At the end of 2014 a first draft of a Heroku buildpack for crystal was createad. This was truly great. Being able to run crystal apps in the Heroku stack was charm idea.

While we continued to develop the language, the tools, and the standard library, the community around Crystal grew. Many were interested in trying their Crystal-powered web apps in Heroku. The network graph at github for the buildpack is quite big, especially for just a bunch of bash scripts!

However, due to some flaws in the approach, the buildpack failed to stay up to date with the latest versions of Crystal; but that is now solved! Yay!

While efforts to develop web frameworks continue nowadays, we wanted to share the very basic steps to use the Crystal buildpack to deploy a web application in Heroku without the need for any additional dependencies.

Create a Crystal project

This assumes you already have crystal installed.

Use crystal init app to create the app.

$ crystal init app heroku101
    create  heroku101/.gitignore
    create  heroku101/LICENSE
    create  heroku101/README.md
    create  heroku101/.travis.yml
    create  heroku101/shard.yml
    create  heroku101/src/heroku101.cr
    create  heroku101/src/heroku101/version.cr
    create  heroku101/spec/spec_helper.cr
    create  heroku101/spec/heroku101_spec.cr
Initialized empty Git repository in /Users/bcardiff/Work/Manas/crystal/heroku101/.git/

$ cd heroku101/

Note: During the rest of the post all the commands are executed from the heroku101/ directory.

The shard.yml file declares the name of the project as heroku101. This will be used by the buildpack to determine the main source file to compile: ./src/heroku101.cr.

$ cat shard.yml
name: heroku101
version: 0.1.0
...

To create a simple http server edit the src/heroku101.cr file and add the following content:

# file: src/heroku101.cr
require "http/server"

bind = "0.0.0.0"
port = 8080

server = HTTP::Server.new(bind, port) do |context|
  context.response.content_type = "text/plain"
  context.response << "Hello world, got #{context.request.path}"
end

puts "Listening on http://#{bind}:#{port}"
server.listen

To build and run the program:

$ crystal src/heroku101.cr
Listening on http://0.0.0.0:8080

Open your browser at http://0.0.0.0:8080.

To stop the server just terminate the process by pressing Ctrl+C.

Herokufy it

Right now the project knows nothing about Heroku. To get started, a Heroku application needs first to be registered. The easiest way to do this is via the Heroku toolbelt:

$ heroku create --buildpack https://github.com/crystal-lang/heroku-buildpack-crystal.git
Creating app... done, ⬢ sleepy-thicket-16179
Setting buildpack to https://github.com/crystal-lang/heroku-buildpack-crystal.git... done
https://sleepy-thicket-16179.herokuapp.com/ | https://git.heroku.com/sleepy-thicket-16179.git

The above command will generate a random app name. Check the docs to give your app a name from the beginning.

Before deploying, we need to make a small change. Heroku randomly assigns a port number to be used by the app. Thanks to the buildpack, this will be informed in a --port option when running the application.

So, add a require "option_parser" at the beginning of src/heroku101.cr and override the port variable default with:

OptionParser.parse! do |opts|
  opts.on("-p PORT", "--port PORT", "define port to run server") do |opt|
    port = opt.to_i
  end
end

The full src/heroku101.cr should be:

# file: src/heroku101.cr
require "http/server"
require "option_parser"

bind = "0.0.0.0"
port = 8080

OptionParser.parse! do |opts|
  opts.on("-p PORT", "--port PORT", "define port to run server") do |opt|
    port = opt.to_i
  end
end

server = HTTP::Server.new(bind, port) do |context|
  context.response.content_type = "text/plain"
  context.response << "Hello world, got #{context.request.path}"
end

puts "Listening on http://#{bind}:#{port}"
server.listen

To build and run with --port option:

$ crystal src/heroku101.cr -- --port 9090
Listening on http://0.0.0.0:9090

Or build an optimised release locally and execute it via:

$ crystal build src/heroku101.cr --release
$ ./heroku101
Listening on http://0.0.0.0:8080
^C
$ ./heroku101 --port 9090
Listening on http://0.0.0.0:9090
^C

Deploy!

When you are ready to go live with your app just deploy it the usual way with git push heroku master.

$ git push heroku master
Counting objects: 22, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (17/17), done.
Writing objects: 100% (22/22), 2.85 KiB | 0 bytes/s, done.
Total 22 (delta 3), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Fetching set buildpack https://github.com/crystal-lang/heroku-buildpack-crystal.git... done
remote: -----> Crystal app detected
remote: -----> Installing Crystal (0.17.3 due to latest release at https://github.com/crystal-lang/crystal)
remote: -----> Installing Dependencies
remote: -----> Compiling src/heroku101.cr (auto-detected from shard.yml)
remote:
remote: -----> Discovering process types
remote:        Procfile declares types     -> (none)
remote:        Default types for buildpack -> web
remote:
remote: -----> Compressing...
remote:        Done: 289.4K
remote: -----> Launching...
remote:        Released v3
remote:        https://sleepy-thicket-16179.herokuapp.com/ deployed to Heroku
remote:
remote: Verifying deploy.... done.
To https://git.heroku.com/sleepy-thicket-16179.git
 * [new branch]      master -> master

The buildpack will:

1. Install the latest crystal release.
2. Install project dependencies via shards.
3. Compile the main source file in release mode.
4. Run the web server process with --port option.

Specify the crystal version

If you want to use a different Crystal version, create a .crystal-version file with the desired version, following crenv’s convention.

$ echo '0.17.1' > .crystal-version

Commit the changes in .crystal-version and deploy.

$ git push heroku master
Counting objects: 3, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 301 bytes | 0 bytes/s, done.
Total 3 (delta 1), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Fetching set buildpack https://github.com/crystal-lang/heroku-buildpack-crystal.git... done
remote: -----> Crystal app detected
remote: -----> Installing Crystal (0.17.1 due to .crystal-version file)
remote: -----> Installing Dependencies
remote: -----> Compiling src/heroku101.cr (auto-detected from shard.yml)
...

You will now notice the (0.17.1 due to .crystal-version file) legend.

Whenever you are ready to upgrade to the latest crystal version, update the content of the file or just remove it and deploy again.

Show me the code!

Find all the sample source code used at https://github.com/bcardiff/sample-crystal-heroku101.

To contribute to crystal buildpack, just fork it. Contributions are welcome!