Hi Datta,
I recommend you to try to develop everything in flow language, as you need to deliver your solution ASAP.
As Jab said, if there is any built in service available for a particular functionality, you can go for a custom java code.
After finish the development, I usually do a performance test and try to identify what are the bottlenecks. Usually some services performance can be increased by replacing the flow service by a java code.
Another point that you must keep in mind is that flow services are easier to maintain than java codes.
Regards,
Rodrigo
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