One of my applications is a pure-JavaScript UI for a JSON API. This UI is an entirely different project that communicates with a public API using an OpenAPI specification.

Our public API is huge and complex: To set up the public API, I need a database, sample data, and three other private API servers that perform individual tasks as directed by the public API. Worse, I would need to set up a lot of different test scenarios with different kinds of data.

It would be a lot easier to set up a mock public API that I could use to test my UI, and it turns out that Mojolicious makes this very easy.

So let's set up a simple Mojolicious::Lite app that responds to a path with a JSON response:

# test-api.pl
use Mojolicious::Lite;
get '/servers' => sub {
    my ( $c ) = @_;
    return $c->render(
        json => [
            { ip => '10.0.0.1', os => 'Debian 9' },
            { ip => '10.0.0.2', os => 'Debian 8' }
        ],
    );
};
app->start;

Now I can fetch that JSON response by starting the web application and going to /servers or by using the get command:

$ perl test-api.pl get /servers
[{"ip":"10.0.0.1","os":"Debian 9"},{"ip":"10.0.0.2","os":"Debian 8"}

$ perl test-api.pl daemon
Server available at http://127.0.0.1:3000

That's pretty easy and shows how easy Mojolicious can be to get started. But I have dozens of routes in my application! Combined with all the possible data and its thousands of routes. How do I make all of them work without copy-pasting code for every single route?

Let's match the whole path of the route and then create a template with the given path. Mojolicious lets us match the whole path using the * placeholder in the route path. Then we can use that path to look up the template, which we'll put in the __DATA__ section.

# test-api.pl
use Mojolicious::Lite;
any '/*path' => sub {
    my ( $c ) = @_;
    return $c->render(
        template => $c->stash( 'path' ),
        format => 'json',
    );
};
app->start;
__DATA__
@@ servers.json.ep
[
    { "ip": "10.0.0.1", "os": "Debian 9" },
    { "ip": "10.0.0.2", "os": "Debian 8" }
]

Again, we can use the get command to test that we get the right data:

$ perl test-api.pl get /servers
[
    { "ip": "10.0.0.1", "os": "Debian 9" },
    { "ip": "10.0.0.2", "os": "Debian 8" }
]

So now I can write a bunch of JSON in my script and it will be exposed as an API. But I'd like it to be easier to make lists of things: REST APIs often have one endpoint as a list and another as an individual item in that list. We can make a list by composing our individual parts using Mojolicious templates and the include template helper:

# test-api.pl
use Mojolicious::Lite;
any '/*path' => sub {
    my ( $c ) = @_;
    return $c->render(
        template => $c->stash( 'path' ),
        format => 'json',
    );
};
app->start;
__DATA__
@@ servers.json.ep
[
    <%== include 'servers/1' %>,
    <%== include 'servers/2' %>
]
@@ servers/1.json.ep
{ "ip": "10.0.0.1", "os": "Debian 9" }
@@ servers/2.json.ep
{ "ip": "10.0.0.2", "os": "Debian 8" }

Now I can test the list endpoint again:

$ perl test-api.pl get /servers
[
    { "ip": "10.0.0.1", "os": "Debian 9" }
,
    { "ip": "10.0.0.2", "os": "Debian 8" }
]

And also one of the individual item endpoints:

$ perl test-api.pl get /servers/1
{ "ip": "10.0.0.1", "os": "Debian 9" }

Currently we handle all request methods (GET, POST, PUT, DELETE) the same, but my API doesn't work like that. So, I need to be able to provide different data for different request methods. To do that, let's add the request method to the template path:

# test-api.pl
use Mojolicious::Lite;
any '/*path' => sub {
    my ( $c ) = @_;
    return $c->render(
        template => join( '/', uc $c->req->method, $c->stash( 'path' ) ),
        format => 'json',
    );
};
app->start;
__DATA__
@@ GET/servers.json.ep
[
    <%== include 'GET/servers/1' %>,
    <%== include 'GET/servers/2' %>
]
@@ GET/servers/1.json.ep
{ "ip": "10.0.0.1", "os": "Debian 9" }
@@ GET/servers/2.json.ep
{ "ip": "10.0.0.2", "os": "Debian 8" }
@@ POST/servers.json.ep
{ "status": "success", "id": 3, "server": <%== $c->req->body %> }

Now all our template paths start with the HTTP request method (GET), allowing us to add different routes for POST requests and other HTTP methods.

