Running a Clojure Pedestal application on Raspberry Pi model B revision 2

Complete guide to deploying a Clojure Pedestal web application on Raspberry Pi, including Java 8 installation, uberjar compilation, SSH setup, and deployment instructions for ARM-based hardware.

Compiling a fat JAR (uberjar) from the Pedestal application

Given that we already have a working application with an endpoint (/temperature) we now can deploy it and run it. Thinking on where this app should be run, I noticed that I have sitting around a Raspberry Pi Model B Rev 2. For those who might ask what processing power this little device has, this is fitted with 512MB RAM and an ARM CPU running at 700MHz. Not a very powerful device, but good enough for our little Clojure app.

If you have a Raspberry Pi sitting around and you want to find out what version it is, just type in the command line:

pi@raspberrypi:~ $ cat /proc/device-tree/model

And you should see something similar on your screen:

Raspberry Pi Model B Rev 2pi@raspberrypi:~ $

On the development machine, just go to the root folder of your Pedestal application and run:

➜  pedestal-api git:(master) lein uberjar
Compiling pedestal-api.server
INFO  org.eclipse.jetty.util.log  - Logging initialized @20621ms to org.eclipse.jetty.util.log.Slf4jLog
Created /Users/x/Projects/clojure/pedestal-api/target/pedestal-api-0.0.1-SNAPSHOT.jar
Created /Users/x/Projects/clojure/pedestal-api/target/pedestal-api-0.0.1-SNAPSHOT-standalone.jar

And the output should be a JAR file in target/pedestal-api-0.0.1-SNAPSHOT.jar. This file contains everything that is needed for the app to run on a system capable of running Java 8 applications.

Install Java 8 on Raspberry Pi Model B Rev 2

In this case, the CPU architecture restricts our options in which JRE (Java Runtime Environment) to install. I struggled a little bit until I managed to find a version that works on Raspbian Buster Lite (my OS on Raspberry Pi).

Let’s start clean and remove any traces of the OpenJDK:

sudo apt-get purge openjdk-8-jre-headles

Now let’s install Java 8:

sudo apt-get install openjdk-8-jre-headless
sudo apt-get install openjdk-8-jre

In the end the output should show Java 8 installed

java -version
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (build 1.8.0_212-8u212-b01-1+rpi1-b01)
OpenJDK Client VM (build 25.212-b01, mixed mode)

SSH into the Raspberry Pi

Until we will setup continuous deployments onto our little server (Raspberry Pi) we will do it by hand. We just ssh into the box and copy the uber JAR.

Enable SSH onto Raspberry Pi

My distro of Raspbian comes with SSH disabled by default. This is a headless distro and all you need to do to enable SSH at boot time is to place a file named ssh onto the boot partition of the SD card. No file content is required and no extension. Raspbian will pickup the file at boot time and enable SSH.

More information can be found into official documentation.

Copy the uber JAR via SSH

To copy the uber JAR we will be using scp (scp – secure copy (remote file copy program)).

scp target/pedestal-api-0.0.1-SNAPSHOT-standalone.jar pi@192.168.1.151:/home/pi/clojure-jars

Arguments passed to the scp command:

File to copy: target/pedestal-api-0.0.1-SNAPSHOT-standalone.jar
Destination: username@host:location

Start the Pedestal app on the Raspberry Pi

To start the Clojure app we need to SSH into the server and run the JAR as any Java archive.

➜  pedestal-api git:(master) ✗ ssh pi@192.168.1.151
pi@raspberrypi:~ $ java -jar clojure-jars/pedestal-api-0.0.1-SNAPSHOT-standalone.jar

It might seem that the command is stuck, but be patient. We need to reminder that this is a less powerful device and it needs some time to start. After the server starts, you should see something like this into your terminal:

pi@raspberrypi:~ $ java -jar clojure-jars/pedestal-api-0.0.1-SNAPSHOT-standalone.jar
INFO  org.eclipse.jetty.util.log  - Logging initialized @93062ms to org.eclipse.jetty.util.log.Slf4jLog

Creating your server...
INFO  org.eclipse.jetty.server.Server  - jetty-9.4.z-SNAPSHOT; built: 2019-04-29T20:42:08.989Z; git: e1bc35120a6617ee3df052294e433f3a25ce7097; jvm 1.8.0_212-8u212-b01-1+rpi1-b01
INFO  o.e.j.server.handler.ContextHandler  - Started o.e.j.s.ServletContextHandler@40e9cd{/,null,AVAILABLE}
INFO  o.e.jetty.server.AbstractConnector  - Started ServerConnector@ce284d{HTTP/1.1,[http/1.1, h2c]}{localhost:8080}
INFO  org.eclipse.jetty.server.Server  - Started @97551ms

This means that your server is started and is ready to accept connections.

Test the app via `curl`

At this stage, the application is not accessible outside of the Raspberry Pi box, so a way to test it is to use curl (curl - transfer a URL).

pi@raspberrypi:~ $ curl localhost:8080/temperature
{"celsius":0,"fahrenheit":32}

What we achieved so far

We packaged a Pedestal web service into an uber JAR
We installed Java8 on Raspberry Pi Model B Rev 2
We enabled SSH onto the Raspberry Pi and we connected to it
We copied the uber JAR onto the server via scp
We started the web service and tested via curl