Transactional RFC (tRFC)
Transactional RFC (tRFC, also originally known as asynchronous RFC) is an asynchronous communication method that executes the called function module in the RFC server only once. The remote system need not be available at the time when the RFC client program is executing a tRFC. The tRFC component stores the called RFC function, together with the corresponding data, in the SAP (Systems Applications and Products) database under a unique transaction ID (TID).
Queued RFC (qRFC)
To guarantee that multiple LUWs are processed in the order specified by the application, tRFC can be serialized using queues (inbound and outbound queues). This type of RFC is called queued RFC (qRFC).
qRFC is therefore an extension of tRFC. It transfers an LUW (transaction) only if it has no predecessors (in reference to the sequence defined in different application programs) in the participating queues.
Implementation of qRFC is recommended if you want to guarantee that several transactions are processed in a predefined order.
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