Data is transferred directly (simultaneously, synchronously) from program to program via CPI-C communication.

Both communications partners must be available at the same time. Because the central SAP system in a typical SAP installation does not run round the clock, but the linked systems are frequently in operation 24 hours a day, asynchronous data transfer is necessary.

Synchronous data transfer has the following disadvantages:

• Transfer is not possible is the partner system or the line is not available.
• A data backlog in the reciever system causes a data backlog in the sender system. Processing in the application is delayed.
• If a connection is broken, it may be necessary to perform a recovery in both systems.

With asynchronous, or buffered, data transfer, data is temporarily stored in a sequential queue.

Asynchronous transfer has the following advantages:

• Wait times in the sender system are avoided.
• A recovery is automatically performed in the sender system.
• Transfer need not be performed during online time. This avoids placing unnecessary load on the system and thus helps to reduce costs.

The Queue Application Programming Interface (Q-API) is an SAP interface for asynchronous data transfer.

Data is buffered sequentially and processed immediately or later by an asynchronously running process.

One possible processing method is to send data to an external partner system via CPI-C. Data units that belong together can be stored in accordance with transaction and sent to a communications partner.

Data is buffered in queues before it is transferred to the target system.

In an R/3 System, the queues are stored in a relational database. All R/3 database systems are supported.

The ABAP interface is implemented as follows:

The C interface to R/3 is implemented as a library which uses R/3 function modules through a Remote Function Call (RFC).

The following topics explain the basic terms and concepts of the SAP interface Q-API.