In several places (1, 2) I find the following statement as to why I should use the pool package () to manage my database connections in a Shiny app:
Opening only one connection per app … cannot handle simultaneous requests (e.g. two sessions open, both querying the database at the same time);
My understanding of shiny apps is that they run in a single-threaded R process, hence there can never be two requests at the same time. Do I miss something here? Why would I want a pool of multiple connections per app if only a single one is used at any time anyways?
(I understand that a pool with a single connection may still be useful as the pool package handles automatic re-connection in case the connection drops.)
-- Thanks, David
21 Answer
Shiny server can serve multiple end-users with a single-threaded R process. Please see the diagram and description at the start of this article. The R process alternates which user it handles at any particular instant. These multiple users can all be making different requests to your DB during overlapping time intervals.
Suppose you have a block of code that makes several requests to the DB. You might think of this big block as a single thing to compute, but maybe the R process thinks it should pause in the middle and serve a different user.
3
