How to Connect Docker Containers

When using Docker to containerize your applications, it is common practice to run each component of the application in a separate container. For example, a website might have a web server, application, and database, each running in its own container.

Configuring the containers to communicate with each other and the host machine can be a challenge. This guide will use a simple example app to demonstrate the basics of Docker container communication. The app will consist of a Node.js app that reads data from a PostgreSQL database.

Before You Begin

Install Docker CE

You will need a Linode with Docker CE installed to follow along with the steps in this guide.

To install Docker CE (Community Edition), follow the instructions within one of the guides below:

For complete instructions on even more Linux distributions, reference the Install Docker Engine section of Docker’s official documentation.

Example Node.js Application

The example app used throughout this guide will be a simple Node.js app that will read “Hello world” from a PostgreSQL database and print it to the console. In this section, you will build and test the app on your Linode without using containers.

Install and Configure PostgreSQL

Update your system:
```
sudo apt update && sudo apt upgrade
```

Install PostGreSQL:

sudo apt install postgresql postgresql-contrib

Change the postgres user’s password:
```
sudo passwd postgres
```

Set a password for the postgres database user:

su - postgres
psql -d template1 -c "ALTER USER postgres WITH PASSWORD 'newpassword';"

Create a database for the example app and connect to it:
```
createdb nodejs
psql nodejs
```

Add “Hello world” to the database:

nodejs=# CREATE TABLE hello (message varchar);
nodejs=# INSERT INTO hello VALUES ('Hello world');
nodejs=# \q

Create a dump of the database for later use:
```
pg_dumpall > backup.sql
```
Sign out as the postgres Linux user:
```
exit
```

Copy the data dump to your home directory:

sudo cp /var/lib/postgresql/backup.sql ~/.

Since you will be connecting to this database from a container (which will have an IP address other than locahost), you will need to edit the PostgreSQL config file to allow connections from remote addresses. Open /etc/postgresql/9.5/main/postgresql.conf in a text editor. Uncomment the listen_addresses line and set it to ‘*’:

File: /etc/postgresql/9.5/main/postgresql.conf

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#------------------------------------------------------------------------------
# CONNECTIONS AND AUTHENTICATION
#------------------------------------------------------------------------------

# - Connection Settings -

listen_addresses = '*'                  # what IP address(es) to listen on;

Enable and start the postgresql service:

sudo systemctl enable postgresql
sudo systemctl start postgresql

Create a Hello World App

Install Node and NPM:

curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
sudo apt-get install nodejs

Navigate to the home directory and create a directory:
```
cd
mkdir app && cd app
```

Using a text editor, create app.js and add the following content:

File: app.js

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const { Client } = require('pg')

const client = new Client({
  user: 'postgres',
  host: 'localhost',
  database: 'nodejs',
  password: 'newpassword',
  port: 5432
})

client.connect()

client.query('SELECT * FROM hello', (err, res) => {
  console.log(res.rows[0].message)
  client.end()
})

This app uses the pg NPM module (node-postgres) to connect to the database created in the previous section. It then queries the ‘hello’ table (which returns the “Hello world” message) and logs the response to the console. Replace 'newpassword' with the postgres database user password you set in the previous section.

Note
The pg module can also use environment variables to configure the client connection. This is the recommended option for production apps. Read more about environment variables in the node-postgres documentation.

Install the pg module:
```
npm install pg
```
Test the app:
```
node app.js
```
If the database is configured correctly, “Hello world” will be displayed on the console.

Connect Container to Docker Host

This section illustrates a use case where the Node.js app is run from a Docker container, and connects to a database that is running on the Docker host.

Set Up Docker Container

Return to your home directory:
```
cd
```

Create a Dockerfile to run the Node.js app:

File: Dockerfile

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FROM debian

RUN apt update -y && apt install -y gnupg curl
RUN curl -sL https://deb.nodesource.com/setup_8.x | bash - && apt install -y nodejs
COPY app/ /home/

ENTRYPOINT tail -F /dev/null

The image built from this Dockerfile will copy the app/ directory to the new image. Edit app.js to allow the app to connect to the database host instead of localhost:

File: app/app.js

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const client = new Client({
  user: 'postgres',
  host: 'database',
  database: 'nodejs',
  password: 'newpassword',
  port: 5432
})

Build an image from the Dockerfile:
```
docker build -t node_image .
```

Connect Container to Database

Docker automatically sets up a default bridge network, accessed through the docker0 network interface. Use ifconfig or ip to view this interface:

ifconfig docker0

The output will resemble the following:

docker0   Link encap:Ethernet  HWaddr 02:42:1e:e8:39:54
  inet addr:172.17.0.1  Bcast:0.0.0.0  Mask:255.255.0.0
  inet6 addr: fe80::42:1eff:fee8:3954/64 Scope:Link
  UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
  RX packets:3848 errors:0 dropped:0 overruns:0 frame:0
  TX packets:5084 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:0
  RX bytes:246416 (246.4 KB)  TX bytes:94809688 (94.8 MB)

The internal IP address of the Docker host (your Linode) is 172.17.0.1.

