Exploring Plack Middlewares

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Exploring Plack Middlewares Oleg Komarov 2013-03-15

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Section 1 Overview: PSGI & Plack

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PSGI Perl Web Server Gateway Interface https://metacpan.org/module/PSGI web servers PSGI server/application container provides environment applications mappings: environment -> response

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Beneﬁts portability reusability/composability frameworks support Mojolicious::Plugin::MountPSGI $self->plugin("MountPSGI", {"/", => "ext/MyApp/app.psgi"} ); Catalyst: use MyApp; MyApp->setup_engine("PSGI"); my $app = sub {MyApp->run(@_)};

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Frameworks Figure: with Dancer

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Hello world sub { my $env = shift; return [ 200, ["Content-Type" => "text/plain"], ["Hello World"], ]; }

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$env An unblessed hashref CGI keys without period REQUEST METHOD SCRIPT NAME QUERY STRING . . . PSGI-speciﬁc keys psgi.version psgi.url scheme psgi.input psgi.errors psgi.multithread psgi.multiprocess psgi.run once psgi.nonblocking psgi.streaming

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Middleware package Plack::Middleware::Foo; use parent qw(Plack::Middleware); sub call { my ($self, $env) = @_; # pre-process $env my $response = $self->app->($env); # post-process $response return $response; } # or for delayed response Plack::Util::response_cb($response, sub { my $response = shift; ... })

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How to use it use Plack::Middleware::Foo; ... $app = Plack::Middleware::Foo->wrap($app, %options);

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Easier with Plack::Builder builder { enable "Foo", foo => 1, bar => 2; enable "Baz"; ... $app; };

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Access to %options package Plack::Middleware::Foo; use parent qw(Plack::Middleware); use Plack::Util::Accessor qw(foo bar); sub call { ... $self->foo $self->bar ... }

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Prepare sub prepare_app { my $self = shift; die "it’s boring" unless $self->fun; }

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Top-level logic builder { if ($location eq "Rome") { enable "Behaviour::Roman"; } ... };

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Top-level logic builder { ... enable_if {$visitor_count++ == 1E6} "ShowBanner"; ... };

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Top-level logic builder { enable_if {$_[0]->{QUERY_STRING} =~ /&debug=1/} "Debug" ... };

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Top-level logic builder { enable "Foo", cb => \&a_cunning_plan; ... };

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Section 2 Examples

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Replacing parts of Apache Access Auth:: ErrorDocument Rewrite SSI

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And even more Cache Header logging proﬁling SizeLimit Throttle

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Developer tools Debug Delay InteractiveDebugger REPL

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Developer tools

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Aspect (lack of) Figure: use Aspect!

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unless is production() unless (is_production()) { enable "DevTools"; }

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DevTools with Aspect package Plack::Middleware::DevTools; use parent qw(Plack::Middleware); use Aspect (); sub call { my ($self, $env) = @_; my @hooks; ... # hooks ... my $response = $self->app->($env); # extract data from $env undef $env; return $response; }

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Tracing hook $env->{"devtools.http"} = []; push @hooks, Aspect::before { my (undef, $url) = $_->args; push @{$env->{"devtools.http"}}, $url; } Aspect::call qr/^LWP::UserAgent::(?:get|post)$/;

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Faking hook push @hooks, Aspect::after { my @args = $_->args; my $true_result = $_->return_value; my $fake_result = get_fake_result($true_result, @args); $_->return_value($fake_result); } Aspect::call "API::method" & Aspect::returning;

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Find missing translations push @hooks, Aspect::after { if ($cur_lang ne $default_lang) { my $msgid = [$_->args]->[0]; my $msgstr = [$_->return_value]->[0]; if ($msgid eq $msgstr) { # replace into db } else { # delete from db } } } Aspect::call qr/::gettext$/;

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Section 3 Refactoring existing code

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Dependencies Plack::Builder might have provided information about outer middlewares (but it doesn’t) Communication via $env P::M::MyAuth sets $env->{"myauth.user id"} P::M::MyUser fetches user data from DB using $env->{"myauth.user id"} and puts it in $env->{"myuser.user"}

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Special-purpose middlewares E.g. P::M::MyAuth and P::M::MyUser Captcha FakeAuth MailFatals Reqid

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Thank you Questions? https://github.com/komarov