docker-compose hostname to communicate between containers works with postgres but not app - docker

Question

I have the following docker-compose.yml:
services:
postgres:
image: "postgres:11.0-alpine"
app:
build: .
ports:
- "4000:4000"
depends_on:
- postgres
- nuxt
nuxt:
image: node:latest
ports:
- "3000:3000"
I need nuxt service to communicate with app.
Within the nuxt service (docker-compose run --rm --service-ports nuxt bash), if I run
root#62cafc299e8a:/app# ping postgres
PING postgres (172.18.0.2) 56(84) bytes of data.
64 bytes from avril_postgres_1.avril_default (172.18.0.2): icmp_seq=1 ttl=64 time=0.283 ms
64 bytes from avril_postgres_1.avril_default (172.18.0.2): icmp_seq=2 ttl=64 time=0.130 ms
64 bytes from avril_postgres_1.avril_default (172.18.0.2): icmp_seq=3 ttl=64 time=0.103 ms
but if I do:
root#62cafc299e8a:/app# ping app
ping: app: No address associated with hostname
Why does it work for postgres but not with app?
If I do docker network inspect 4fcb63b4b1c9, they appear to all be on the same network:
[
{
"Name": "myapp_default",
"Id": "4fcb63b4b1c9fe37ebb26e9d4d22c359c9d5ed6153bd390b6f0b63ffeb0d5c37",
"Created": "2019-05-16T16:46:27.820758377+02:00",
"Scope": "local",
"Driver": "bridge",
"EnableIPv6": false,
"IPAM": {
"Driver": "default",
"Options": null,
"Config": [
{
"Subnet": "172.18.0.0/16",
"Gateway": "172.18.0.1"
}
]
},
"Internal": false,
"Attachable": true,
"Ingress": false,
"ConfigFrom": {
"Network": ""
},
"ConfigOnly": false,
"Containers": {
"53b726bdd01159b5f18e8dcb858e979e6e2f8ef68c62e049b824899a74b186c3": {
"Name": "myapp_app_run_c82e91ca4ba0",
"EndpointID": "b535b6ca855a5dea19060b2f7c1bd82247b94740d4699eff1c8669c5b0677f78",
"MacAddress": "02:42:ac:12:00:03",
"IPv4Address": "172.18.0.3/16",
"IPv6Address": ""
},
"62cafc299e8a90fd39530bbe4a6af8b86098405e54e4c9e61128539ffd5ba928": {
"Name": "myapp_nuxt_run_3fb01bb2f778",
"EndpointID": "7eb8f5f8798baee4d65cbbfe5f0f5372790374b48f599f32490700198fa6d54c",
"MacAddress": "02:42:ac:12:00:04",
"IPv4Address": "172.18.0.4/16",
"IPv6Address": ""
},
"9dc1c848b2e347876292650c312e8aaf3f469f2efa96710fb50d033b797124b4": {
"Name": "myapp_postgres_1",
"EndpointID": "a925438ad5644c03731b7f7c926cff095709b2689fd5f404e9ac4e04c2fbc26a",
"MacAddress": "02:42:ac:12:00:02",
"IPv4Address": "172.18.0.2/16",
"IPv6Address": ""
}
},
"Options": {},
"Labels": {
"com.docker.compose.network": "default",
"com.docker.compose.project": "myapp",
"com.docker.compose.version": "1.23.2"
}
}
]
So why is that? Also tried with aliases, without success. :(

Answer 1

Your app container is most likely not running. Its appearance in docker network inspect means that the container exists but it may be exited (i.e. is not running). You can check with docker ps -a, for example:
$ docker ps -a
CONTAINER ID ... STATUS ... NAMES
fe908e014fdd Exited (0) Less than a second ago so_app_1
3b2ca418c051 Up 2 minutes so_postgres_1
container app exists but is not running: you won't be able to ping it even if it exists in the network
container postgres exists and is running: you will be able to ping it
It's probably due to the fact that docker-compose run --rm --service-ports nuxt bash will only create and run the nuxt container, it won't run app nor postgres. You are able to ping postgres because it was already running before you used docker-compose run nuxt bash
To be able to ping other containers after running docker-compose run nuxt ..., you should either:
Have the other containers already running before (such as by using docker-compose up -d)
Have the other containers depends_on the container you are trying to run, for example:
nuxt:
image: node:latest
ports:
- "3000:3000"
# this will ensure posgres and app are run as well when using docker-compose run
depends_on:
- app
- nuxt
Even with that, your container may fail to start (or exit right after start) and you won't be able to ping it. Check with docker ps -a that it is running and docker logs to see why it may have exited.

Answer 2

As #Pierre said, most probably your container is not running.
From below docker-compose from your question, it seems you are not doing anything in that container such as running a server or uwsgi to keep it alive.
app:
build: .
ports:
- "4000:4000"
depends_on:
- postgres
- nuxt
To keep it running in docker compose, add command directive like below.
app:
build: .
command: tail -f /dev/null #trick to stop immediate exit and keep the container alive.
ports:
- "4000:4000"
depends_on:
- postgres
- nuxt
It should be 'pingable' container now.
If you wish to run via docker run, use -t, which creates a psuedo-tty
docker run -t -d <image> <command>