Allow PostgreSQL to accept connections from the Docker interface. Open /etc/postgresql/9.5/main/pg_hba.conf in a text editor and add the following line:
File: /etc/postgresql/9.5/main/pg_hba.conf
1
host all postgres 172.17.0.0/16 password
Since 172.17.0.1 is the IP of the Docker host, all of the containers on the host will have an IP address in the range 172.17.0.0/16.
Restart the database:
```
sudo systemctl restart postgresql
```
Start the container:
```
docker run -d --add-host=database:172.17.0.1 --name node_container node_image
```
The --add-host option defines a database host, which points to the IP address of the Docker host. Declaring the database host at runtime, rather than hard-coding the IP address in the app, helps keep the container reusable.
From within the container, use ping to test the connection to the database host:
```
docker exec -it node_container ping database
```
Each Docker container is also assigned its own IP address from within the 172.17.0.0/16 block. Find the IP address of this container with ip:
```
docker exec -it node_container ip addr show eth0
```
You can test this connection by pinging this address from the Docker host.

Run the app:

docker exec -it node_container node home/app.js

If the configuration was successful, the program should display the “Hello world” console output as before.

Connect Two Containers

In this section, both the app and database will be running in separate containers. You can use the official postgres image from Docker Hub and load in the SQL dump created earlier.

Caution
You should not store production database data inside a Docker container. Containers should be treated as ephemeral entities: if a container unexpectedly crashes or is restarted, all data in the database will be lost.

Stop and remove the Node.js container:

docker stop node_container
docker rm node_container

Pull the postgres image:
```
docker pull postgres
```
Make sure your backup.sql file is in your current working directory, then run the postgres image:
```
docker run -d -v `pwd`:/backup/ --name pg_container postgres
```
The -v option mounts your current working directory to the /backup/ directory on the new container.
The new container will automatically start the postgres database and create the postgres user. Enter the container and load the SQL dump:
```
docker exec -it pg_container bash
cd backup
psql -U postgres -f backup.sql postgres
exit
```
Run the node image again. This time, instead of --add-host, use the --link option to connect the container to pg_container:
```
docker run -d --name node_container --link=pg_container:database node_image
```
This will link the pg_container under the hostname database.
Open /etc/hosts in node_container to confirm that the link has been made:
```
docker exec -it node_container cat /etc/hosts
```
There should be a line similar to the following:
File: /etc/hosts
```
172.17.0.2  database  pg_container
```
This shows that pg_container has been assigned to the IP address 172.17.0.2, and is linked to this container via the hostname database, as expected.
Since the Node.js app is still expecting to connect to a PostgreSQL database on the database host, no further changes are necessary. You should be able to run the app as before:
```
docker exec -it node_container node home/app.js
```

Using Docker Compose

Using the --link or --host options every time you launch your containers can be cumbersome. If your server or any of the containers crash, they must be manually reconnected. This is not an ideal situation for any application that requires constant availability. Fortunately, Docker provides Docker Compose to manage multiple containers and automatically link them together when they are launched. This section will use Docker Compose to reproduce the results of the previous section.

Note
For a more comprehensive explanation of Docker Compose and how to write docker-compose.yml configuration files, see our complete Docker Compose guide.

Install Docker Compose:

sudo curl -L https://github.com/docker/compose/releases/download/1.17.0/docker-compose-`uname -s`-`uname -m` -o /usr/local/bin/docker-compose
sudo chmod +x /usr/local/bin/docker-compose

In the same directory as your Dockerfile, create a docker-compose.yml file with the following content:

File: docker-compose.yml

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version: '3'

services:
  database:
    image: postgres
    container_name: pg_container
    volumes:
       - pgdata:/var/lib/postgresql/data

  app:
    build: .
    container_name: node_container
    links:
     - database
    environment:
      - PGPASSWORD=newpassword
      - PGUSER=postgres
      - PGDATABASE=nodejs
      - PGHOST=database
      - PGPORT=5432
    depends_on:
      - database

volumes:
  pgdata: {}

When you run Docker Compose with this file, it will create the pg_container and node_container from the previous section. As before, the database container will use the official PostgreSQL image, while the app container will be built from your Dockerfile. The links entry serves the same function as the --link option in the run command used earlier.

Docker Compose also allows you to set up environment values, so you can simplify the app to use these rather than having the values hard-coded. Edit app.js to remove these values:

File: app.js

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const express = require('express')
const { Client } = require('pg')

const client = new Client()

client.connect()

client.query('SELECT * FROM hello', (err, res) => {
  console.log(res.rows[0].message)
  client.end()
})

Remove the previous containers:

docker rm -f node_container pg_container

Use Docker Compose to bring up the containers:
```
docker-compose up -d
```

Load the example data into the new container:

docker cp backup.sql pg_container:/
docker exec -it pg_container psql -U postgres -f backup.sql postgres

Run app.js from the app container:

docker exec -it node_container node home/app.js

The app should run as before.

Conclusion

By default, Docker automatically assigns an IP address to each container and to the Docker host. You can manually connect services between containers by using these addresses (assuming that your firewall allows the connection).

However, Docker also provides a number of convenient wrappers around these connections to help you speed up and simplify the connection process. You can connect your Docker host to a container with a unique hostname, or directly link two containers. Using Docker Compose can simplify this process even further by allowing you to declare connections in the docker-compose.yml file so that they are automatically established when the containers are brought up.

There are other connection options that were not covered in this guide. For example, you can run a container using --net="host", which will share that container’s network stack with the Docker host: localhost on the container will point to localhost on the Docker host. You can also expose ports on each Docker container, or configure the default bridge network for more flexibility. For a more in-depth discussion of these options, see the links in the More Info section below.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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How to Connect Docker Containers

Before You Begin

Install Docker CE

Example Node.js Application

Install and Configure PostgreSQL

Create a Hello World App

Connect Container to Docker Host

Set Up Docker Container

Connect Container to Database

Connect Two Containers

Using Docker Compose

Conclusion

More Information

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