Stable container name for the web service

[miharekar]

Use case: I'd like to connect from Prometheus to my web service without going through the internet.

I can connect to other accessories like [service-name]-cadvisor:9200 but [service-name]-web:80 goes through Kamal Proxy and that one rejects it, because it doesn't recognize the host.

The actual container name of the web service changes with each deploy, so I can't really hardcode it in prometheus' config.yml.

Any other suggestions you might have of how I might achieve that?

[askadityapandey]

Docker network alias, umm could be a good one?

[djmb]

We run otel_collector as an accessory and connect prometheus to it. The collector is configured to pick up the app containers using docker discovery.

There's more info here - https://dev.37signals.com/kamal-prometheus/

[jonasbark]

I'm really not sure why this ticket was closed, as I've got the same issue and I don't think this is properly covered.

docker network inspect kamal will indeed tell you which endpoint can be used:

...
"f490b65d1ab7622c6a307c50a8a5144db41a55c3343582685c8726f5322de97d": {
                "Name": "cloudconnect-web-efd125f9a55a6f33627761e48ddf907031ee16bb",
...

but as @miharekar noted that name will change with each git commit.

docker network disconnect kamal f490b65d1ab7622c6a307c50a8a5144db41a55c3343582685c8726f5322de97d
docker network connect --alias cloudconnect kamal f490b65d1ab7622c6a307c50a8a5144db41a55c3343582685c8726f5322de97

Will work indeed, but I have not seen this option exposed.

An example post-deploy hook:

CONTAINER_ID=$(kamal server exec -q docker ps --filter "ancestor=ghcr.io/..." --format "{{.ID}}" | tail -n +2)
echo "Container ID: $CONTAINER_ID"
kamal server exec docker network disconnect kamal "$CONTAINER_ID";
kamal server exec docker network connect --alias cloudconnect kamal "$CONTAINER_ID";

[huseyin]

I ran into the same issue when setting up Prometheus without otel collector and solved it adding a post-deploy hook that adds an alias on every deploy (something similar to @jonasbark's approach). That said, I don’t think this case is unique to Prometheus. We could be emitting anything through any port with a listener consuming it, and in all those cases, we should be able to connect to the container using a stable hostname. Having an option to add aliases right in the config would indeed be good.

[pjpires]

An example post-deploy hook:

CONTAINER_ID=$(kamal server exec -q docker ps --filter "ancestor=ghcr.io/..." --format "{{.ID}}" | tail -n +2)
echo "Container ID: $CONTAINER_ID"
kamal server exec docker network disconnect kamal "$CONTAINER_ID";
kamal server exec docker network connect --alias cloudconnect kamal "$CONTAINER_ID";

@jonasbark this was super useful for me to be able to get an "accessory" (anycable-go) to connect with a service (RPC) running as a "server" (in kamal's terms), thank you very much! 🙏

However, in the end I think I found a simpler solution, so I'll leave it here in case someone finds it useful. Since kamal forwards the options: to the underlying docker run ... command, I was able to do this:

servers:
  anycable-rpc:
    hosts: <%= ENV['HOST'] %>
    options:
      network-alias: my-app-anycable-rpc

accessories:
  anycable-go:
    # ... other configs
    env:
      clear:
        ANYCABLE_RPC_HOST: "my-app-anycable-rpc:50051"

Note: I initially tried the name option instead, it's still forwarded to docker run but would make kamal fail in a subsequent docker container ls command as it is expecting to find the container by a different name.

Edit: fixed the example with the proper hosts: key, as pointed out in #1278 (comment) (thanks!).