How do I connect to the localhost of a docker container (from inside the container)

Question

I have a nodejs app that connects to a blockchain on the same server. Normally I use 127.0.0.1 + the port number (each chain gets a different port).

I decided to put the chain and the app in the same container, so that the frontend developers don't have to bother with setting up the chain.

However, When I build the image the chain should start. When I run the image it isn't. Furthermore, when I do go in the container and try to run it manually it says "besluitChain2@xxx.xx.x.2:PORT". So I thought instead of 127.0.0.1 I needed to connect to the port on 127.0.0.2, but that doesn't seem to work.

I'm sure connecting like this isn't new, and should work the same with a database. Can anyone help? The first piece of advice would be how to debug these images, because I have no idea where it goes wrong.

here is my dockerfile

FROM ubuntu:16.04
RUN apt-get update
RUN apt-get install -y curl
RUN apt-get install -y apt-utils
RUN apt-get install -y build-essential
RUN curl -sL https://deb.nodesource.com/setup_6.x | bash -
RUN apt-get install -y nodejs
ADD workfolder/app /root/applications/app
ADD .multichain /root/.multichain
RUN npm install \
    && apt-get upgrade -q -y \
        && apt-get dist-upgrade -q -y \
        && apt-get install -q -y wget curl \
        && apt-get clean \
        && rm -rf /var/lib/apt/lists/* \
        && cd /tmp \
        && wget http://www.multichain.com/download/multichain-1.0-beta-1.tar.gz \
        && tar -xvzf multichain-1.0-beta-1.tar.gz \
        && cd multichain-1.0-beta-1 \
        && mv multichaind multichain-cli multichain-util /usr/local/bin \
        && cd /tmp \
        && rm -Rf multichain*
RUN multichaind Chain -daemon
RUN cd /root/applications/app && npm install
CMD cd /root/applications/app && npm start
EXPOSE 8080

btw due to policies I can only connect to the server at port 80 to check if it works. When I run the docker image I can go to my /api-docs but not to any of the endpoints where I start interacting with the blockchain.

Answer 1

I decided to put the chain and the app in the same container

That was a mistake, I think.

Docker is not a virtual machine. It's a virtual application or process instance.

A Docker container runs a linux distro under the hood, but this is a detail that should be ignored when thinking about the purpose of Docker.

You should think of a Docker container as a single application process, not as a full virtual machine to run generally run multiple processes. This is evidenced by the way Docker will shut the container down once the main process shuts down (the process with PID 1).

I've got a longer post about this, here: https://derickbailey.com/2016/08/29/so-youre-saying-docker-isnt-a-virtual-machine/

Additionally, the RUN multichaind instruction in your dockerfile doesn't run the chain in your image / container. It tells the image to run this instruction during the build process.

A Dockerfile is a list of instructions for building an image. The wording here is important. An image is not executed, it is built. An image is a static, immutable template from which a Container is executed.

RUN multichaind Chain -daemon

By putting this RUN instruction in your image, you are temporarily starting the chain, but it is immediately halted (forcefully) when the image layer is done building. It will not remain running, because an image is not executed, it is built.

My advice is to put the chain in a separate image.

You'll have one image for the chain, and one for the node.js app.

You can use docker-compose to make it easier to run containers from both of these at the same time. Or you can run containers manually from them. Either way, you need two images.