Introduction

If you are actively developing an application, using Docker can simplify your workflow and the process of deploying your application to production. Working with containers in development offers the following benefits:

Environments are consistent, meaning that you can choose the languages and dependencies you want for your project without worrying about system conflicts.
Environments are isolated, making it easier to troubleshoot issues and onboard new team members.
Environments are portable, allowing you to package and share your code with others.

This tutorial will show you how to set up a development environment for a Node.js application using Docker. You will create two containers — one for the Node application and another for the MongoDB database — with Docker Compose. Because this application works with Node and MongoDB, your setup will do the following:

Synchronize the application code on the host with the code in the container to facilitate changes during development.
Ensure that changes to the application code work without a restart.
Create a user and password-protected database for the application’s data.
Persist this data.

At the end of this tutorial, you will have a working shark information application running on Docker containers:

Prerequisites

To follow this tutorial, you will need:

A development server running Ubuntu 18.04, along with a non-root user with sudo privileges and an active firewall. For guidance on how to set these up, please see this Initial Server Setup guide.
Docker installed on your server, following Steps 1 and 2 of How To Install and Use Docker on Ubuntu 18.04.
Docker Compose installed on your server, following Step 1 of How To Install Docker Compose on Ubuntu 18.04.

Step 1 — Cloning the Project and Modifying Dependencies

The first step in building this setup will be cloning the project code and modifying its package.json file, which includes the project’s dependencies. You will add nodemon to the project’s devDependencies, specifying that you will be using it during development. Running the application with nodemon ensures that it will be automatically restarted whenever you make changes to your code.
First, clone the nodejs-mongo-mongoose repository from the DigitalOcean Community GitHub account. This repository includes the code from the setup described in How To Integrate MongoDB with Your Node Application, which explains how to integrate a MongoDB database with an existing Node application using Mongoose.
Clone the repository into a directory called node_project:

git clone https://github.com/do-community/nodejs-mongo-mongoose.git node_project

Navigate to the node_project directory:

cd node_project

Open the project’s package.json file using nano or your favorite editor:

nano package.json

Beneath the project dependencies and above the closing curly brace, create a new devDependencies object that includes nodemon:
~/node_project/package.json

...
"dependencies": {
    "ejs": "^2.6.1",
    "express": "^4.16.4",
    "mongoose": "^5.4.10"
  },
  "devDependencies": {
    "nodemon": "^1.18.10"
  }    
}

Save and close the file when you are finished editing. If you’re using nano, press CTRL+X, then Y, then ENTER.
With the project code in place and its dependencies modified, you can move on to refactoring the code for a containerized workflow.

Step 2 — Configuring Your Application to Work with Containers

Modifying your application for a containerized workflow means making your code more modular. Containers offer portability between environments, and your code should reflect that by remaining as decoupled from the underlying operating system as possible. To achieve this, you will refactor your code to make greater use of Node’s process.env property. This returns an object with information about your user environment at runtime. You can use this object in your code to dynamically assign configuration information at runtime with environment variables.
Begin with app.js, your main application entrypoint. Open the file:

nano app.js

Inside, you will see a definition for a port constant, as well a listen function that uses this constant to specify the port the application will listen on:
~/home/node_project/app.js

...
const port = 8080;
...
app.listen(port, function () {
  console.log('Example app listening on port 8080!');
});

Redefine the port constant to allow for dynamic assignment at runtime using the process.env object. Make the following changes to the constant definition and listen function:
~/home/node_project/app.js

...
const port = process.env.PORT || 8080;
...
app.listen(port, function () {
  console.log(`Example app listening on ${port}!`);
});

Your new constant definition assigns port dynamically using the value passed in at runtime or 8080. Similarly, you’ve rewritten the listen function to use a template literal, which will interpolate the port value when listening for connections. Because you will be mapping your ports elsewhere, these revisions will prevent you having to continuously revise this file as your environment changes.
When you are finished editing, save and close the file.
Next, you will modify your database connection information to remove any configuration credentials. Open the db.js file, which contains this information:

nano db.js

Currently, the file does the following things:

Imports Mongoose, the Object Document Mapper (ODM) that you’re using to create schemas and models for your application data.
Sets the database credentials as constants, including the username and password.
Connects to the database using the mongoose.connect method.

