Getting Started with Clojure on Heroku

Introduction

This tutorial will have you deploying a Clojure app in minutes.

Hang on for a few more minutes to learn how it all works, so you can make the most out of Heroku.

The tutorial assumes that you have a free Heroku account and Clojure & Leiningen installed.

Set up

In this step you will install the Heroku Toolbelt. This provides you access to the Heroku Command Line utility, as well as git, tools you’ll use in later steps.

Download the Heroku Toolbelt

Once installed, you can use the heroku command from your command shell.

On Windows, start the Command Prompt (cmd.exe) or Powershell to access the command shell.

$ heroku login
Enter your Heroku credentials.
Email: lein@example.com
Password:
Authentication successful.

Authenticating is required to allow both the heroku and git commands to operate.

Note that if you’re behind a firewall that requires use of a proxy to connect with external HTTP/HTTPS services, you can set the HTTP_PROXY or HTTPS_PROXY environment variables before running the heroku command.

Prepare the app

In this step, you will prepare a simple application that can be deployed.

Execute the following commands to clone the sample application:

$ git clone https://github.com/heroku/clojure-getting-started.git
$ cd clojure-getting-started

You now have a functioning git repository that contains a simple application as well as a project.clj file, which is used by Clojure’s dependency manager.

Deploy the app

In this step you will deploy the app to Heroku.

Create an app on Heroku, which prepares Heroku to receive your source code:

$ heroku create
Creating sharp-rain-871 in organization heroku... done, stack is cedar-14
http://sharp-rain-871.herokuapp.com/ | https://git.heroku.com/sharp-rain-871.git
Git remote heroku added

When you create an app, a git remote (called heroku) is also created and associated with your local git repository.

Heroku generates a random name (in this case sharp-rain-871) for your app - you can pass a parameter to specify your own, or rename it later with heroku apps:rename.

Now deploy your code:

$ git push heroku master
Initializing repository, done.
Counting objects: 15, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (13/13), done.
Writing objects: 100% (15/15), 3.89 KiB, done.
Total 15 (delta 1), reused 0 (delta 0)

-----> Clojure (Leiningen 2) app detected
-----> Installing OpenJDK 1.6...done
-----> Installing Leiningen
       Downloading: leiningen-2.4.2-standalone.jar
       Writing: lein script
-----> Building with Leiningen
       Running: lein uberjar
       (Retrieving clj-http/clj-http/0.3.6/clj-http-0.3.6.jar from clojars)
       [... many more dependencies ...]
       Created /tmp/build_5d250765-909f-4f06-88cb-4e9ba7d0f5ef/target/clojure-getting-started-1.0.0-SNAPSHOT.jar
       Created /tmp/build_5d250765-909f-4f06-88cb-4e9ba7d0f5ef/target/clojure-getting-started-standalone.jar
-----> Discovering process types
       Procfile declares types -> web

-----> Compressing... done, 54.3MB
-----> Launching... done, v3
       http://sharp-rain-871.herokuapp.com/ deployed to Heroku

To git@heroku.com:sharp-rain-871.git
 * [new branch]      master -> master

The application is now deployed. Ensure that at least one instance of the app is running:

$ heroku ps:scale web=1

Now visit the app at the URL generated by its app name. As a handy shortcut, you can open the website as follows:

$ heroku open

View logs

Heroku treats logs as streams of time-ordered events aggregated from the output streams of all your app and Heroku components, providing a single channel for all of the events.

View information about your running app using one of the logging commands, heroku logs:

$ heroku logs --tail
2014-08-20T18:10:08.028873+00:00 heroku[web.1]: Starting process with command `java $JVM_OPTS -cp target/clojure-getting-started-standalone.jar clojure.main -m clojure-getting-started.web`
2014-08-20T18:10:16.463481+00:00 heroku[web.1]: State changed from starting to up
2014-08-20T18:10:16.355604+00:00 app[web.1]: 2014-08-20 18:10:16.354:INFO:oejs.Server:jetty-7.x.y-SNAPSHOT
2014-08-20T18:10:16.398993+00:00 app[web.1]: 2014-08-20 18:10:16.398:INFO:oejs.AbstractConnector:Started SelectChannelConnector@0.0.0.0:43029
2014-08-20T18:10:18.084422+00:00 heroku[router]: at=info method=GET path="/" host=sharp-rain-871.herokuapp.com request_id=1976513e-905b-454e-af1c-01d7a0ced3fb fwd="54.87.110.203" dyno=web.1 connect=10ms service=239ms status=200 bytes=411

Visit your application in the browser again, and you’ll see another log message generated.

Press Control+C to stop streaming the logs.

