[Rd] concurrent requests (Rook, but I think the question is more general)

[Dan Tenenbaum]

On Wed, Oct 24, 2012 at 11:13 AM, Richard D. Morey <r.d.morey at rug.nl> wrote:
> This question involves Rook, but I think the answer will be general enough
> that it pays to post here. At any rate, I don't know enough to know whether
> this is a Rook only issue or a general R issue.
>
> Here's what I'd like to do (and indeed, have code that should do this):
>
> 1. Start R, Rook
> 2. Start an analysis via a HTTP request to Rook. This analysis uses .Call()
> to some compiled C code, if that matters. The C code calls a callback
> function to update a variable with its progress.
> 3. While the analysis is happening, use Rook to obtain current status with
> an HTTP request
>
> The problem is that once the analysis starts, Rook does not respond to
> requests. All of the status requests to Rook pile up, and then are answered
> when the analysis (step 2) is done. Here is some example code to demonstrate
> what the issue:
>
> ##########
>
> library(Rook)
> s <- Rhttpd$new()
> s$add(
>   name="pingpong",
>   app=Rook::URLMap$new(
>     '/ping' = function(env){
>       req <- Rook::Request$new(env)
>       res <- Rook::Response$new()
>       res$write('This is ping.')
>       Sys.sleep(20)
>       res$finish()
>     },
>     '/pong' = function(env){
>       req <- Rook::Request$new(env)
>       res <- Rook::Response$new()
>       res$write("This is pong.")
>       res$finish()
>     },
>     '/?' = function(env){
>       req <- Rook::Request$new(env)
>       res <- Rook::Response$new()
>       res$redirect(req$to_url('/pong'))
>       res$finish()
>     }
>   )
> )
>
> s$start(quiet=TRUE)
> s$browse('pingpong')
>
> #############################
>
> If you request /ping, R runs Sys.sleep() for 20 seconds. This is where my
> .Call() statement would be. While the .Call() (Sys.sleep()) function is
> doing its thing, I need to get Rook to respond on /pong (which would simply
> respond with the progress), but if you run this code, request /ping, then
> immediately request /pong, you'll see that the /pong request will not be
> answered until the Sys.sleep() is done.
>
> Of course, for a progress report to be useful, the requests have to be
> answered immediately. Is this a Rook issue, or an R issue? Or am I asking
> something unreasonable?

One answer would be to start an Rserve instance on your local machine.
When your web app initiates processing, it actually starts the
long-running task on the server with RS.eval(wait=FALSE). See ?RCC
with the RS.client package loaded.
Then when you check for task completion, call RS.collect () with a
short timeout, and if it has something for you it will give it to you.

That doesn't give you a numeric progress report, but perhaps if your
long-running task writes its status somewhere (to a file?) the
progress-checking task could look there as well.

Dan


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