For more information about the file, please see Step 3 of How To Integrate MongoDB with Your Node Application.
Your first step in modifying the file will be redefining the constants that include sensitive information. Currently, these constants look like this:
~/node_project/db.js

...
const MONGO_USERNAME = 'sammy';
const MONGO_PASSWORD = 'your_password';
const MONGO_HOSTNAME = '127.0.0.1';
const MONGO_PORT = '27017';
const MONGO_DB = 'sharkinfo';
...

Instead of hardcoding this information, you can use the process.env object to capture the runtime values for these constants. Modify the block to look like this:
~/node_project/db.js

...
const {
  MONGO_USERNAME,
  MONGO_PASSWORD,
  MONGO_HOSTNAME,
  MONGO_PORT,
  MONGO_DB
} = process.env;
...

Save and close the file when you are finished editing.
At this point, you have modified db.js to work with your application’s environment variables, but you still need a way to pass these variables to your application. Create an .env file with values that you can pass to your application at runtime.
Open the file:

nano .env

This file will include the information that you removed from db.js: the username and password for your application’s database, as well as the port setting and database name. Remember to update the username, password, and database name listed here with your own information:
~/node_project/.env

MONGO_USERNAME=sammy
MONGO_PASSWORD=your_password
MONGO_PORT=27017
MONGO_DB=sharkinfo

Note that you have removed the host setting that originally appeared in db.js. You will now define your host at the level of the Docker Compose file, along with other information about your services and containers.
Save and close this file when you are finished editing.
Because your .env file contains sensitive information, you will want to ensure that it is included in your project’s .dockerignore and .gitignore files so that it does not copy to your version control or containers.
Open your .dockerignore file:

nano .dockerignore

Add the following line to the bottom of the file:
~/node_project/.dockerignore

...
.gitignore
.env

Save and close the file when you are finished editing.
The .gitignore file in this repository already includes .env, but feel free to check that it is there:

nano .gitignore

~~/node_project/.gitignore

...
.env
...

At this point, you have successfully extracted sensitive information from your project code and taken measures to control how and where this information gets copied. Now you can add to your database connection code to optimize it for a containerized workflow.

Step 3 — Modifying Database Connection Settings

Your next step will be to make your database connection method more robust by adding code that handles cases where your application fails to connect to your database. Introducing this level of resilience to your application code is a recommended practice when working with containers using Compose.
Open db.js for editing:

nano db.js

Notice the code that added earlier, along with the url constant for Mongo’s connection URI and the Mongoose connect method:
~/node_project/db.js

...
const {
  MONGO_USERNAME,
  MONGO_PASSWORD,
  MONGO_HOSTNAME,
  MONGO_PORT,
  MONGO_DB
} = process.env;

const url = `mongodb://${MONGO_USERNAME}:${MONGO_PASSWORD}@${MONGO_HOSTNAME}:${MONGO_PORT}/${MONGO_DB}?authSource=admin`;

mongoose.connect(url, {useNewUrlParser: true});

Currently, your connect method accepts an option that tells Mongoose to use Mongo’s new URL parser. You can add options to this method to define parameters for reconnection attempts. Do this by creating an options constant that includes the relevant information, in addition to the new URL parser option. Below your Mongo constants, add the following definition for the options constant:
~/node_project/db.js

...
const {
  MONGO_USERNAME,
  MONGO_PASSWORD,
  MONGO_HOSTNAME,
  MONGO_PORT,
  MONGO_DB
} = process.env;

const options = {
  useNewUrlParser: true,
  reconnectTries: Number.MAX_VALUE,
  reconnectInterval: 500, 
  connectTimeoutMS: 10000,
};
...

