Docker : java.net.ConnectException: Connection refused - Application running at port 8083 is not able to access other application on port 3000

Question

I have to consume an external rest API(using restTemplate.exchange) with Spring Boot. My rest API is running on port 8083 with URL http://localhost:8083/myrest (Docker command : docker run -p 8083:8083 myrest-app)

External API is available in form of public docker image and after running below command , I am able to pull and run it locally.

 docker pull dockerExternalId/external-rest-api docker
    run -d -p 3000:3000 dockerExternalId/external-rest-api

a) If I enter external rest API URL, for example http://localhost:3000/externalrestapi/testresource directly in chrome, then I get valid JSON data.

b) If I invoke it with myrest application from eclipse(Spring Boot Application), still I am getting valid JSON Response. (I am using Windows Platform to test this)

c) But if I run it on Docker and execute myrest service (say http://localhost:8083/myrest), then i am facing java.net.ConnectException: Connection refused

More details :

org.springframework.web.client.ResourceAccessException: I/O error on GET request for "http://localhost:3000/externalrestapi/testresource": Connection refused (Connection refused); nested exception is java.net.ConnectException: Connection refused (Connection refused)

P.S - I am using Docker on Windows.

Answer 1

# The problem

You run with:

docker run -p 8083:8083 myrest-app

But you need to run like:

docker run --network "host" --name "app" myrest-app

So passing the flag --network with value host will allow you container to access your computer network.

Please ignore my first approach, instead use a better alternative that does not expose the container to the entire host network... is possible to make it work, but is not a best practice.

A Better Alternative

Create a network to be used by both containers:

docker network create external-api

Then run both containers with the flag --network external-api.

docker run --network "external-api" --name "app" -p 8083:8083 myrest-app

and

docker run -d --network "external-api" --name "api" -p 3000:3000 dockerExternalId/external-rest-api

The use of flag -p to publish the ports for the api container are only necessary if you want to access it from your computers browser, otherwise just leave them out, because they aren't needed for 2 containers to communicate in the external-api network.

TIP: docker pull is not necessary, once docker run will try to pull the image if does not found it in your computer. Let me know how it went...

Call the External API

So in both solutions I have added the --name flag so that we can reach the other container in the network.

So to reach the external api from my rest app you need to use the url http://api:3000/externalrestapi/testresource.

Notice how I have replaced localhost by api that matches the value for --name flag in the docker run command for your external api.

Answer 2

From your myrest-app container if you try to access http://localhost:3000/externalrestapi/testresource, it will try to access 3000 port of the same myrest-app container.

Because each container is a separate running Operating System and it has its own network interface, file system, etc.

Docker is all about Isolation.

There are 3 ways by which you can access an API from another container.

1. Instead of localhost, provide the IP address of the external host machine (i.e the IP address of your machine on which docker is running)
2. Create a docker network and attach these two containers. Then you can provide the container_name instead of localhost.
3. Use --link while starting the container (deprecated)