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Heroku buildpack: Clojure

This is a 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 current master.

Usage

Example usage for an app already stored in git:

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

$ heroku create

$ git push heroku master
...
-----> Heroku receiving push
-----> Fetching custom buildpack
-----> Clojure app detected
-----> Installing Leiningen
       Downloading: leiningen-2.0.0-preview10-standalone.jar
       Writing: lein script
-----> Building with Leiningen
       Running: with-profile production compile :all
       Downloading: org/clojure/clojure/1.2.1/clojure-1.2.1.pom from central
       Downloading: org/clojure/clojure/1.2.1/clojure-1.2.1.jar from central
       Copying 1 file to /tmp/build_2e5yol0778bcw/lib
-----> Discovering process types
       Procfile declares types -> core
-----> Compiled slug size is 10.0MB
-----> Launching... done, v4
       http://gentle-water-8841.herokuapp.com deployed to Heroku

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. Leiningen 1.7.1 will be used by default, but if you have :min-lein-version "2.0.0" in project.clj then Leiningen 2.x will be used instead.

Configuration

By default your project is built by running lein deps under Leiningen 1.x and lein compile :all under Leiningen 2.x. To customize this, check in a bin/build script into your project and it will be run instead of invoking lein directly.

If you are using Leiningen 2.x, it's highly recommended that you use the :production profile in your Procfile and/or bin/build to avoid having tests and development dependencies on your classpath in production.

If you don't need to add anything to the :production profile then you can leave it out and the one from opt/profiles.clj in the buildpack will be used. If you do need to add something, it's recommended you include :mirrors for faster dependency resolution from S3 for Central:

:production {:app-specific "config"
             :mirrors {"central" "http://s3pository.herokuapp.com/maven-central"}}

Since Clojars currently mixes snapshots and releases it's currently not appropriate to mirror to S3 unless you know for sure you're not using any snapshots even transitively.

You should reduce memory consumption by using the trampoline task in your Procfile. This will cause Leiningen to calculate the classpath and code to run for your project, then exit and execute your project's JVM:

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

JDK Version

By default you will get OpenJDK 1.6. 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 an uberjar at build time.

Open bin/compile in your editor, and replace the block labeled "fetch deps with lein" with something like this:

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