[dancer-users] Delayed and asynchronous Dancer2

Andrew Solomon andrew at geekuni.com
Tue Oct 6 23:26:03 BST 2015

I've had no experience of Dancer2's async yet (apart from dancing along at
Sawyer's async web-scraping performance at YAPC::EU last month:) so I'll
tell you the naive approach I've taken.

* send the job off to the queue and respond immediately with a 'waiting
* the 'waiting page' has this bit of jQuery which is checking with the
server if the job is complete

  function startRefresh() {
    setTimeout(startRefresh,5000); // remember Javascript isn't waiting for
this before continuing...
    jQuery.post('/json-get-job-status', {},
      function(data) {
        if (data.job_status != 'done') {
          window.location = 'http://example.com/job_done',
  window.onload = function () {

NOTE: http://mybox/json-get-job-status is being called by the browser
without reloading the page

Does that address your question, or am I completely off on a tangent?


On Tue, Oct 6, 2015 at 8:54 PM, Amelia Ireland <aireland at lbl.gov> wrote:

> Hello again,
> What are my options for running a potentially lengthy process using
> Dancer? I'm adapting old CGI.pm-based code, which uses one of two
> techniques:
> 1. the CGI creates a job that it sends to an externally-managed queue;
> users have a workspace area where they can check to see if their job has
> finished;
> 2. the CGI keeps the connection to the browser open while performing the
> process using a hack involving sending small chunks of data from time to
> time.
> Another common technique (similar to 1 above) is to fork off a child
> process and have the browser periodically refresh the page, which triggers
> a check to see if the child process has finished (e.g. by checking for the
> presence of a results file) and the results can be loaded.
> Do Dancer2's delayed / async / non-blocking capabilities offer any
> additional ways to deal with long-running processes, or maybe a more
> elegant way to do option 2?
> Thank you for any information!
> Amelia.
> On 2 October 2015 at 10:50, Amelia Ireland <aireland at lbl.gov> wrote:
>> Hello,
>> I'm very interested in using Dancer2's streaming and async capabilities,
>> and have a number of questions about using Dancer2 asynchronously:
>> - Does the whole app have to be written to be asynchronous /
>> non-blocking? (I assume so)
>> - Do all apps running from the same .psgi file have to be non-blocking?
>> E.g. in app.psgi:
>> builder {
>>    mount '/app1' => $nonblocking_app;
>>    mount '/app2' => $blocking_app;
>> };
>> - Are hooks safe to use in a non-blocking Dancer2 app or do they
>> introduce a blocking element?
>> - When I run the drums example in a browser (Safari), it waits until the
>> entire drum loop has completed and then prints the content. Is that what is
>> expected to happen? I expected the content to be written by the browser as
>> it received it. The docs aren't very clear on what Dancer is actually doing
>> under the hood -- it would be useful to have more explanation of what's
>> going on. I'd be happy to write up something myself, but I don't know
>> what's going on. ;-)
>> Thanks!
>> Amelia.
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Andrew Solomon

Mentor at Geekuni http://geekuni.com/
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