Allow Kamal to run without traefik

[yoelcabo]

Related discussion: #345

I know Kamal is designed for deploying web applications, as DHH was mentioning here: #191 (comment)

That said, the code change is small, and I think there is a valid use case for not having a web role:

Our use case

We are slowly migrating out of Heroku/AWS. Our first step was migrating some background workers.

If a worker is unreliable, jobs will be picked by "someone else", but if a web container is unreliable, it can cause serious downtime. That's why we are keeping the web in Heroku for now, but the workers are slowly being moved to Hetzner with Kamal.

A workaround for us was running a web container that was never accessed. However, this makes the configuration confusing, creates unnecessary connections to the database and makes the deployment slower.

[djmb]

Hi @yoelcabo!

I agree with your reasoning for this change. I've asked for a couple of changes. I think you'll also need to make a few changes to get this working again after #577 was merged.

[djmb]

Could we raise an exception here instead and protect the callers with if statements? If someone managed to call this without a web role, I think it would be better to fail then to silently pass the check.

[djmb]

Again could we raise here instead of returning?

[yoelcabo]

Thanks a lot for the feedback 🙏 @djmb

I've addressed your comments and resolved the conflicts.

Due to #577 there are some important changes in my PR:

• Rename primary_web_role configuration variable to primary_role. This is a breaking change for anyone who adopted Support customizing the primary_web_role #577
• primary_role enables traefik by default. Support customizing the primary_web_role #577 was raising if it was not enabled. Since my PR makes it possible to run without traefik, I think it makes sense that enabling it for the primary is the default.

cc @mdkent

Regarding your comment on the running_traefik? check. This is my change:

• The check happens now in the deploy command instead. If traefik is disabled in the primary, the healthcheck is not performed.
• I added a raise for whenever someone invokes kamal healthcheck perform manually.
• I removed the check on the command builder because it doesn't seem a possible code path.

[yoelcabo]

KAMAL.config.role(KAMAL.config.primary_role) might be too verbose.

In my original changes, KAMAL.config.primary_role was an instance of Role.

However, with #577, KAMAL.config.primary_web_role was introduced as a String.

I didn't want to change that to make the review simpler and focusing on what matters. However, if you think a cleanup might be interesting, I can submit another PR afterwards

[djmb]

Hi @yoelcabo - yes if you could raise another PR to convert KAMAL.config.primary_role to a Role that would be great. I'll merge this PR now 🙏