Fly.io vs Kamal for Solo Developers

Quick Comparison

Fly.io Overview

Fly.io deploys Docker containers as micro VMs across a global network of edge servers. Configure your app with fly.toml, deploy with a single command, and your app runs close to users worldwide. The platform handles networking, TLS, scaling, and load balancing. You get managed Postgres, persistent volumes, private service networking, and a free tier with 3 shared VMs.

Fly.io's strength is removing infrastructure decisions. You don't choose a server, configure a reverse proxy, or manage SSL certificates. The platform handles it. For solo developers who want to deploy and move on, it's an efficient workflow.

Kamal Overview

Kamal is an open-source deployment tool created by the team behind Rails. It takes a different approach entirely. Instead of providing infrastructure, Kamal deploys Docker containers to any server you can SSH into. You bring the server. Kamal handles building your Docker image, pushing it to a registry, SSHing into your machine, and swapping containers with zero downtime.

The workflow is centered around a deploy.yml config file. You define your app, the server addresses, environment variables, and accessories (databases, Redis). Then kamal setup bootstraps the server and kamal deploy pushes updates. The Kamal proxy handles routing and zero-downtime container swaps.

Kamal doesn't care where your server lives. Hetzner, DigitalOcean, AWS, a Raspberry Pi in your closet. If it has Docker and you can SSH in, Kamal will deploy to it. That flexibility is the whole point.

Key Differences

Platform vs. tool. Fly.io is a complete hosting platform. Kamal is a deployment tool. Fly.io includes servers, networking, databases, and monitoring. Kamal gives you a deploy pipeline and nothing else. You provide everything Kamal deploys to.

Server responsibility. With Fly.io, server management isn't your concern. Hardware failures, OS updates, security patches. Fly.io handles it. With Kamal, you manage the servers. If your VPS has a kernel vulnerability, you're patching it. If the disk fills up, you're fixing it. This is the fundamental tradeoff.

Global deployment. Fly.io deploys to 30+ regions with a config line. Kamal deploys to whatever servers you list in your config. You could theoretically achieve multi-region deployment with Kamal by adding servers in different locations, but you'd need to manage load balancing, DNS routing, and synchronization yourself. Fly.io makes this trivial.

Cost. Kamal on a $5/month VPS beats Fly.io on price for single-region deployments. A $6 Hetzner box running a Kamal-deployed app with a Docker Postgres container handles what might cost $15-25/month on Fly.io. The more services you run, the bigger the gap.

Deployment speed. Fly.io builds happen on their infrastructure. Push and wait 1-2 minutes. Kamal builds Docker images locally (or in CI), pushes to a registry, then deploys. The total time depends on your build speed and registry upload time. For large images, Kamal can be slower. For small images with a fast connection, they're comparable.

Zero downtime. Both achieve zero-downtime deploys. Fly.io does it through rolling VM updates. Kamal does it through the Kamal proxy, which routes traffic to the new container before stopping the old one. Both work well in practice.

Database management. Fly.io offers managed Postgres deployed as a Fly app. Kamal deploys databases as Docker accessories on your server. Neither is a fully managed database service like RDS. With Kamal, you get direct access to the container and full control over configuration. With Fly.io, you get a more abstracted but slightly easier setup.

When to Choose Fly.io

• You want managed infrastructure without server management
• Global multi-region deployment is a requirement
• You prefer deploying without maintaining your own servers
• Built-in networking and secrets management matters
• The free tier is useful for your use case

When to Choose Kamal

• You already have servers or VPS credits
• Long-term cost savings outweigh setup convenience
• You want complete infrastructure independence
• You're comfortable with Docker, SSH, and basic server admin
• You want to deploy to any provider without platform lock-in

The Verdict

Fly.io is the managed option. Kamal is the DIY option. Both deploy Docker containers. Both achieve zero downtime. The difference is who manages the servers.

For solo developers who value their time above everything else, Fly.io is the pragmatic choice. You deploy and forget. No server maintenance, no security patches, no disk space monitoring. The platform tax you pay in monthly fees buys back hours of operational work.

For solo developers who are comfortable with servers and want maximum control over their stack, Kamal is an excellent tool. It's lean, opinionated in the right ways, and works with any infrastructure provider. The combination of Kamal plus a cheap VPS gives you a professional deployment pipeline at minimal cost.

My recommendation: if you have a VPS sitting around or you're cost-sensitive, learn Kamal. It's a skill that pays for itself. If you want to focus purely on building and don't want to think about servers, Fly.io is the faster path. Both are solid choices for solo developers.