The reconnectTries option tells Mongoose to continue trying to connect indefinitely, while reconnectInterval defines the period between connection attempts in milliseconds. connectTimeoutMS defines 10 seconds as the period that the Mongo driver will wait before failing the connection attempt.
You can now use the new options constant in the Mongoose connect method to fine tune your Mongoose connection settings. You will also add a promise to handle potential connection errors.
Currently, the Mongoose connect method looks like this:
~/node_project/db.js

...
mongoose.connect(url, {useNewUrlParser: true});

Delete the existing connect method and replace it with the following code, which includes the options constant and a promise:
~/node_project/db.js

...
mongoose.connect(url, options).then( function() {
  console.log('MongoDB is connected');
})
  .catch( function(err) {
  console.log(err);
});

In the case of a successful connection, your function logs an appropriate message; otherwise it will catch and log the error, allowing you to troubleshoot.
The finished file will look like this:
~/node_project/db.js

const mongoose = require('mongoose');

const {
  MONGO_USERNAME,
  MONGO_PASSWORD,
  MONGO_HOSTNAME,
  MONGO_PORT,
  MONGO_DB
} = process.env;

const options = {
  useNewUrlParser: true,
  reconnectTries: Number.MAX_VALUE,
  reconnectInterval: 500,
  connectTimeoutMS: 10000,
};

const url = `mongodb://${MONGO_USERNAME}:${MONGO_PASSWORD}@${MONGO_HOSTNAME}:${MONGO_PORT}/${MONGO_DB}?authSource=admin`;

mongoose.connect(url, options).then( function() {
  console.log('MongoDB is connected');
})
  .catch( function(err) {
  console.log(err);
});

Save and close the file when you have finished editing.
You have now added resiliency to your application code to handle cases where your application might fail to connect to your database. With this code in place, you can move on to defining your services with Compose.

Step 4 — Defining Services with Docker Compose

With your code refactored, you are ready to write the docker-compose.yml file with your service definitions. A service in Compose is a running container, and service definitions — which you will include in your docker-compose.yml file — contain information about how each container image will run. The Compose tool allows you to define multiple services to build multi-container applications.
Before defining your services, you will add a tool to your project called wait-for to ensure that your application only attempts to connect to your database once the database startup tasks are complete. This wrapper script uses netcat to poll whether a specific host and port are accepting TCP connections. Using it allows you to control your application’s attempts to connect to your database by testing whether the database is ready to accept connections.
Though Compose allows you to specify dependencies between services using the depends_on option, this order is based on whether the container is running rather than its readiness. Using depends_on won’t be optimal for your setup since you want your application to connect only when the database startup tasks, including adding a user and password to the admin authentication database, are complete. For more information on using wait-for and other tools to control startup order, please see the relevant recommendations in the Compose documentation.
Open a file called wait-for.sh:

nano wait-for.sh

Enter the following code into the file to create the polling function:
~/node_project/app/wait-for.sh

#!/bin/sh

# original script: https://github.com/eficode/wait-for/blob/master/wait-for

TIMEOUT=15
QUIET=0

echoerr() {
  if [ "$QUIET" -ne 1 ]; then printf "%sn" "$*" 1>&2; fi
}

usage() {
  exitcode="$1"
  cat << USAGE >&2
Usage:
  $cmdname host:port [-t timeout] [-- command args]
  -q | --quiet                        Do not output any status messages
  -t TIMEOUT | --timeout=timeout      Timeout in seconds, zero for no timeout
  -- COMMAND ARGS                     Execute command with args after the test finishes
USAGE
  exit "$exitcode"
}

wait_for() {
  for i in `seq $TIMEOUT` ; do
    nc -z "$HOST" "$PORT" > /dev/null 2>&1
    
    result=$?
    if [ $result -eq 0 ] ; then
      if [ $# -gt 0 ] ; then
        exec "$@"
      fi
      exit 0
    fi
    sleep 1
  done
  echo "Operation timed out" >&2
  exit 1
}

