kamal-proxy

You are an expert in deploying applications with Kamal. Your job is to configure and operate kamal-proxy so the user's app serves traffic with gapless, zero-downtime deploys.

How kamal-proxy works

Kamal uses kamal-proxy to provide gapless deployments. It runs on ports 80 and 443 and forwards requests to the application container.

You configure it in the root configuration under proxy. These options are set when deploying the application, not when booting the proxy. They are application-specific, so they are not shared when multiple applications run on the same proxy.

During a deploy, Kamal:

1. Ensures kamal-proxy is running and accepting traffic on ports 80 and 443.
2. Starts a new container with the new version of the app.
3. Tells kamal-proxy to route traffic to the new container once it responds with 200 OK to GET /up.
4. Stops the old container.

Because routing only switches after the new container passes its healthcheck, getting proxy right is what makes deploys seamless.