Plack::Runner and plackup

plackup starts a PSGI server which executes a PSGI application. However the script itself is just a very small wrapper around Plack::Runner which does all the heavy lifting. Plack::Runner

  1. parses the command line options.
  2. instantiates the chosen loader class (which is in the Plack::Loader namespace).
  3. instatiates the chosen server library (which is in the Plack::Handler namespace).
  4. starts the PSGI server and passes it a PSGI application


Loaders are responsible for instantiating and running the PSGI server. Here are the more interesting capabilities a $loader object has:

  • $loader->guess() guesses which server library should be loaded by looking at command line opts, $ENV, and %INC.
  • $loader->load() instantiates the server library and returns the object.
  • $loader->run() starts the server.

The Plack::Loader namespace contains 3 kinds of loaders:

  • Plack::Loader::Delayed - delays compilation of the web app until the first request occurs
  • Plack::Loader::Restarter - reloads the server if any files are changed
  • Plack::Loader::Shotgun - foreach request, forks a child which compiles the web app and runs it

I can choose which loader I want using plack --loader


The PSGI spec tells me that PSGI defines the interface between an application and a server. Because the PSGI spec is (intentionally) very minimal, there is a good deal of wiggle room to interpret how an application and a server might want to play together.

A library in the Plack::Handler namespace is the place where the application meets the server. This layer contains all the wiggling.

Lets say I wrote a new server called AngryBrontosaurus and I want to be able to use it with plackup --server AngryBrontosaurus. I could implement a small class like this:

package Plack::Handler::AngryBrontosaurus
use strict;
use AngryBrontosaurus;

sub new {
    my $class = shift;
    bless { @_ }, $class;

sub run {
    my ($self, $app) = @_; 
    AngryBrontosaurus->new->run($app, $self);

Then, to make sure AngryBrontosaurus and Plack::Handler::AngryBrontosaurus correctly implement the PSGI spec, I should also test my code with Plack::Test::Suite.

use Test::More;
use Plack::Test::Suite;

Notice that while the Plack::Handler namespace contains classes for several PSGI servers like Plack::Handler::Starman or Plack::Handler::Twiggy, it also includes some classes like Plack::Handler::Apache2 and Plack::Handler::FCGI. Clearly Apache2 was not written with PSGI compliance in mind, but there is glue in the Plack::Handler::Apache2 layer to enable it to speak with PSGI compliant applications.

Sequence diagram

This diagram describes how Plack::Runner, Plack::Handler, and Plack::Loader interact.