Multiple Docker containers

Running multiple containers is really common - and that's why it a really good idea to learn about it. This is an article following an online tutorial.

Kalle Tolonen
Aug. 31, 2022

Cloning the source repository

First I cloned the demo repo to my home folder.

cd
git clone https://github.com/prakhar1989/FoodTrucks

The containers

The tutorial splits the demo app into two containers - one for the Flask process and one for the database(?). I had a previous Flask container so all that was needed was a container for Elasticsearch.

sudo docker search elasticsearch
sudo docker pull docker.elastic.co/elasticsearch/elasticsearch:8.4.0
sudo docker ps

The container is available for use, so we can try it out.

$sudo docker ps
CONTAINER ID   IMAGE                                                 COMMAND                  CREATED              STATUS              PORTS                                                                                  NAMES
9afb4c55be8d   docker.elastic.co/elasticsearch/elasticsearch:8.4.0   "/bin/tini -- /usr/l…"   About a minute ago   Up About a minute   0.0.0.0:9200->9200/tcp, :::9200->9200/tcp, 0.0.0.0:9300->9300/tcp, :::9300->9300/tcp   es
c9e14692f9dc   prakhar1989/static-site                               "./wrapper.sh"           26 hours ago         Up 26 hours         0.0.0.0:49154->80/tcp, :::49154->80/tcp, 0.0.0.0:49153->443/tcp, :::49153->443/tcp     static-site

sudo docker run -d --name es -p 9200:9200 -p 9300:9300 -e "discovery.type=single-node" docker.elastic.co/elasticsearch/elasticsearch:8.4.0

Since the container is now named as “es”, you can refer to it with that name instead of the id. I could not get the container to answer from the tutorial’s IP, so I tried to do things again with the same version as the tutorial had.

docker pull docker.elastic.co/elasticsearch/elasticsearch:6.3.2
sudo docker run -d --name es -p 9200:9200 -p 9300:9300 -e "discovery.type=single-node" docker.elastic.co/elasticsearch/elasticsearch:6.3.2

This time around I managed to reproduce the results.

  "name" : "gvHSlS9",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "0WMvJrL1Q3OQD-VVTGZynQ",
  "version" : {
    "number" : "6.3.2",
    "build_flavor" : "default",
    "build_type" : "tar",
    "build_hash" : "053779d",
    "build_date" : "2018-07-20T05:20:23.451332Z",
    "build_snapshot" : false,
    "lucene_version" : "7.3.1",
    "minimum_wire_compatibility_version" : "5.6.0",
    "minimum_index_compatibility_version" : "5.0.0"
  },
  "tagline" : "You Know, for Search"
}

Dockerfile

The Dockerfile is as follows:

# start from base
FROM ubuntu:18.04

LABEL maintainer="Prakhar Srivastav <prakhar@prakhar.me>"

# install system-wide deps for python and node
RUN apt-get -yqq update
RUN apt-get -yqq install python3-pip python3-dev curl gnupg
RUN curl -sL https://deb.nodesource.com/setup_10.x | bash
RUN apt-get install -yq nodejs

# copy our application code
ADD flask-app /opt/flask-app
WORKDIR /opt/flask-app

# fetch app specific deps
RUN npm install
RUN npm run build
RUN pip3 install -r requirements.txt

# expose port
EXPOSE 5000

# start app
CMD [ "python3", "./app.py" ]

Notice how piping commands can be done and a working directory can be set for subsequent commands.

sudo docker build -t kalletolonen/foodtruckdemo .

The flag “t” tells that the following string will be the tag for the image and the last period indicates that everything from the working directory should be included in the container.

Connecting the 2 containers

Let’s examine the networks Docker has creted for us.

sudo docker network ls

NETWORK ID     NAME      DRIVER    SCOPE
041be8b608b8   bridge    bridge    local
6337965bc456   host      host      local
8d10629d0230   none      null      local

Bridge is the network that is that used by default by containers. This can be validated by testing it:

sudo docker network inspect bridge

The resulting JSON tells us that the IP-address of the container.