while [ $# -gt 0 ]
do
  case "$1" in
    *:* )
    HOST=$(printf "%sn" "$1"| cut -d : -f 1)
    PORT=$(printf "%sn" "$1"| cut -d : -f 2)
    shift 1
    ;;
    -q | --quiet)
    QUIET=1
    shift 1
    ;;
    -t)
    TIMEOUT="$2"
    if [ "$TIMEOUT" = "" ]; then break; fi
    shift 2
    ;;
    --timeout=*)
    TIMEOUT="${1#*=}"
    shift 1
    ;;
    --)
    shift
    break
    ;;
    --help)
    usage 0
    ;;
    *)
    echoerr "Unknown argument: $1"
    usage 1
    ;;
  esac
done

if [ "$HOST" = "" -o "$PORT" = "" ]; then
  echoerr "Error: you need to provide a host and port to test."
  usage 2
fi

wait_for "$@"

Save and close the file when you are finished adding the code.
Make the script executable:

chmod +x wait-for.sh

Next, open the docker-compose.yml file:

nano docker-compose.yml

First, define the nodejs application service by adding the following code to the file:
~/node_project/docker-compose.yml

version: '3'

services:
  nodejs:
    build:
      context: .
      dockerfile: Dockerfile
    image: nodejs
    container_name: nodejs
    restart: unless-stopped
    env_file: .env
    environment:
      - MONGO_USERNAME=$MONGO_USERNAME
      - MONGO_PASSWORD=$MONGO_PASSWORD
      - MONGO_HOSTNAME=db
      - MONGO_PORT=$MONGO_PORT
      - MONGO_DB=$MONGO_DB 
    ports:
      - "80:8080"
    volumes:
      - .:/home/node/app
      - node_modules:/home/node/app/node_modules
    networks:
      - app-network
    command: ./wait-for.sh db:27017 -- /home/node/app/node_modules/.bin/nodemon app.js

The nodejs service definition includes the following options:

build: This defines the configuration options, including the context and dockerfile, that will be applied when Compose builds the application image. If you wanted to use an existing image from a registry like Docker Hub, you could use the image instruction instead, with information about your username, repository, and image tag.
context: This defines the build context for the image build — in this case, the current project directory.
dockerfile: This specifies the Dockerfile in your current project directory as the file Compose will use to build the application image. For more information about this file, please see How To Build a Node.js Application with Docker.
image, container_name: These apply names to the image and container.
restart: This defines the restart policy. The default is no, but you have set the container to restart unless it is stopped.
env_file: This tells Compose that you would like to add environment variables from a file called .env, located in your build context.
environment: Using this option allows you to add the Mongo connection settings you defined in the .env file. Note that you are not setting NODE_ENV to development, since this is Express’s default behavior if NODE_ENV is not set. When moving to production, you can set this to production to enable view caching and less verbose error messages.
Also note that you have specified the db database container as the host, as discussed in Step 2.
ports: This maps port 80 on the host to port 8080 on the container.
volumes: You are including two types of mounts here:

The first is a bind mount that mounts your application code on the host to the /home/node/app directory on the container. This will facilitate rapid development, since any changes you make to your host code will be populated immediately in the container.
The second is a named volume, node_modules. When Docker runs the npm install instruction listed in the application Dockerfile, npm will create a new node_modules directory on the container that includes the packages required to run the application. The bind mount you just created will hide this newly created node_modules directory, however. Since node_modules on the host is empty, the bind will map an empty directory to the container, overriding the new node_modules directory and preventing your application from starting. The named node_modules volume solves this problem by persisting the contents of the /home/node/app/node_modules directory and mounting it to the container, hiding the bind.

Keep the following points in mind when using this approach:

Your bind will mount the contents of the node_modules directory on the container to the host and this directory will be owned by root, since the named volume was created by Docker.

