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Kickbet

Kickbet is a web app where you can guess the results of football matches and compare your guesses with those made by your friends. I built Kickbet specifically for the 2022 FIFA World Cup.

The rules are simple. You score:

• 3 points if you correctly guessed the result;
• 2 points if you guessed the goal difference;
• 1 point if you guessed the winner;
• 0 points otherwise.

When the tournament is finished, the person with the most points wins.

The architecture

• Kickbet app exposes a REST API and serves the UI;
• Redis is used as user session storage;
• SendGrid is used for sending emails via SMTP.

The deployment

I use an ansible playbook to set up the VM with:

• Docker + docker-compose;
• NGINX;
• certbot;

NGINX is configured to handle HTTPS and to rewrite the domain in the cookies to/from localhost to the production domain.

Whenever I need to deploy another version of the application, I use another playbook which deploys the docker-compose.yml with the application (a specific docker image tag) and its dependencies.

I also set up two cron jobs:

• one which creates a backup of the database and sends it to S3 once a day;
• another one for renewing the SSL certificates;

Everything runs on a single $6 / month DigitalOcean droplet.

The tech stack for the app

Kotlin, Spring Boot, React. The React app is bundled and served as static content by the Kotlin app.

The fun stuff

• hexagonal architecture on the server side;
• business logic for automatic assignment of teams to a particular knockout stage match. Also, the logic that moves the teams up the knockout stage ladder;
• a database-based task queue based on a SELECT ... FOR UPDATE SKIP LOCKED query;
• a tree of the knockout stage made only with a couple of <div> and a splash of CSS;

Lessons learned

Redis not really needed, but doesnt’t hurt at all to use it

For such a simple app with such a small number of users, I believe I could simply use the database to store the user sessions. Anyway, I think I spent less time configuring redis than I would’ve spent setting up the DB schema for storing the sessions.

Remember to set up the firewall

I initially forgot to set up the firewall on the VM. It didn’t even take 24 hours to kill my redis instance. I believe this stackoverflow post describes what happened to my server.