Numaflow provides some built-in functions that can be used directly.
cat builtin UDF does nothing but return the same messages it receives, it is very useful for debugging and testing.
spec: vertices: - name: cat-vertex udf: builtin: name: cat
filter built-in UDF does filter the message based on expression.
payload keyword represents message object.
see documentation for expression here
spec: vertices: - name: filter-vertex udf: builtin: name: filter kwargs: expression: int(object(payload).id) > 100