Great question.

"know that they were unsuccessful, while still remaining at the comment box"

Form submission is a redirection so user will land on whatever you response.redirect(...) them to. In order to keep the page from being navigated, you need to send the form in JavaScript. AJAX is the basic format of how this is done. jQuery is a library to simplify this process:
https://api.jquery.com/jquery.ajax/

It is easy to ramp up, and may lend great help for the Capstone project as well.

Originally posted by @Alfredhgoog in #8

Not an immediate actionable item requested, but datastore service might be unavailable due to network conditions, service linkage etc. A failure will result in the servlet throwing an exception and showing a stacktrace to the response webpage, not so good looking as a web browsing experience. I would do a try {} catch {} for the specific exceptions, and return a friendly error message.

Originally posted by @Alfredhgoog in #8