I have an use case where, I read in the newline json elements stored in google cloud storage and start processing each json. While processing each json, I have to call an external API for doing de-duplication whether that json element was discovered previously. I'm doing a `ParDo` with a `DoFn` on each json. 

I haven't seen any online tutorial saying how to call an external API endpoint from apache beam `DoFn` Dataflow. 

I'm using `JAVA` SDK of Beam. Some of the tutorial I studied explained that using `startBundle` and `FinishBundle` but I'm not clear on how to use it