1. The problem I’m having:
Hi!
By default, when a service is scaled in Docker Compose using the deploy.replicas property (as can be seen in the example in the docker-compose.yml file), multiple instances of the same service are created and registered in the Docker DNS server with the same domain name but pointing to different IPs.
In this scenario, when configuring a reverse proxy to that service in Caddy Server, it is usually done as follows:
:80 {
reverse_proxy whoami:8000
}
With that configuration, when several consecutive calls are made to the final service through Caddy Server (calling localhost:8080), the requests are always resolved towards the same service and the same IP is always used.
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
~ ❯ curl http://localhost:8080
I'm fa9fca8a5293
On the other hand, if a dynamic upstream is used, as shown in the following configuration, the requests are balanced between the different instances created.
:80 {
reverse_proxy {
lb_policy round_robin
dynamic a {
name whoami
port 8000
refresh 1s
}
}
}
~ ❯ curl http://localhost:8080
I'm ece316ecfe32
~ ❯ curl http://localhost:8080
I'm b5cfbdb0ce4c
~ ❯ curl http://localhost:8080
I'm 8c63835eace1
~ ❯ curl http://localhost:8080
I'm b5cfbdb0ce4c
~ ❯ curl http://localhost:8080
I'm ece316ecfe32
~ ❯ curl http://localhost:8080
I'm b5cfbdb0ce4c
~ ❯ curl http://localhost:8080
I'm b5cfbdb0ce4c
The question is: when simply using reverse_proxy whoami:8000, does Caddy Server cache the first IP returned by the DNS server and does not make a balancing between the different entries that are registered in the Docker DNS server? To make a balancing between the different instances the only solution is to use a dynamic upstream? Services like Nginx by default perform that balancing between all DNS entries for a service, hence my question.
Thank you very much.
2. Error messages and/or full log output:
N/A
3. Caddy version:
v2.8.4 running in docker container
4. How I installed and ran Caddy:
Using docker with the following image:
caddy:latest
a. System environment:
- OS:
Ubuntu 24.04 - Docker Version: `Docker version 27.3.1, build ce12230``
b. Command:
docker compose up
c. docker-compose.yml:
services:
caddy:
image: caddy:latest
container_name: caddy
ports:
- 8080:80
volumes:
- ./Caddyfile:/etc/caddy/Caddyfile
whoami:
image: jwilder/whoami
deploy:
replicas: 3
d. Caddyfile:
- With standard reverse proxy
:80 {
reverse_proxy whoami:8000
}
- With dynamic upstream reverse proxy
:80 {
reverse_proxy {
lb_policy round_robin
dynamic a {
name whoami
port 8000
refresh 1s
}
}
}
5. Links to relevant resources:
Another post with a related question: Loadbalance docker compose replicas