We also added a POST/servers.json.ep template that shows us getting a successful response from adding a new server via the API. It even correctly gives us back the data we submitted, like our original API might do.

We can test our added POST /servers method with the get command again:

$ perl test-api.pl get -M POST -c '{ "ip": "10.0.0.3" }' /servers
{ "status": "success", "id": 3, "server": { "ip": "10.0.0.3" } }

Now what if I want to test what happens when the API gives me an error? Mojolicious has an easy way to layer on additional templates to use for certain routes: Template variants. These variant templates will be used instead of the original template, but only if they are available. Read more on how to use template variants yesterday on the advent calendar.

By setting the template variant to the application "mode", we can easily switch between multiple sets of templates by adding -m <mode> to the command we run.

# test-api.pl
use Mojolicious::Lite;
any '/*path' => sub {
    my ( $c ) = @_;
    return $c->render(
        template => join( '/', uc $c->req->method, $c->stash( 'path' ) ),
        variant => $c->app->mode,
        format => 'json',
    );
};
app->start;
__DATA__
@@ GET/servers.json.ep
[
    <%== include 'GET/servers/1' %>,
    <%== include 'GET/servers/2' %>
]
@@ GET/servers/1.json.ep
{ "ip": "10.0.0.1", "os": "Debian 9" }
@@ GET/servers/2.json.ep
{ "ip": "10.0.0.2", "os": "Debian 8" }
@@ POST/servers.json.ep
{ "status": "success", "id": 3, "server": <%== $c->req->body %> }
@@ POST/servers.json+error.ep
% $c->res->code( 400 );
{ "status": "error", "error": "Bad request" }

$ perl test-api.pl get -m error -M POST -c '{}' /servers
{ "status": "error", "error": "Bad request" }

And finally, since I'm using this to test an AJAX web application, I need to allow the preflight OPTIONS request to succeed and I need to make sure that all of the correct Access-Control-* headers are set to allow for cross-origin requests.

# test-api.pl
use Mojolicious::Lite;
hook after_build_tx => sub {
    my ($tx, $app) = @_;
    $tx->res->headers->header( 'Access-Control-Allow-Origin' => '*' );
    $tx->res->headers->header( 'Access-Control-Allow-Methods' => 'GET, POST, PUT, PATCH, DELETE, OPTIONS' );
    $tx->res->headers->header( 'Access-Control-Max-Age' => 3600 );
    $tx->res->headers->header( 'Access-Control-Allow-Headers' => 'Content-Type, Authorization, X-Requested-With' );
};
any '/*path' => sub {
    my ( $c ) = @_;
    # Allow preflight OPTIONS request for XmlHttpRequest to succeed
    return $c->rendered( 204 ) if $c->req->method eq 'OPTIONS';
    return $c->render(
        template => join( '/', uc $c->req->method, $c->stash( 'path' ) ),
        variant => $c->app->mode,
        format => 'json',
    );
};
app->start;
__DATA__
@@ GET/servers.json.ep
[
    <%== include 'GET/servers/1' %>,
    <%== include 'GET/servers/2' %>
]
@@ GET/servers/1.json.ep
{ "ip": "10.0.0.1", "os": "Debian 9" }
@@ GET/servers/2.json.ep
{ "ip": "10.0.0.2", "os": "Debian 8" }
@@ POST/servers.json.ep
{ "status": "success", "id": 3, "server": <%== $c->req->body %> }
@@ POST/servers.json+error.ep
% $c->res->code( 400 );
{ "status": "error", "error": "Bad request" }

Now I have 20 lines of code that can be made to mock any JSON API I write. Mojolicious makes everything easy!