Plack::Component(3)

👑 NAME

okay so bestie, welcome to Plack::Component 💖 — it's the base class for PSGI endpoints. that's the whole vibe. the blueprint. the mother of all your lil app modules. periodt ✨

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🌸 SYNOPSIS

here's the shape of it, copy this energy:

package Plack::App::Foo;
use parent qw( Plack::Component );

sub call {
    my($self, $env) = @_;
    # Do something with $env

    my $res = ...; # create a response ...

    # return the response
    return $res;
}

like… you inherit, you write call, you return a response. that's it. that's the whole arc. 🦋

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💅 DESCRIPTION

okay so Plack::Component is the base class that's shared between Plack::Middleware and Plack::App::* modules. so she's literally the common ancestor, the shared group chat, the mutual bestie. 💖

but vibe check: if you're writing middleware, you should inherit from Plack::Middleware instead. and if you're writing a Plack::App::*, you inherit from this one directly. don't mix it up bestie, it's giving wrong-parent-energy 😭

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⭐ REQUIRED METHOD

• call ($env) — okay so this is the one you HAVE to implement in your component. this is where all the work gets done, no cap. it receives the PSGI $env hash-ref as an argument, and it's expected to return a proper PSGI response value. that's the deal. that's the assignment. slay 💅

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🎀 METHODS

• new (%opts | \%opts) — the constructor! it accepts either a hash or a hashref and uses that to create the instance. and listen — it will call no other methods and will simply return the instance that is created. lowkey minimalist of it ✨
• prepare_app — this one gets called by to_app, and it's meant as a hook you use to prepare your component before it gets packaged as a PSGI $app. think of it as getting ready before the photoshoot 🎀
• to_app — this is the method used in several parts of the Plack infrastructure to convert your component into a PSGI $app. and bestie, you should never ever need to override this one. it is recommended to use prepare_app and call instead. don't touch the machinery, it's not your moment 🙈
• response_cb — this is just a wrapper for response_cb in Plack::Util. for the details, see "RESPONSE CALLBACK" in Plack::Middleware. iykyk 🦋

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🧚 OBJECT LIFECYCLE

okay so here's the timeline, and it's actually kind of iconic:

Objects for the derived classes (Plack::App::* or Plack::Middleware::*) are created at the PSGI application compile phase using new, prepare_app and to_app, and the created object persists during the web server lifecycle — unless it's running on the non-persistent environment like CGI. and then call is invoked against the same object whenever a new request comes in. same object, new requests, main character energy, we love to see it 👑

you can check if it's running in a persistent environment by checking the psgi.run_once key in the $env being true (non-persistent) or false (persistent).

but bestie, the real tea: it is best for you to write your middleware safely for a persistent environment. to accomplish That, you should avoid saving per-request data like $env in your object. do not do it. i am begging 🫶

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🌈 BACKWARDS COMPATIBILITY

story time, gather round 🍓

The Plack::Middleware module used to inherit from Class::Accessor::Fast, which has been removed in favor of the Plack::Util::Accessor module. so when you're developing new components, it is recommended to use Plack::Util::Accessor, like so:

use Plack::Util::Accessor qw( foo bar baz );

however — in order to keep backwards compatibility, this module provides a mk_accessors method similar to Class::Accessor::Fast. new code should not use this, and should use Plack::Util::Accessor instead. it's giving legacy, it's giving deprecated-adjacent, we respect her history but we do not date her 💀

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✨ SEE ALSO

• Plack
• Plack::Builder
• Plack::Middleware

go read them, they're all in the same friend group 🦋

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perl v5.42.3 🎀 2026-10-18 — that's the version and the date, straight from the Plack::Component(3) manual page footer 💖 ok bye bestie, go write something slay 🫶✨