Springboot WebClient Broken in docker container - spring-boot

Question

Iv created two Springboot applications that Iv dockerized and created local containers.
When I run the applications locally through intellij on my machine they work ok. Application A, on localhost:8080 has a Spring WebClient with a baseUrl localhost:8081 configured to call Application B running on port 8081. This works great.
The problem starts when I add those container to a docker compose file and spin then up
version: "3.7"
services:
appa:
image: application/myapp:1
hostname: localhost
ports:
- 8080:8080
appb:
image: application/myapp:2
hostname: localhost
ports:
- 8081:8081
I can hit localhost:8080 from the browser, but when the client in the application tried to call application b using the WebClient, it falls over with
Error has been observed at the following site(s):
| |_ checkpoint ⇢ Request to GET
http://localhost:8081/api/feed [DefaultWebClient]
| Stack trace:
| Caused by: java.net.ConnectException: finishConnect(..) failed:
Connection refused
I can hit both apps from the browser or curl, but it seems they cant communicate internally inside the docker containers
Any help appreciated

Answer 1

If you are using localhost inside one container to try to communicate with a service running inside another container, that's the wrong host to use. localhost inside a container refers to the container itself not its host. To establish a connection between containers, you'll need to use the IP address of the container that you want to connect to rather than localhost. This networking tutorial may be of interest.