We saw a simple example serving static files using Plack/PSGI. We can use that simple example to see how to write tests for any web applications that using PSGI. For examle plain PSGI based applications or Dancer based applications.

In every PSGI based application there is (or can be) a app.psgi file that is used to launch the application. In our case all the code is in that file, but this is not a requirement for this to work.


use strict;
use warnings;

use Plack::Builder;

use Plack::App::File;
my $app = Plack::App::File->new(root => "www")->to_app;

builder {
      enable "DirIndex", dir_index => 'index.html';

We also have an html file in the www subdirectory which is being served:


<!DOCTYPE html>
<html lang="en">
  <meta charset="utf-8">
  <meta name="viewport"
     content="width=device-width, initial-scale=1, user-scalable=yes">
  <title>Web Crawling Sendbox</title>

  <li><a href="/">self</a></li>
  <li><a href="/abc">to abc</a> that does not exist</li>


Finally we have the test script located in the t/ directory and having a .t extension.


use strict;
use warnings;
use Test::More;
use Plack::Test;
use HTTP::Request::Common qw(GET);
use Path::Tiny qw(path);

plan tests => 5;

my $app = do 'app.psgi';

my $test = Plack::Test->create($app);

my $main = path('www/index.html')->slurp_utf8;

my $res = $test->request(GET "/"); # HTTP::Response
is $res->code, 200;
is $res->message, 'OK';
#diag $res->headers; #HTTP::Headers
#diag explain [ $res->headers->header_field_names ];
is $res->header('Content-Length'), length $main;
is $res->header('Content-Type'), 'text/html; charset=utf-8';
diag $res->header('Last-Modified');
is $res->content, $main;

Test::More is just the standard test modules used by most of the Perl modules and Perl-based applications. If you'd like to learn more about it check out the testing series.

The two modules Plack::Test and HTTP::Request::Common are the core of our test script.

Path::Tiny is only used as a helper module to make it easy to slurp in the content of a file.

By running my $app = do 'app.psgi'; we load the content of the app.psgi file and assign its return value to the $app variable. In every PSGI-based application, and so in ours too, the app.psgi file is expected to return a reference to a function that represents the application.

Plack::Test is going to use this reference to function to run the application on our behalf and in our process.

This might need to be clarified here. In this solution we don't launch a separate server for testing. Our test script contains both the test code and by that do call also the web application.

In the next line in my $main = path('www/index.html')->slurp_utf8; we just read in the content of the www/index.html file so later we can compare that to what we get from the web application.

In the next line: my $res = $test->request(GET "/"); is the actual execution of a call to the web application. Here we sent an HTTP GET request to the / page. The result that is assigned to $res is an instance of HTTP::Response. The GET keyword is actually a function imported from HTTP::Request::Common. We could have also imported POST, but we did not need it in our test script.

We can now interrogate the response to see if we got what we have expected.

We don't have to do all these checks, I am just showing a few of the possibilities:

is $res->code, 200; checks if the response code was 200, that represents success for HTTP requests.

is $res->message, 'OK'; checks the actual response.

$res->headers; will return an instance of HTTP::Headers that can be checked separately.

Specifically the header_field_names returns the names of all the headers. If we really want to make sure is that everything is as expected, we could test those too, but I only used to list the actual headers this request returned. Then I used the header method to fetch the value of the given attribute in the header. Again, this is probably way to much for a regular web page, but there can be cases, when we would like to make sure a given header is returned to some of the requests. For example when we started to add a stand alone Ajax client to our application we had to add the Access-Control-Allow-Origin to the header. We also added a test to check if the request indeed returned that header. (In other cases you might want to make sure that certain headers are not part of a respopnse. For example you'd like to make sure that none of the responses include the Access-Control-Allow-Origin header.

The ultimate and most important check in most cases, is seeing ig the content resembles the expected content.

In our case we have the luxury, that we can compare the actual result to the whole expected file:

is $res->content, $main;

However this is usually not the case. In most cases we will either need to use a regex match to see if certain strings or patterns can be found in the response, or to make sure certain strings don't appear. We can use the like function for this instead of is. We can even go further an use an HTML Parser to see if specific HTML snippets are in the response.

The whole directory structure is quite simple:

$ tree
├── app.psgi
├── t
│   └── test.t
└── www
    └── index.html