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Dodona is an online exercise platform for learning to code. It wants to teach students how to program in the most meaningful and effective way possible. Dodona acts as an online co-teacher, designed to give every student access to high quality education. The focus is on automatic corrections and giving meaningful feedback on the submitted solutions from students.

This repository contains the source code of the web application. If you simply want to use Dodona, please go to

The documentation for end users can be found at

Supporting Dodona

Dodona is free to use for schools and we would like to keep it that way! Keeping this platform up and running takes a lot of time, just as supporting hundreds of schools and thousands of students. If you would like to fund Dodona, you can find more information on or get in touch by emailing us at

Contacting us

There are several ways to contact us:

Local development

If you want to help with development, issues tagged with the student label are a good starting point.

Development Setup

  1. Install and start mysql or mariadb.
  2. If using mysql, change the sql-mode in the mysqld configuration block:
  3. Create a dodona user with access to the dodona and dodona_test-N databases. You will need as much test databases as you have CPU threads.
    CREATE USER 'dodona'@'localhost' IDENTIFIED BY 'dodona';
    GRANT ALL ON dodona.* TO 'dodona'@'localhost';
    GRANT ALL ON dodona_test.* TO 'dodona'@'localhost';
    GRANT ALL ON `dodona_test-0`.* TO 'dodona'@'localhost';
    GRANT ALL ON `dodona_test-N`.* TO 'dodona'@'localhost';
  4. Install the correct ruby version using RVM (the currently used version can be found here).
  5. Install the correct node version using nvm and yarn (any modern node version should do).
  6. Run bundle install and yarn install
  7. Create and seed the database with rails db:setup. (If something goes wrong with the database, you can use rails db:reset to drop, rebuild and reseed the database.) If the error "Could not initialize python judge" arises, use SKIP_PYTHON_JUDGE=true rails db:setup
  8. Run bin/server to start the server. More information on how to start the development setup can be found here. Dodona will be available on a subdomain of localhost: http://dodona.localhost:3000.
  9. CAS authentication does not work in development. Instead the seed database from step 7 provides three users via login tokens. The relevant login links are displayed in an announcement on the home page of Dodona.

Evaluating exercises locally

These steps are not required to run the server, but you need docker to actually evaluate exercises.

  1. Install and start docker.
  2. Run docker pull dodona/dodona-python

If you want to build the docker images yourself:

  1. Clone the docker-images repository.
  2. Build a docker image. The scripts builds all images. But with the initial data, only dodona-python is needed. You can build this image with docker build --pull --force-rm -t "dodona-python" -f "dodona-python.dockerfile" ..

Loading visualisations locally

These steps are not required to run the server, but are needed to let the visualisations load.

  1. Install and start memcached.
  2. Create the following file tmp/caching-dev.txt in the root of the project.


Some gems and dependencies (such as memcached) do not work on Windows. You should use WSL 2 instead, and run everything inside WSL. This means you use WSL for the database, memcached, git, Docker, etc.

Starting a local server

The simplest way to start the server is with the rails s command. But this will not process the submission queue, and javascript will be compiled by webpack in the background (without output when something goes wrong).

  • To process the submission queue, delayed job needs to be started with the bin/delayed_job start command.
  • With yarn build:css --watch your css is reloaded live (use without the --watch flag when you need to build the stylesheets and live reload is not wanted).
  • With yarn build:js --watch your javascript is reloaded live (use without the --watch flag when you need to build the javascript and live reload is not wanted).

To start the rails server, delayed job, css bundling and js bundling at the same time, simply run bin/server.

This has one letdown: debugging with byebug is broken.

Localhost subdomain

Dodona uses subdomains in order to sandbox exercise descriptions (which are arbitrary HTML pages and could be used for malicious purposes if not properly sandboxed). We serve the main application in development from http://dodona.localhost:3000 and exercise descriptions from http://sandbox.localhost:3000.

If this does not work out of the box you can add the following lines to your /etc/hosts file:             dodona.localhost             sandbox.localhost

Running linters and tests

To lint the code, run rubocop for Ruby and yarn lint for JavaScript.

We have tests in JavaScript, Ruby, and system tests:

  • For JavaScript, run yarn test
  • For the system tests, run bundle exec rails test:system
  • For the ruby tests, run bundle exec rails test

There is also a type check for code written in TypeScript. This can be executed with yarn typeCheck.


  • Use the PARALLEL_WORKERS ENV var to specify the number of threads to use.
  • Use TestProf to profile the ruby tests
  • Use bundle exec rails test filename to run a single test file, use bundle exec rails test filename:linenumber to run a specific test