Containers": {
            "648bbcc410082baa4bd1e94a8ae27e9c91c7ded0b38a6040094997465a226bb7": {
                "Name": "es",
                "EndpointID": "245b53d50d38837c84370d694d6e1d51f3b5199df603d4f8a9534a06c4a2f8b1",
                "MacAddress": "02:42:ac:11:00:03",
                "IPv4Address": "172.17.0.3/16",
                "IPv6Address": ""
            }

Next - let’s try connectiog to the es-container with the flask-container.

sudo docker run -it --rm kalletolonen/foodtrucktest bash

The it-flag makes the connection look like a regular terminal connection and rm-flag indicates that the container gets cleaned up after it’s exited.

root@8eda7f42cdc4:/opt/flask-app# pwd
/opt/flask-app
root@8eda7f42cdc4:/opt/flask-app# curl 172.17.0.3:9200
{
  "name" : "gvHSlS9",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "0WMvJrL1Q3OQD-VVTGZynQ",
  "version" : {
    "number" : "6.3.2",
    "build_flavor" : "default",
    "build_type" : "tar",
    "build_hash" : "053779d",
    "build_date" : "2018-07-20T05:20:23.451332Z",
    "build_snapshot" : false,
    "lucene_version" : "7.3.1",
    "minimum_wire_compatibility_version" : "5.6.0",
    "minimum_index_compatibility_version" : "5.0.0"
  },
  "tagline" : "You Know, for Search"
}

That worked as expected.

Creating a Docker network

The previous solution was great for testing the connection, and now we need to create a dedicated network.

sudo docker network create foodtrucknet

~/FoodTrucks$ sudo docker network ls
NETWORK ID     NAME           DRIVER    SCOPE
041be8b608b8   bridge         bridge    local
946d41a01d2d   foodtrucknet   bridge    local
6337965bc456   host           host      local
8d10629d0230   none           null      local

Next it’s time to remove the es container and make a new one with a –net flag to put it into our brand new net.

sudo docker container stop es
sudo docker container rm es
sudo docker run -d --name es --net foodtrucknet -p 9200:9200 -p 9300:9300 -e "discovery.type=single-node" docker.elastic.co/elasticsearch/elasticsearch:6.3.2
sudo docker network inspect foodtrucknet

"Containers": {
            "c96fd38ec00e5bfa4b5c8fabd98fe1cfd7ae012e3aed0dbfcaff39b143870d76": {
                "Name": "es",
                "EndpointID": "e9e4fe67c6c9a8ab2624eaf01936295969eaf3c9a422af1ac05b27eabc770d44",
                "MacAddress": "02:42:ac:12:00:02",
                "IPv4Address": "172.18.0.2/16",
                "IPv6Address": ""
            }

Foodtrucknet has the es container running in it. So let’s see what happens when we launch the Flask-container in the same network.

sudo docker run -it --rm --net foodtrucknet kalletolonen/foodtrucktest bash

root@04f3227e581d:/opt/flask-app# curl es:9200
{
  "name" : "OoIkiJY",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "wUf-xq3oQwqR8qHVkXzOCA",
  "version" : {
    "number" : "6.3.2",
    "build_flavor" : "default",
    "build_type" : "tar",
    "build_hash" : "053779d",
    "build_date" : "2018-07-20T05:20:23.451332Z",
    "build_snapshot" : false,
    "lucene_version" : "7.3.1",
    "minimum_wire_compatibility_version" : "5.6.0",
    "minimum_index_compatibility_version" : "5.0.0"
  },
  "tagline" : "You Know, for Search"
}

To launch the Flask container without the bash, you just need to change some flags.

sudo docker run -d --net foodtrucknet -p 5000:5000 --name foodtrucktest kalletolonen/foodtrucktest

The -d flag tell Docker to run in detached mode, ie. in the background.

Source(s)

Docker-curriculum
Elastic.co