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Building a microservice in Perl, part 2: Up and running

Mark Gardner
I help professional Perl developers to engineer modern, disciplined applications in the cloud.
Originally published at on ・9 min read

In part 1 we designed our API using OpenAPI/Swagger. Now it’s time to write some tests and wire it up using Mojolicious::Plugin::OpenAPI. This is a much longer article; buckle up!

If you haven’t already, you’ll need to install Perl. Using Linux or macOS? You’ve already got Perl installed on your system. On Windows? I recommend you install Strawberry Perl as it lets you develop Perl applications using the same tools that our Unix-based brethren use.

(Advanced users may want to investigate using perlbrew, plenv, or berrybrew for managing multiple versions of Perl and installing more recent versions than are included on your system.)

Once you have Perl installed, it’s a simple matter of using either the cpan or cpanm tools to install Mojolicious and Mojolicious::Plugin::OpenAPI. The latter will install some dependencies as well. Here’s a transcript of installing cpanm and the two modules:

% curl -L | perl - App::cpanminus
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
100 295k 100 295k 0 0 97765 0 0:00:03 0:00:03 --:--:-- 97765
--> Working on App::cpanminus
Fetching ... OK
Configuring App-cpanminus-1.7044 ... OK
Building and testing App-cpanminus-1.7044 ... OK
Successfully installed App-cpanminus-1.7044
1 distribution installed

% cpanm Mojolicious Mojolicious::Plugin::OpenAPI
--> Working on Mojolicious
Fetching ... OK
Configuring Mojolicious-9.01 ... OK
Building and testing Mojolicious-9.01 ... OK
Successfully installed Mojolicious-9.01
--> Working on Mojolicious::Plugin::OpenAPI
Fetching ... OK
Configuring Mojolicious-Plugin-OpenAPI-4.00 ... OK
==> Found dependencies: JSON::Validator
--> Working on JSON::Validator
Fetching ... OK
Configuring JSON-Validator-4.14 ... OK
==> Found dependencies: YAML::PP, Test::Deep
--> Working on YAML::PP
Fetching ... OK
Configuring YAML-PP-0.026 ... OK
==> Found dependencies: Test::Warn, Test::Deep
--> Working on Test::Warn
Fetching ... OK
Configuring Test-Warn-0.36 ... OK
==> Found dependencies: Sub::Uplevel
--> Working on Sub::Uplevel
Fetching ... OK
Configuring Sub-Uplevel-0.2800 ... OK
Building and testing Sub-Uplevel-0.2800 ... OK
Successfully installed Sub-Uplevel-0.2800
Building and testing Test-Warn-0.36 ... OK
Successfully installed Test-Warn-0.36
--> Working on Test::Deep
Fetching ... OK
Configuring Test-Deep-1.130 ... OK
Building and testing Test-Deep-1.130 ... OK
Successfully installed Test-Deep-1.130
Building and testing YAML-PP-0.026 ... OK
Successfully installed YAML-PP-0.026
Building and testing JSON-Validator-4.14 ... OK
Successfully installed JSON-Validator-4.14
Building and testing Mojolicious-Plugin-OpenAPI-4.00 ... OK
Successfully installed Mojolicious-Plugin-OpenAPI-4.00
7 distributions installed
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(The versions listed above may differ as this article gets progressively out of date.)

Next, make a folder somewhere to place our new microservice project. We’ll call it ~/Projects/blog here, but any place will do as long as you know how to get to it from your text editor and your command line.

After that, go into that directory and use the newly-installed mojo command to build the basics of our project:

% cd ~/Projects/blog

% mojo generate app Local::Dictionary::Microservice
  [mkdir] /Users/mgardner/Projects/blog/local_dictionary_microservice/script
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/script/local_dictionary_microservice
  [chmod] /Users/mgardner/Projects/blog/local_dictionary_microservice/script/local_dictionary_microservice 744
  [mkdir] /Users/mgardner/Projects/blog/local_dictionary_microservice/lib/Local/Dictionary
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/lib/Local/Dictionary/
  [exist] /Users/mgardner/Projects/blog/local_dictionary_microservice
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/local-dictionary-microservice.yml
  [mkdir] /Users/mgardner/Projects/blog/local_dictionary_microservice/lib/Local/Dictionary/Microservice/Controller
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/lib/Local/Dictionary/Microservice/Controller/
  [mkdir] /Users/mgardner/Projects/blog/local_dictionary_microservice/t
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/t/basic.t
  [mkdir] /Users/mgardner/Projects/blog/local_dictionary_microservice/public
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/public/index.html
  [mkdir] /Users/mgardner/Projects/local_dictionary_microservice/templates/layouts
  [write] /Users/mgardner/Projects/local_dictionary_microservice/templates/layouts/default.html.ep
  [mkdir] /Users/mgardner/Projects/blog/local_dictionary_microservice/templates/example
  [write] /Users/mgardner/Projects/blog/local_dictionary_microservice/templates/example/welcome.html.ep
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This command builds the scaffolding for our new project. We won’t be using all of it, but it does provide some useful starting points.