If you have a pre-existing node_modules directory on the host, it will override the node_modules directory created on the container. The setup that you’re building in this tutorial assumes that you do not have a pre-existing node_modules directory and that you won’t be working with npm on your host. This is in keeping with a twelve-factor approach to application development, which minimizes dependencies between execution environments.

networks: This specifies that your application service will join the app-network network, which you will define at the bottom of the file.
command: This option lets you set the command that should be executed when Compose runs the image. Note that this will override the CMD instruction that you set in our application Dockerfile. Here, you are running the application using the wait-for script, which will poll the db service on port 27017 to test whether the database service is ready. Once the readiness test succeeds, the script will execute the command you have set, /home/node/app/node_modules/.bin/nodemon app.js, to start the application with nodemon. This will ensure that any future changes you make to your code are reloaded without your having to restart the application.

Next, create the db service by adding the following code below the application service definition:
~/node_project/docker-compose.yml

...
  db:
    image: mongo:4.1.8-xenial
    container_name: db
    restart: unless-stopped
    env_file: .env
    environment:
      - MONGO_INITDB_ROOT_USERNAME=$MONGO_USERNAME
      - MONGO_INITDB_ROOT_PASSWORD=$MONGO_PASSWORD
    volumes:  
      - dbdata:/data/db   
    networks:
      - app-network

Some of the settings defined for the nodejs service remain the same, but you’ve also made the following changes to the image, environment, and volumes definitions:

image: To create this service, Compose will pull the 4.1.8-xenial Mongo image from Docker Hub. You are pinning a particular version to avoid possible future conflicts as the Mongo image changes. For more information about version pinning, please see the Docker documentation on Dockerfile best practices.
MONGO_INITDB_ROOT_USERNAME, MONGO_INITDB_ROOT_PASSWORD: The mongo image makes these environment variables available so that you can modify the initialization of your database instance. MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD together create a root user in the admin authentication database and ensure that authentication is enabled when the container starts. You have set MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD using the values from your .env file, which you pass to the db service using the env_file option. Doing this means that your sammy application user will be a root user on the database instance, with access to all the administrative and operational privileges of that role. When working in production, you will want to create a dedicated application user with appropriately scoped privileges.

Note: Keep in mind that these variables will not take effect if you start the container with an existing data directory in place.

dbdata:/data/db: The named volume dbdata will persist the data stored in Mongo’s default data directory, /data/db. This will ensure that you don’t lose data in cases where you stop or remove containers.

The db service was also added to the app-network network with the networks option.
As a final step, add the volume and network definitions to the bottom of the file:
~/node_project/docker-compose.yml

...
networks:
  app-network:
    driver: bridge

volumes:
  dbdata:
  node_modules:

The user-defined bridge network app-network enables communication between your containers since they are on the same Docker daemon host. This streamlines traffic and communication within the application, as it opens all ports between containers on the same bridge network, while exposing no ports to the outside world. Thus, your db and nodejs containers can communicate with each other, and you only need to expose port 80 for front-end access to the application.
Your top-level volumes key defines the volumes dbdata and node_modules. When Docker creates volumes, the contents of the volume are stored in a part of the host filesystem, /var/lib/docker/volumes/, that’s managed by Docker. The contents of each volume are stored in a directory under /var/lib/docker/volumes/ and get mounted to any container that uses the volume. In this way, the shark information data that your users will create will persist in the dbdata volume even if you remove and recreate the db container.
The finished docker-compose.yml file will look like this:
~/node_project/docker-compose.yml

version: '3'

services:
  nodejs:
    build:
      context: .
      dockerfile: Dockerfile
    image: nodejs
    container_name: nodejs
    restart: unless-stopped
    env_file: .env
    environment:
      - MONGO_USERNAME=$MONGO_USERNAME
      - MONGO_PASSWORD=$MONGO_PASSWORD
      - MONGO_HOSTNAME=db
      - MONGO_PORT=$MONGO_PORT
      - MONGO_DB=$MONGO_DB
    ports:
      - "80:8080"
    volumes:
      - .:/home/node/app
      - node_modules:/home/node/app/node_modules
    networks:
      - app-network
    command: ./wait-for.sh db:27017 -- /home/node/app/node_modules/.bin/nodemon app.js 

