Curl application running in a container from another container - docker

Question

I have created a docker network with:
docker network create --driver bridge sample-network
Next I start two containers on that network:
docker run -it --network sample-network -p 8080:8080 --name frontend-container frontend-image
docker run -it --network sample-network -p 8082:8080 --name backend-container backend-image
and the result:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
e5328faa21db frontend-image "docker-entrypoint.s…" 33 minutes ago Up 33 minutes 0.0.0.0:8080->8080/tcp frontend-container
e13d798edbec backend-image "java -jar backend-0…" About an hour ago Up About an hour 0.0.0.0:8082->8080/tcp backend-container
Where the backend-container runs a spring boot web application
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v2.1.7.RELEASE)
2019-10-21 18:29:40.487 INFO 1 --- [ main] hello.Application : Starting Application v0.1.0-SNAPSHOT on e13d798edbec with PID 1 (/backend-0.1.0-SNAPSHOT.jar started by root in /)
2019-10-21 18:29:40.489 INFO 1 --- [ main] hello.Application : No active profile set, falling back to default profiles: default
2019-10-21 18:29:41.289 INFO 1 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 8080 (http)
2019-10-21 18:29:41.321 INFO 1 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2019-10-21 18:29:41.321 INFO 1 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet engine: [Apache Tomcat/9.0.22]
2019-10-21 18:29:41.399 INFO 1 --- [ main] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2019-10-21 18:29:41.399 INFO 1 --- [ main] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 873 ms
2019-10-21 18:29:41.609 INFO 1 --- [ main] o.s.s.concurrent.ThreadPoolTaskExecutor : Initializing ExecutorService 'applicationTaskExecutor'
2019-10-21 18:29:41.758 INFO 1 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 8080 (http) with context path ''
2019-10-21 18:29:41.761 INFO 1 --- [ main] hello.Application : Started Application in 1.604 seconds (JVM running for 1.889)
Next I would like to curl the backend-container from the frontend-container:
$ docker exec -it frontend-container /bin/bash
bash-4.4# curl backend-container:8082
curl: (7) Failed to connect to backend-container port 8082: Connection refused
But why do I get a connection refused? They are both on the same network.

Answer 1

Both of your containers run applications on port 8080. So to connect to the backend application over your network you should use backend-container:8080 as host.
It seems that you published port 8082 of backend-container to your host - but that does not mean that you can connect to app on this port from another container - it would work if you wanted to access backend-container from host using localhost:8082.
On how -p option works refer to container networking :
By default, when you create a container, it does not publish any of its ports to the outside world. To make a port available to services outside of Docker, or to Docker containers which are not connected to the container’s network, use the --publish or -p flag. This creates a firewall rule which maps a container port to a port on the Docker host.