Socket connections between docker containers fails - docker

Question

I have multiple containers being deployed through a docker-compose file seen below
version: '3'
services:
module2:
restart: always
build:
dockerfile: Dockerfile
context: ./Module-2
ports:
- '16667:16667'
module3:
build:
dockerfile: Dockerfile
context: ./Module-3
ports:
- '16669:16669'
Module 2 takes a socket request from an outside source and works as intended. The trouble begins when module 2 tries to connect with module 3
Module 2 code (JAVA)
private int socket_port = 16669;
private String server = "127.0.0.1";
public TextOutputSocket() {
}
public TextOutputSocket(String host, int socket_port) {
this.server = host;
this.socket_port = socket_port;
}
public void sendText(String textToSend) {
OutputStream os = null;
Socket sock = null;
try {
System.out.println("Connecting to " + server + ":" + socket_port);
sock = new Socket(server, socket_port);
os = sock.getOutputStream();
module 3 code (GO)
ln, err := net.Listen("tcp", ":16669")
if err != nil {
fmt.Println(err)
// handle error
}
Module 2 recieves a connection refused error when ever i try to send the request.
I feel I don't have the best understanding of docker networks and i assume this is where the problem lies.
Thank you for the help in advance

Answer 1

In your case, when you spin up docker-compose, module2 and module3 2 containers will be in the same docker network and they can connect to each other using their DNS names i.e. module2 and module3 respectively.
As a result, you should update your module2 code to be like this
private int socket_port = 16669;
private String server = "module3";
public TextOutputSocket() {
}
...
Note that you will not need to do a port mapping like - '16667:16667' or - '16669:16669' in order for these 2 modules to talk to each other.

Answer 2

First you need to understand how docker containers work. Each of you applications are deployed in two seperate containers. So when trying to connect to a different container you need to give the ip or the hostname of that specific container.
Here you have tried to connect to 1669 of localhost, instead what you should be doing is try to connect to the other container. This can be done by setting the container name of the module3 container and docker dns will resolve the ip address for you.
Simple replace 127.0.0.1 with module3