Define a Procfile

Use a Procfile, a text file in the root directory of your application, to explicitly declare what command should be executed to start your app.

The Procfile in the example app you deployed looks like this:

web: java $JVM_OPTS -cp target/clojure-getting-started-standalone.jar clojure.main -m clojure-getting-started.web

This declares a single process type, web, and the command needed to run it. The name web is important here. It declares that this process type will be attached to the HTTP routing stack of Heroku, and receive web traffic when deployed.

Procfiles can contain additional process types. For example, you might declare one for a background worker process that processes items off of a queue.

Scale the app

Right now, your app is running on a single web dyno. Think of a dyno as a lightweight container that runs the command specified in the Procfile.

You can check how many dynos are running using the ps command:

$ heroku ps
=== web (1X): `java $JVM_OPTS -cp target/clojure-getting-started-standalone.jar clojure.main -m clojure-getting-started.web`
web.1: up 2014/04/25 16:26:38 (~ 1s ago)

Having only a single web dyno running will result in the dyno going to sleep after one hour of inactivity. This causes a delay of a few seconds for the first request upon waking. Subsequent requests will perform normally.

To avoid this, you can scale to more than one web dyno. This will also increase your throughput. For example:

$ heroku ps:scale web=2

For abuse prevention, scaling the application may require account verification. If your account has not been verified, you will be directed to visit the verification site.

For each application, Heroku provides 750 free dyno-hours. Running your app at 2 dynos would exceed this free, monthly allowance, so scale back:

$ heroku ps:scale web=1

Declare app dependencies

Heroku recognizes an app as Clojure by the existence of a project.clj file in the root directory.

The demo app you deployed already has a project.clj, and it looks something like this:

(defproject clojure-getting-started "1.0.0-SNAPSHOT"
  :description "Demo Clojure web app"
  :url "http://clojure-getting-started.herokuapp.com"
  :license {:name "Eclipse Public License v1.0"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.6.0"]
                 [compojure "1.1.8"]
                 [ring/ring-jetty-adapter "1.2.2"]
                 [environ "0.5.0"]]
  :min-lein-version "2.0.0"
  :plugins [[environ/environ.lein "0.2.1"]]
  :hooks [environ.leiningen.hooks]
  :uberjar-name "clojure-getting-started-standalone.jar"
  :profiles {:uberjar {:aot :all}})

The project.clj file determines the versions of the dependencies that should be installed with your application, including that of Clojure itself.

Run the app locally

Now start a repl session using Leiningen:

$ lein repl
nREPL server started on port 48885 on host 127.0.0.1 - nrepl://127.0.0.1:48885
REPL-y 0.3.1
Clojure 1.6.0
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e

user=>

At this point you can evaluate any Clojure code in the context of your app, but let’s start by booting the embedded Jetty HTTP server:

user=> (require 'clojure-getting-started.web)
nil
user=> (def server (clojure-getting-started.web/-main))
2014-08-19 16:14:56.170:INFO:oejs.Server:jetty-7.6.8.v20121106
2014-08-19 16:14:56.220:INFO:oejs.AbstractConnector:Started SelectChannelConnector@0.0.0.0:5000
#'user/server
user=>

Your app will now be running at localhost:5000. Use a web browser to test that it’s working.

Most Clojure users run their repls inside their editor, whether this is Emacs, Vim, Eclipse, etc, but if you haven’t gotten that set up yet it’s simplest just to stick with the command-line lein repl invocations.

You can also start a server using lein run -m clojure-getting-started.web, but this is not recommended during development since you have to restart it every time you make a change. Running from the repl is a lot more convenient.

Push local changes

In this step you’ll learn how to propagate a local change to the application through to Heroku. As an example, you’ll modify the application to add an additional dependency and the code to use it.

Modify project.clj to include a dependency for camel-snake-kebab:

  :dependencies [[org.clojure/clojure "1.6.0"]
                 [compojure "1.1.8"]
                 [ring/ring-jetty-adapter "1.2.2"]
                 [environ "0.5.0"]
                 [camel-snake-kebab "0.2.4"]]

Modify src/clojure_getting_started/web.clj so that it :requires this library:

(ns clojure-getting-started.web
  (:require [compojure.core :refer [defroutes GET PUT POST DELETE ANY]]
            [compojure.handler :refer [site]]
            [compojure.route :as route]
            [clojure.java.io :as io]
            [ring.adapter.jetty :as jetty]
            [environ.core :refer [env]]
            [camel-snake-kebab.core :as kebab]))

Now modify app to use it:

(defroutes app
  (GET "/camel" {{input :input} :params}
       {:status 200
        :headers {"Content-Type" "text/plain"}
        :body (kebab/->CamelCase input)})
  (GET "/snake" {{input :input} :params}
       {:status 200
        :headers {"Content-Type" "text/plain"}
        :body (kebab/->snake_case input)})
  (GET "/kebab" {{input :input} :params}
       {:status 200
        :headers {"Content-Type" "text/plain"}
        :body (kebab/->kebab-case input)})
  (GET "/" []
       (splash))
  (ANY "*" []
       (route/not-found (slurp (io/resource "404.html")))))

This will add /camel, /snake, and /kebab routes which accept an ?input=... query parameter and change that string into CamelCase, snake_case, or kebab-case, respectively.