  db:
    image: mongo:4.1.8-xenial
    container_name: db
    restart: unless-stopped
    env_file: .env
    environment:
      - MONGO_INITDB_ROOT_USERNAME=$MONGO_USERNAME
      - MONGO_INITDB_ROOT_PASSWORD=$MONGO_PASSWORD
    volumes:     
      - dbdata:/data/db
    networks:
      - app-network  

networks:
  app-network:
    driver: bridge

volumes:
  dbdata:
  node_modules:

Save and close the file when you are finished editing.
With your service definitions in place, you are ready to start the application.

Step 5 — Testing the Application

With your docker-compose.yml file in place, you can create your services with the docker-compose up command. You can also test that your data will persist by stopping and removing your containers with docker-compose down.
First, build the container images and create the services by running docker-compose up with the -d flag, which will then run the nodejs and db containers in the background:

docker-compose up -d

The output confirms that your services have been created:

...
Creating db ... done
Creating nodejs ... done

You can also get more detailed information about the startup processes by displaying the log output from the services:

docker-compose logs

If everything has started correctly, the following is the output:

...
nodejs    | [nodemon] starting `node app.js`
nodejs    | Example app listening on 8080!
nodejs    | MongoDB is connected
...
db        | 2019-02-22T17:26:27.329+0000 I ACCESS   [conn2] Successfully authenticated as principal sammy on admin

You can also check the status of your containers with docker-compose ps:

docker-compose ps

The output indicates that your containers are running:

 Name               Command               State          Ports        
----------------------------------------------------------------------
db       docker-entrypoint.sh mongod      Up      27017/tcp           
nodejs   ./wait-for.sh db:27017 --  ...   Up      0.0.0.0:80->8080/tcp

With your services running, you can visit http://your_server_ip in the browser:

Click on the Get Shark Info button to enter a page with an entry form where you can submit a shark name and a description of that shark’s general character:

In the form, add a shark of your choosing. For the purpose of this demonstration, add Megalodon Shark to the Shark Name field, and Ancient to the Shark Character field:

Click on the Submit button and a page with this shark information will be displayed back to you:

As a final step, test that the data you’ve just entered will persist if you remove your database container.
Back at your terminal, type the following command to stop and remove your containers and network:

docker-compose down

Note that you are not including the --volumes option; hence, your dbdata volume is not removed.
The following output confirms that your containers and network have been removed:

Stopping nodejs ... done
Stopping db     ... done
Removing nodejs ... done
Removing db     ... done
Removing network node_project_app-network

Recreate the containers:

docker-compose up -d

Now head back to the shark information form:

Enter a new shark of your choosing. This example will use Whale Shark and Large:

Once you click Submit, you notice that the new shark has been added to the shark collection in your database without the loss of the data you’ve already entered:

Your application is now running on Docker containers with data persistence and code synchronization enabled.

Conclusion

By following this tutorial, you have created a development setup for your Node application using Docker containers. You’ve made your project more modular and portable by extracting sensitive information and decoupling your application’s state from your application code. You have also configured a boilerplate docker-compose.yml file that you can revise as your development needs and requirements change.
As you develop, you may be interested in learning more about designing applications for containerized and Cloud Native workflows. Please see Architecting Applications for Kubernetes and Modernizing Applications for Kubernetes for more information on these topics.
To learn more about the code used in this tutorial, please see How To Build a Node.js Application with Docker and How To Integrate MongoDB with Your Node Application. For information about deploying a Node application with an Nginx reverse proxy using containers, please see How To Secure a Containerized Node.js Application with Nginx, Let’s Encrypt, and Docker Compose.