Automatic Restarts

A common annoyance many of us share when developing web applications is restarting our web application every time we edit files.

While we do not need to restart our web server when changing image, JavaScript, CSS, or template files, we still require it when changing our application code.

Dancer 1 had an option to reload the application on file changes but it proved brittle and in some cases caused more annoyance than it prevented.

There are several ways to handle this nowadays. Let's go through them.


The Plack::Middleware::Refresh module allows us to refresh your application and all of its modules.

All we need to do is load this middleware in our handler:

# app.psgi
use MyApp;
use Plack::Builder;

builder {
    enable 'Refresh';

However, this will turn it on all the time under every environment. We can control that by loading it only if we are in the development environment:

# app.psgi
use MyApp;
use Plack::Builder;

builder {
    if ( $ENV{'PLACK_ENV'} eq 'development' ) {
        enable 'Refresh';


However, this requires changing our handler. This will affect all other developers as well, and might cause restarts while we're still working on multiple files and cannot control whether it will restart.

There are other solutions that are likely to yield more flexibility.

plackup -r or plackup -R

Plack comes with a built-in command to start a web server for our web application. It doesn't start all web servers but it works for Perl-based web servers such as Starman or Gazelle.

# start web server, restarting when a file in lib/ changes
plackup -r bin/app.psgi

We may have renamed the lib directory. In this case, we will need to use -R instead:

# assuming we lib/ to routes/
plackup -R bin,routes bin/app.psgi

This will monitor both bin and routes so you can update your code or your handler and the web server will restart automatically.

You can also monitor additional directories. Imagine having having a templating system that uses compiled templates. You generated the compiled ones during a start of the application. The original ones would be stored in templates and compiled and stored in views.

plackup -R routes,templates bin/app.psgi

Any change to routes directory or to templates will force a restart, compiling your templates again and providing you with fresh code.


As a last resort, when the above approach aren't convenient or do not perform well, you can use the Shotgun loader from Plack::Loader::Shotgun.

It's quite simple and you can load it by calling it with -L:

plackup -L Shotgun bin/app.psgi

Instead of watching and restarting, this loader simply forks and runs each request with its own process. If you are loading heavy modules and would like to speed it up, you can do so with -M:

plackup -MMoose -MDBIx::Class -L Shotgun bin/app.psgi


This article has been written by Sawyer X for the Perl Dancer Advent Calendar 2016.


No copyright retained. Enjoy.

Sawyer X.