Part of the application layer in AS ABAP. The application layer can be distributed across multiple application servers, usually instantiated on different computers whose operating systems do not need to be identical. However, multiple application servers of one or more AS ABAPs may run on one computer. Application servers communicate with the presentation layer, the database layer, and with each other using the message server. The most important components of an application server are its work processes, the number and type of which are determined when the AS ABAP is started. An ABAP program is executed by a suitable application server work process. The memory areas of an application server, which can be accessed in the ABAP programs, are the shared memory, the user memory, the ABAP memory, and the roll area in the internal session.