On Mon, Jul 24, 2017 at 9:11 AM, George Neuner <[email protected]> wrote:
> Hi David, > > On 7/24/2017 8:18 AM, David Storrs wrote: > > What happens in the following code? > > (define dbh (postgresql-connect ...)) > > ;; Use the DBH in a new thread > (thread (thunk > (while ...some long-running condition... > (sleep 1) ; don't flood the DB > (query-exec dbh "insert into users ...")))) > > ;; And in the main thread > (query-exec dbh ...) > > I now have a database object that's being shared between two threads, > yes? Or is the object copied when the new thread is created and, if so, > what will that do to the underlying connection to the DB? > > > The single DBMS connection is being shared by the 2 threads. That's a > recipe for disaster if both try to use it simultaneously. > > > New scenario: > > (define dbh (postgresql-connect ...)) > (define current-dbh (make-parameter dbh)) > (query-value (current-dbh) "select ...") > (sleep 1000000) > (query-value (current-dbh) "select ...") > > I would expect the first query-value to work but by the time the second > runs the handle has probably been disconnected by the database and the > query will throw an exception. Am I understanding that properly? > > > After sleeping 11.6 days? Probably. But it depends on the connection > keep-alive settings. By default, Postgresql uses the OS settings, which > are roughly ~130 minutes assuming that the client understands and > participates [which Racket does]. However, on most systems, keep-alive > timeouts can be set far higher. > > Aside: I'm not aware of any published limits on keep-alive settings, only > the defaults are known for various systems. If the settings are just > integral seconds and counts, and (theoretically) could go to MAX_INT, that > would represent ~3^20 years on a 32-bit machine ... for all practical > purposes, infinite. > > > Final scenario, identical to the previous except that the parameter is a > promise: > (define dbh (postgresql-connect ...)) > (define current-dbh (make-parameter (delay dbh))) > (query-value (force (current-dbh)) "select ...") > (sleep 1000000) > (query-value (force (current-dbh)) "select ...") > > Would this make any difference? I can't see why but thought I should ask. > > > There's no difference wrt the above. You aren't delaying the *opening* of > the connection - you are simply performing unnecessary gyrations to get at > the handle. There would be a difference if instead you wrote: > > (define dbh (delay (postgresql-connect ...))) > (define current-dbh (make-parameter dbh)) > (query-value (force (current-dbh)) "select ...") > (sleep ...) > (query-value (force (current-dbh)) "select ...") > > which would try to open the connection at the point of each query. > However, at the 2nd query, if the connection were still open due to > keep-alive, the repeated open attempt might fail. > > > George > Okay, good. That's all as I expected. Thanks, George. > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