(Why did we put Local:: in the beginning of the name? Because nothing you place there will conflict with other modules you install from CPAN.)

Next, put the OpenAPI document we developed in part 1 somewhere in the project. We’ll choose openapi/dictionary_openapi.yml. Create a folder called openapi in the project, and then open your text editor and paste the following document into a file called dictionary_openapi.yml.

Calling the plugin

Right now all our application does is serve a demonstration page; you can test it by running the following command:

% script/local_dictionary_microservice daemon

[2021-02-28 16:17:09.76863] [74167] [info] Listening at "http://*:3000"
Web application available at
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…and then going to that URL on the last line.

After you’ve verified that it works, hold down the Control key and type the letter C in your terminal to stop the script. Edit the lib/Local/Dictionary/ file in your text editor. Replace it with this:

Now when you run the script as a daemon, the URL it reports responds with the JSON version of our OpenAPI document. Progress!

Note the $self->plugin() call towards the end of our class above. That’s the secret sauce that loads our OpenAPI document and tells Mojolicious to create responses from it.

Writing our first tests

Next, we’ll write a couple test scripts. (Why are we writing tests before actually coding our microservice? Because we’re practicing test-driven development, in which we write our tests first, see that they fail, and then write the code to make them succeed.) Here’s a modified t/basic.t script that should succeed right off the bat:

Run it with prove:

% prove -vl t/basic.t

t/basic.t .. [2021-02-28 16:32:54.79287] [74611] [debug] [3kzjyqgi] GET "/"
[2021-02-28 16:32:54.79321] [74611] [debug] [3kzjyqgi] Routing to a callback
[2021-02-28 16:32:54.79536] [74611] [debug] [3kzjyqgi] 200 OK (0.002493s, 401.123/s)

ok 1 - GET /
ok 2 - 200 OK
ok 3 - has value for JSON Pointer "/openapi"
All tests successful.
Files=1, Tests=3, 0 wallclock secs ( 0.02 usr 0.01 sys + 0.45 cusr 0.07 csys = 0.55 CPU)
Result: PASS
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Now for a test that will fail at first. Put this in t/word.t:

And prove it:

% prove -vl t/word.t

t/word.t .. [2021-02-28 16:48:53.26826] [75751] [debug] [SlDSHWVE] POST "/word/foo"
[2021-02-28 16:48:53.26897] [75751] [debug] [SlDSHWVE] Routing to controller "Local::Dictionary::Microservice::Controller::Word" and action "save"
[2021-02-28 16:48:53.27278] [75751] [debug] [SlDSHWVE] Template "word/save.html.ep" not found
[2021-02-28 16:48:53.27287] [75751] [debug] [SlDSHWVE] Nothing has been rendered, expecting delayed response
[2021-02-28 16:49:23.27553] [75751] [debug] Inactivity timeout
# Premature connection close

not ok 1 - POST /word/foo
not ok 2 - 200 OK

# Failed test 'POST /word/foo'
# at t/word.t line 8.

# Failed test '200 OK'
# at t/word.t line 8.
# got: undef
# expected: '200'
# Looks like you failed 2 tests of 2.
Dubious, test returned 2 (wstat 512, 0x200)
Failed 2/2 subtests

Test Summary Report
t/word.t (Wstat: 512 Tests: 2 Failed: 2)
  Failed tests: 1-2
  Non-zero exit status: 2
Files=1, Tests=2, 31 wallclock secs ( 0.02 usr 0.01 sys + 0.45 cusr 0.07 csys = 0.55 CPU)
Result: FAIL
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Note in the second line of output that Mojolicious tried to route to a controller class at Local::Dictionary::Microservice::Controller::Word with an action of save. That’s what our OpenAPI document told it to do with its x-mojo-to: word#save line.

Adding methods to make the tests pass

Now we’ll write the controller class at lib/Local/Dictionary/Microservice/Controller/

The save method above retrieves the word and definition parameters from the URL path and POST data, respectively, and then saves them into a hash that is tied to a Berkeley DB file stored in definitions.db. It then renders an empty response back to the client.

