We have an existing sql server 2012 instance but without full text search. before I add the feature, I want to know if it stops or restart the instance since it is for production.
thanks in advance.
The setup program restarts the target instance when installing full text search.
However, there's an easy way to find out: test it! Don't trust what I say: test it in your test environment.
For anyone wondering about this, I've just tried it with SQL Server 2016.
It doesn't restart the service in a normal sense of the term. All the connection are like frozen or delayed for a few seconds (around 20 seconds in my case) and then the response is returned. That means that you won't get any errors in the application (maybe a timeout or something like that), but the applications might take a bit to respond.
Also, if you have SSMS open and connected for example, the connection is not dropped, it continues working without having to reconnect or resetting to the default database.
The only thing I've noticed though, the SQL Server Agent was not started automatically. It was set as Startup: Automatic, but after the install it was stopped and I had to manually start it again.