Tips for deploying Rails with Dokku on Hetzner

This post is a collection of tips and tricks I’ve gathered over the years, while running my own Rails apps on a VPS with Dokku.

I will be avoiding things you can already find in Dokku’s documentation, which I highly recommend reading through if you haven’t already.

This post is a work-in-progress, I will be updating it as I go. It should only serve as a quick reference.

Performance

Enable YJIT

YJIT is a just-in-time compiler by Shopify that can significantly boost your app’s performance.

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dokku config:set rails-app RUBYOPT="--enable-yjit"

Use jemalloc

jemalloc is an efficient memory allocator that can greatly reduce your app’s memory usage.

Add the jemalloc buildpack to your .buildpack file:

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https://github.com/gaffneyc/heroku-buildpack-jemalloc.git
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https://github.com/heroku/heroku-buildpack-ruby.git

Or set buildpacks via command line:

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dokku buildpacks:add rails-app https://github.com/gaffneyc/heroku-buildpack-jemalloc.git

Then enable the environment variable:

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dokku config:set app JEMALLOC_ENABLED=true

Reliability

Auto-restore

Automatically start/restore your Dokku containers on reboot to reduce downtime. Using systemd, we can create a simple service that runs dokku ps:restore on startup.

Create the systemd service file:

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sudo nano /etc/systemd/system/dokku-restore.service

Paste the following into the file:

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[Unit]
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Description=Restore Dokku Containers at Boot
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After=docker.service
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Requires=docker.service
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[Service]
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Type=oneshot
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ExecStart=/usr/bin/dokku ps:restore
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RemainAfterExit=true
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[Install]
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WantedBy=multi-user.target

Reload systemd and enable the service:

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sudo systemctl daemon-reexec
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sudo systemctl daemon-reload
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sudo systemctl enable dokku-restore.service

Optionally, start it now to test:

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sudo systemctl start dokku-restore.service

This runs dokku ps:restore once at boot, after Docker is ready.

Cache cleanup

Frequent deployments with Dokku can lead to Docker’s cache becoming full, potentially exhausting system disk space. Schedule a daily Docker build-cache cleanup with cron.

Open crontab:

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sudo crontab -e

The first time you run this you may be asked which editor to use. Pick your favorite (nano is fine).

Insert this single line, then save and exit:

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# Purge Docker build cache every day at 04:05 server time
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5 4 * * * /usr/bin/docker builder prune -f 2>&1 | /usr/bin/logger -t docker-prune

5 4 * * * schedules the task to run at minute 5 of hour 4 every day. The output is piped to logger which writes to journald, and you can view the logs later with journalctl -t docker-prune.

Verify it’s registered:

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sudo crontab -l

You should see the line you just added.

Enable Swap

Deployments can consume significant system resources, particularly RAM, during resource-intensive builds. If the system exhausts its memory, it may hang or crash, potentially bringing down your application.

Swap space is a designated portion of your system’s disk that acts as an extension of RAM. It is particularly beneficial for handling long-running or resource-demanding builds.

Enabling Swap

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sudo fallocate -l 2G /swapfile && \
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sudo chmod 600 /swapfile && \
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sudo mkswap /swapfile && \
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sudo swapon /swapfile && \
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echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Verify

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free -h          # swap row ≈ 2.0G
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swapon --show    # lists /swapfile size 2G

Rollback (just in case)

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sudo swapoff /swapfile
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sudo rm /swapfile
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sudo sed -i '/\/swapfile none swap/d' /etc/fstab

If you’re still timing out, you’re probably CPU-bound. Build in CI and dokku tags:deploy or upgrade your VPS.

Security

Configure Firewall

Configure your firewall to allow only HTTPS traffic from Cloudflare. See the guide from Hetzner for more details:

https://community.hetzner.com/tutorials/cloudflare-website-protect

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{
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  "formation": {
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    "web": {
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      "quantity": 1
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    }
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  },
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  "cron": [
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    {
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      "command": "bundle exec rake foo:bar",
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      "schedule": "0 2 * * *"
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    }
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  ],
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  "healthchecks": {
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    "web": [
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      {
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        "type": "startup",
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        "description": "Checking if the app responds to the /up endpoint",
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        "path": "/up",
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        "attempts": 3
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      }
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    ]
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  }
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}

GitHub Auto-Deploy

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name: Deploy to Dokku
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on:
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  push:
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    branches:
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      - main
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  workflow_dispatch:
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jobs:
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  deploy:
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    runs-on: ubuntu-22.04
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    if: ${{ !contains(toJSON(github.event.head_commit.message), '[skip deploy]') }}
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    steps:
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      - name: Cloning repo
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        uses: actions/checkout@v3
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        with:
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          fetch-depth: 0
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      - name: Push to dokku
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        uses: dokku/github-action@master
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        with:
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          branch: main
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          git_remote_url: ${{ vars.GIT_REMOTE_URL }}
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          ssh_private_key: ${{ secrets.SSH_PRIVATE_KEY }}