Now run the test:

% prove -vl t/word.t

t/word.t .. [2021-02-28 18:06:33.77551] [78923] [debug] [VV0L-5rU] POST "/word/foo"
[2021-02-28 18:06:33.77623] [78923] [debug] [VV0L-5rU] Routing to controller "Local::Dictionary::Microservice::Controller::Word" and action "save"
[2021-02-28 18:06:33.77751] [78923] [debug] [VV0L-5rU] 200 OK (0.001994s, 501.505/s)

ok 1 - POST /word/foo
ok 2 - 200 OK
All tests successful.
Files=1, Tests=2, 0 wallclock secs ( 0.02 usr 0.01 sys + 0.43 cusr 0.07 csys = 0.53 CPU)
Result: PASS
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Woohoo, our method works! Let’s add some tests to the same t/word.t script , underneath the first one:

Here we’re testing that we get back the definition we saved, that we can delete the definition, and that when we try to retrieve it again we get a 404 Not Found error.

The test script will fail again, but we can fix that by adding the define and remove methods to our controller class in lib/Local/Dictionary/Microservice/Controller/

Run the test script one last time:

% prove -vl t/word.t

t/word.t .. [2021-02-28 18:13:50.82904] [79162] [debug] [7PprxDOz] POST "/word/foo"
[2021-02-28 18:13:50.82978] [79162] [debug] [7PprxDOz] Routing to controller "Local::Dictionary::Microservice::Controller::Word" and action "save"
[2021-02-28 18:13:50.83225] [79162] [debug] [7PprxDOz] 200 OK (0.003191s, 313.381/s)

ok 1 - POST /word/foo
ok 2 - 200 OK
[2021-02-28 18:13:50.83531] [79162] [debug] [G0qyFA0G] GET "/word/foo"
[2021-02-28 18:13:50.83568] [79162] [debug] [G0qyFA0G] Routing to controller "Local::Dictionary::Microservice::Controller::Word" and action "define"
[2021-02-28 18:13:50.85745] [79162] [debug] [G0qyFA0G] 200 OK (0.022096s, 45.257/s)
ok 3 - GET /word/foo
ok 4 - 200 OK
ok 5 - exact match for content
[2021-02-28 18:13:50.86182] [79162] [debug] [MoedWcOi] DELETE "/word/foo"
[2021-02-28 18:13:50.86224] [79162] [debug] [MoedWcOi] Routing to controller "Local::Dictionary::Microservice::Controller::Word" and action "remove"
[2021-02-28 18:13:50.86337] [79162] [debug] [MoedWcOi] 200 OK (0.001535s, 651.466/s)
ok 6 - DELETE /word/foo
ok 7 - 200 OK
[2021-02-28 18:13:50.86662] [79162] [debug] [G2lH8gyw] GET "/word/foo"
[2021-02-28 18:13:50.86684] [79162] [debug] [G2lH8gyw] Routing to controller "Local::Dictionary::Microservice::Controller::Word" and action "define"
[2021-02-28 18:13:50.86964] [79162] [debug] [G2lH8gyw] 404 Not Found (0.002989s, 334.560/s)
ok 8 - GET /word/foo
ok 9 - 404 Not Found
All tests successful.
Files=1, Tests=9, 0 wallclock secs ( 0.02 usr 0.01 sys + 0.46 cusr 0.10 csys = 0.59 CPU)
Result: PASS
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Congratulations! You now have a simple microservice for storing and retrieving definitions of words. You can now write a web front-end in HTML and JavaScript, or perhaps another microservice that consumes this one.

(As an exercise, write the /health route defined in our OpenAPI document. It calls a heartbeat method in a controller class named Monitoring.)

What next?

  • You may not want to store your definitions in a DB file in your project; consider making it a configurable option. Or use one of the many modules on CPAN to choose a completely different backend.
  • Add methods to list some or all definitions, using hypertext as the engine of application state (HATEOAS) to render links where appropriate.
  • Support multiple definitions of the same word, or expand the API to respond with synonyms and antonyms. (Just make sure to increment the version in the OpenAPI document and add some way for clients to specify the version to indicate you’re making a breaking change!)

Did you have any trouble following along? Got stuck on the installation steps or somewhere else? Please leave a comment below and I’ll try to help.

Discussion (2)

thibaultduponchelle profile image
Tib • Edited

This is a GREAT article ! Big thank you

One little thing, I think it's HATEOAS instead HATEOS

And you make my day with Local:: trick!

mjgardner profile image
Mark Gardner Author

Oops, thanks for the correction. It’s an awkward acronym.

I don’t know why using Local:: isn’t publicized more. It just makes sense for internal projects.