Heroku buildpack: Clojure

This is the official Heroku buildpack for Clojure apps. It uses Leiningen.

Note that you don't have to do anything special to use this buildpack with Clojure apps on Heroku; it will be used by default for all projects containing a project.clj file, though it may be an older revision than what you're currently looking at.

How it works

The buildpack will detect your app as Clojure if it has a project.clj file in the root. If you use the clojure-maven-plugin, the standard Java buildpack should work instead.

Documentation

For more information about using Clojure and buildpacks on Heroku, see these Dev Center articles:

• Getting Started with Clojure on Heroku
• Heroku Clojure Support
• Building a Database-Backed Clojure Web Application
• Database Connection Pooling with Clojure
• Live-Debugging Remote Clojure Apps with Drawbridge
• WebSockets on Heroku with Clojure and Immutant
• Queuing in Clojure with Langohr and RabbitMQ

Usage

Example usage for an app already stored in git:

$ tree
|-- Procfile
|-- project.clj
|-- README
`-- src
    `-- sample
        `-- core.clj

$ heroku create
...

$ git push heroku main
...
remote: -----> Fetching custom tar buildpack... done
remote: -----> Clojure (Leiningen 2) app detected
remote: -----> Installing OpenJDK 1.8...done
remote: -----> Installing Leiningen
remote:        Downloading: leiningen-2.5.2-standalone.jar
remote:        Writing: lein script
remote: -----> Building with Leiningen
remote:        Running: lein uberjar
remote:        Created /tmp/build_37f1ae84b9f8b63c3ddef2a4b691ef41/target/clojure-getting-started-1.0.0-SNAPSHOT.jar
remote:        Created /tmp/build_37f1ae84b9f8b63c3ddef2a4b691ef41/target/clojure-getting-started-standalone.jar
remote: -----> Discovering process types
remote:        Procfile declares types -> web
remote:
remote: -----> Compressing... done, 53.6MB
remote: -----> Launching... done, v5
remote:        https://gentle-water-6857.herokuapp.com/ deployed to Heroku
remote:
remote: Verifying deploy.... done.

Configuration

Leiningen 1.7.1 will be used by default, but if you have :min-lein-version "2.0.0" in project.clj (highly recommended) then the latest Leiningen 2.x release will be used instead.

Your Procfile should declare what process types which make up your app. Often in development Leiningen projects are launched using lein run -m my.project.namespace, but this is not recommended in production because it leaves Leiningen running in addition to your project's process. It also uses profiles that are intended for development, which can let test libraries and test configuration sneak into production.

In order to ensure consistent builds, normally values set with heroku config:add ... (other than LEIN_USERNAME, LEIN_PASSWORD, and LEIN_PASSPHRASE) will not be visible at compile time. To expose more to the compilation process, set a BUILD_CONFIG_ALLOWLIST config var containing a space-delimited list of config var names. Note that this can result in unpredictable behaviour since changing your app's config does not result in a rebuild of your app. So it's easy to get into a situation where your build is broken, but you don't notice it until later when you push. For this reason it's recommended to take care with this feature and always push after changing a allowlisted config value.

Uberjar

If your project.clj contains an :uberjar-name setting, then lein uberjar will run during deploys. If you do this, your Procfile entries should consist of just java invocations.

If your main namespace doesn't have a :gen-class then you can use clojure.main as your entry point and indicate your app's main namespace using the -m argument in your Procfile:

web: java -cp target/myproject-standalone.jar clojure.main -m myproject.web

If you have custom settings you would like to only apply during build, you can place them in an :uberjar profile. This can be useful to use AOT-compiled classes in production but not during development where they can cause reloading issues:

:profiles {:uberjar {:main myproject.web, :aot :all}}

If you need Leiningen in a heroku run session, it will be downloaded on-demand.

Note that if you use Leiningen features which affect runtime like :jvm-opts, extraction of native dependencies, or :java-agents, then you'll need to do a little extra work to ensure your Procfile's java invocation includes these things. In these cases it might be simpler to use Leiningen at runtime instead.

Leiningen at Runtime

Instead of putting a direct java invocation into your Procfile, you can have Leiningen handle launching your app. If you do this, be sure to use the trampoline and with-profile tasks. Trampolining will cause Leiningen to calculate the classpath and code to run for your project, then exit and execute your project's JVM, while with-profile will omit development profiles:

web: lein with-profile production trampoline run -m myapp.web

Including Leiningen in your slug will add about ten megabytes to its size and will add a second or two of overhead to your app's boot time.

Overriding build behavior

If neither of these options get you quite what you need, you can check in your own executable bin/build script into your app's repo and it will be run instead of compile or uberjar after setting up Leiningen.

Leiningen Version

The buildpack will check for a bin/lein script in the repo, and run it instead of the default lein command. This allows you to control the exact version of Leiningen used to build the app.

JDK Version

By default you will get OpenJDK 1.8. To use a different version, you can commit a system.properties file to your app.

$ echo "java.runtime.version=1.7" > system.properties
$ git add system.properties
$ git commit -m "JDK 7"

Hacking

To change this buildpack, fork it on GitHub. Push up changes to your fork, then create a test app with --buildpack YOUR_GITHUB_URL and push to it. If you already have an existing app you may use heroku config:add BUILDPACK_URL=YOUR_GITHUB_URL instead.

For example, you could adapt it to generate a tarball at build time.

Open bin/compile in your editor, and replace the block labeled "Calculate build command" with something like this:

echo "-----> Generating tar with Leiningen:"
echo "       Running: lein tar"
cd $BUILD_DIR
PATH=.lein/bin:/usr/local/bin:/usr/bin:/bin JAVA_OPTS="-Xmx500m -Duser.home=$BUILD_DIR" lein tar 2>&1 | sed -u 's/^/       /'
if [ "${PIPESTATUS[*]}" != "0 0" ]; then
    echo " !     Failed to create tar with Leiningen"
    exit 1
fi

Commit and push the changes to your buildpack to your GitHub fork, then push your sample app to Heroku to test. The output should include:

-----> Generating tar with Leiningen:

If it's something other users would find useful, pull requests are welcome.

Troubleshooting

To see what the buildpack has produced, do heroku run bash and you will be logged into an environment with your compiled app available. From there you can explore the filesystem and run lein commands.