Now test locally. Since you’ve changed your dependencies, you’ll need to exit your existing repl session if it’s still open (ctrl-d) and start another one with lein repl:

user=> (require 'clojure-getting-started.web)
nil
user=> (def server (clojure-getting-started.web/-main))
2014-08-19 16:14:56.170:INFO:oejs.Server:jetty-7.6.8.v20121106
2014-08-19 16:14:56.220:INFO:oejs.AbstractConnector:Started SelectChannelConnector@0.0.0.0:5000
#'user/server

When you navigate to http://localhost:5000/snake?input=HelloWorld, you should see your string in the URL converted to snake_case: hello_world.

Now deploy. Almost every deploy to Heroku follows this same pattern. First, add the modified files to the local git repository:

$ git add .

Now commit the changes to the repository:

$ git commit -m "Add camel-snake-kebab and use it from web.clj."

Now deploy, just as you did previously:

$ git push heroku master

Finally, check that everything is working:

$ curl http://sharp-rain-871.herokuapp.com/camel?input=doner-kebab
DonerKebab

Provision add-ons

Add-ons are third-party cloud services that provide out-of-the-box additional services for your application, from persistence through logging to monitoring and more.

By default, Heroku stores 1500 lines of logs from your application. However, it makes the full log stream available as a service - and several add-on providers have written logging services that provide things such as log persistence, search, and email and SMS alerts when certain conditions are met.

Provision the papertrail logging add-on:

$ heroku addons:add papertrail
Adding papertrail on sharp-rain-871... done, v4 (free)
Welcome to Papertrail. Questions and ideas are welcome (support@papertrailapp.com). Happy logging!
Use `heroku addons:docs papertrail` to view documentation.

To help with abuse prevention, provisioning an add-on may require account verification. If your account has not been verified, you will be directed to visit the verification site.

The add-on is now deployed and configured for your application. You can list add-ons for your app like so:

$ heroku addons

To see this particular add-on in action, visit your application’s Heroku URL a few times. Each visit will generate more log messages, which should now get routed to the papertrail add-on. Visit the papertrail console to see the log messages:

$ heroku addons:open papertrail

A console will open up, showing the latest log events, and providing you with an interface to search and set up alerts:

Alt text

Start a repl on a dyno

You can run a command, typically scripts and applications that are part of your app, in a one-off dyno using the heroku run command. It can also be used to launch a REPL process attached to your local terminal for experimenting in your app’s environment:

$ heroku run lein repl
Running `lein repl` attached to terminal... up, run.1459
user=>

If you receive an error, Error connecting to process, then you may need to configure your firewall.

The REPL functions just like a local lein repl, but it’s remote.

You will be be able to run the following:

user=> (require '[camel-snake-kebab.core :as kebab])
nil
user=> (kebab/->kebab-case "JustSomeStuff")
"just-some-stuff"

To get a real feel for how dynos work, you can create another one-off dyno and run the bash command, which opens up a shell on that dyno. You can then execute commands there. Each dyno has its own ephemeral filespace, populated with your app and its dependencies - once the command completes (in this case, bash), the dyno is removed.

$ heroku run bash
Running `bash` attached to terminal... up, run.3052
~ $ ls
Procfile  README.md  project.clj  resources  src  target  test

Don’t forget to hit ctrl-d (or exit) to exit the shell and terminate the dyno.

Define config vars

Heroku lets you externalise configuration - storing data such as encryption keys or external resource addresses in config vars.

At runtime, config vars are exposed as environment variables to the application. The environ library exposes this in a convenient way, though of course you can use raw System/getenv calls if you prefer.

Let’s add links from the splash page, defined in src/clojure_getting_started/web.clj to sample conversions:

(def sample (env :sample "sample-string-thing"))

(defn splash []
  {:status 200
   :headers {"Content-Type" "text/html"}
   :body (for [kind ["camel" "snake" "kebab"]]
           (format "<a href=\"/%s?input=%s\">%s %s</a><br />"
                   kind sample kind sample))})

The sample var contains the string that will be used in the sample links. Environ handles translating from the SAMPLE environment variable to (env :sample) with an optional default value.

