warning: I'm pretty new to R, sniny and co ==> I don't realize whether this question is interesting.
update : It turns out that it is a shiny question and it seems to be a frequent problem, that is not obvious. Please read all answers, they don't address the same cases.
I have a Data base in DB. Is there a difference between DBtoto <- reactive({DB()}) and DBtoto <- reactive({DB}) ? If so, what is it ?
In fact I don't see what BD() (with parentheses) means.
Yes, there's a difference. DB() is a call to the function named DB. DB is the function itself. If it's not a function, then DB() doesn't make sense, and will trigger a run-time error (unless there's another object somewhere which is a function).
reactive() is a Shiny function, that says the value of its argument may change over time. Usually it would make more sense to think the value of the function call would change, but it's (remotely) possible that the function itself could change.
I also found the first answer of What is “object of type ‘closure’ is not subsettable” error in Shiny? addresses this question. To summary, everything created with 'reactive()' in shiny must be referred as a function.
In my example, if DB was reactive (for instance DB <- reactive(read_DataBase())), then DB() must be referred with parenthesis. For instance, to get the attribute 'x', you must write BD()$x. In my 'DBtoto' example above the first expression holds in the case DB is itself reactive.