This document talks about custom BAPI creation and generating the ALE interface for the Custom BAPI.
BAPIs are integrated into the ALE communication model. ALE communication is asynchronous and message-based. ALE communication is the preferred way to integrate distributed SAP Systems, for example, for distributing master data. Since Release 4.0 BAPIs are the standardized interfaces for ALE-supported communication. ALE services, such as asynchronous BAPI calls, distribution model, monitoring and error handling can be used for BAPIs. The IDoc types required for the ALE services can be generated from BAPIs.
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ALE - BAPI Intefaces with screenshots.