YAPI is an acronym for Y-chart Application Programmer's Interface. The goal of YAPI is to enable reuse of stream processing applications in two ways namely composability and portability. Composability refers to the use of existing stream processing applications in the development of new stream processing applications. Portability refers to the use of stream processing applications on different architectures. To this end, YAPI separates the concerns of the application designer, who specifies the functionality of the system, and the system designer, who implements the functionality.