I'm having problems getting two Docker containers to talk to each other using Docker Compose. In an attempt to reduce the problem down to the smallest reproducible example, I have created the following three files:
docker-compose.yml
services:
db:
build: webSQL
ports:
- "3000:3000"
client-test:
build: clientTest
webSQL/Dockerfile
FROM ubuntu
RUN apt update
RUN apt install netcat-traditional
RUN nc -l 3000
clientTest/Dockerfile
FROM ubuntu
RUN apt update
RUN apt install curl -y
RUN curl -X "hello" http://db:3000
When I run docker-compose up in the directory containing the docker-compose.yml file, I get the following output:
[+] Building 4.1s (11/11) FINISHED docker:default
=> [client-test internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 131B 0.0s
=> [db internal] load metadata for docker.io/library/ubuntu:latest 0.0s
=> [client-test internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [db internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 122B 0.0s
=> [db 1/4] FROM docker.io/library/ubuntu:latest 0.0s
=> CACHED [db 2/4] RUN apt update 0.0s
=> CACHED [client-test 3/4] RUN apt install curl -y 0.0s
=> ERROR [client-test 4/4] RUN curl -X "hello" http://db:3000 4.0s
=> [db internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> CACHED [db 3/4] RUN apt install netcat-traditional 0.0s
=> CANCELED [db 4/4] RUN nc -l 3000 4.0s
------
> [client-test 4/4] RUN curl -X "hello" http://db:3000:
0.641 % Total % Received % Xferd Average Speed Time Time Time Current
0.641 Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- 0:00:02 --:--:-- 0curl: (6) Could not resolve host: db
------
failed to solve: process "/bin/sh -c curl -X \"hello\" http://db:3000" did not complete successfully: exit code: 6
My understanding of Docker Compose is that every container can automatically make requests to every other container in the same Docker Compose file. (Assuming you're not specifying different networks for the containers to run in) What am I doing wrong?
RUN statements are executed at build time. At build time, the Docker network isn't available and you can't connect to other containers.
The Docker network is set up at run-time and it's at that time that containers can connect to each other.
To specify a command that runs at run-time, you use the ENTRYPOINT statement in connection with the CMD statement.
When you have containers that depend on other containers, you can specify that relationship in depends_on statements in your Docker Compose file. Note that depends_on only waits for the containers to be started. Not for the containers to be ready for connections. Often containers like databases do some work at startup and are not immediately available for connections.
To fix your setup, we can add depends_on to your compose file
services:
db:
build: webSQL
ports:
- "3000:3000"
client-test:
build: clientTest
depends_on:
- db
And your db Dockerfile
FROM ubuntu
RUN apt update
RUN apt install netcat-traditional
CMD nc -l 3000
And your clientTest Dockerfile
FROM ubuntu
RUN apt update
RUN apt install curl -y
CMD curl -X "hello" http://db:3000