When all tasks associated with an OpenAPI function are completed, OpenAPI notifies the application by means of a callback function. When the callback function is invoked, OpenAPI sends return status and other information to the application in the delayed output parameters. This “other” information is needed by the application to make subsequent function calls.

Closure is the means for an application to pass any information it wishes to the callback function.

The application creates the closure parameter and passes it as an input parameter to the parameter block. OpenAPI does not care about the contents of the closure parameter; it simply passes it to the callback function when the function completes.

An asynchronous OpenAPI function always requests a callback function to notify the application when the OpenAPI function tasks are completed. If the application does not provide a callback function, OpenAPI assumes that the application is polling for the function completion.