To set the config var on Heroku, execute the following:

$ heroku config:set SAMPLE=this_one_started_as_snake_case
Setting config vars and restarting sharp-rain-871... done, v10
SAMPLE: this_one_started_as_snake_case

View the config vars that are set using heroku config:

$ heroku config
== sharp-rain-871 Config Vars
PAPERTRAIL_API_TOKEN: [REDACTED]
SAMPLE: this_one_started_as_snake_case

Deploy your changed application to Heroku to see this in action.

Provision a database

The add-on marketplace has a large number of data stores, from Redis and MongoDB providers, to Postgres and MySQL. In this step you will add a free Heroku Postgres Starter Tier dev database to your app.

Add the database:

$ heroku addons:add heroku-postgresql:hobby-dev
Adding heroku-postgresql:hobby-dev... done, v3 (free)

This creates a database, and sets a DATABASE_URL environment variable (you can check by running heroku config).

Edit your project.clj file to add the java.jdbc library and postgresql driver to your dependencies:

  :dependencies [[org.clojure/clojure "1.6.0"]
                 [compojure "1.1.8"]
                 [ring/ring-jetty-adapter "1.2.2"]
                 [environ "0.5.0"]
                 [camel-snake-kebab "0.2.4"]
                 [org.clojure/java.jdbc "0.3.5"]
                 [postgresql "9.1-901-1.jdbc4"]]

Now edit your web.clj file to load this library. Add [clojure.java.jdbc :as db] to the require section of ns. Next, read and write to the DB:

(defn splash []
  {:status 200
   :headers {"Content-Type" "text/html"}
   :body (concat (for [kind ["camel" "snake" "kebab"]]
                   (format "<a href=\"/%s?input=%s\">%s %s</a><br />"
                           kind sample kind sample))
                 ["<hr /><ul>"]
                 (for [s (db/query (env :database-url)
                                   ["select content from sayings"])]
                   (format "<li>%s</li>" (:content s)))
                 ["</ul>"])})

(defn record [input]
  (db/insert! (env :database-url "postgres://localhost:5432/kebabs")
              :sayings {:content input}))

(defroutes app
  (GET "/camel" {{input :input} :params}
       (record input)
       {:status 200
        :headers {"Content-Type" "text/plain"}
        :body (kebab/->CamelCase input)})
  (GET "/snake" {{input :input} :params}
       (record input)
       {:status 200
        :headers {"Content-Type" "text/plain"}
        :body (kebab/->snake_case input)})
  (GET "/kebab" {{input :input} :params}
       (record input)
       {:status 200
        :headers {"Content-Type" "text/plain"}
        :body (kebab/->kebab-case input)})
  (GET "/" []
       (splash))
  (ANY "*" []
       (route/not-found (slurp (io/resource "404.html")))))

This ensures that all invocations will be recorded and listed in the sayings table. They will be shown on the splash page.

Deploy this to Heroku. If you access / you’ll get a blank page–to get the real story you’ll need to run heroku logs. There’s a big stack trace, but at the top it says:

org.postgresql.util.PSQLException: ERROR: relation "sayings" does not exist|  Position: 21

This means we haven’t gotten around to creating the table yet. Note that in a real-world application we would have ring middleware set up to catch exceptions and either display the stack trace in the browser (during development) or show a user-friendly error message (in production), but that is outside the scope of this article.

Assuming that you have Postgres installed locally, use the heroku pg:psql command to connect to the remote database, create a table and insert a row:

$ heroku pg:psql
psql (9.3.2, server 9.3.3)
SSL connection (cipher: DHE-RSA-AES256-SHA, bits: 256)
Type "help" for help.
=> create table sayings (id serial primary key, content text);
CREATE TABLE
=> insert into sayings values (1, 'HelloComputer');
INSERT 0 1
=> \q

Now seed it with a few sample invocations:

$ curl http://sharp-rain-871.herokuapp.com/camel?input=doner-kebab
doner_kebab
$ curl http://sharp-rain-871.herokuapp.com/kebab?input=snake_like_charms
snake-like-charms

When you access your app’s / route, you will see something like this below the sample links:

HelloComputer
doner-kebab
snake_like_charms

Next steps

You now know how to deploy an app, change its configuration, view logs, scale, and attach add-ons.

Here’s some recommended reading. The first, an article, will give you a firmer understanding of the basics. The second is a pointer to the main Clojure category here on Dev Center:

Read How Heroku Works for a technical overview of the concepts you’ll encounter while writing, configuring, deploying and running applications.
Read Deploying Clojure Apps on Heroku to understand how to take an existing Clojure app and deploy it to Heroku.
Visit the Clojure category to learn more about developing and deploying Clojure applications.