Docker is an amazing tool for developers. It allows us to build and replicate images on any host, removing the inconsistencies of dev environments and reducing onboarding timelines considerably.

To provide an example of how you might move to containerized development, I built a simple todo API using NodeJS, Express, and PostgreSQL using Docker Compose for development, testing, and eventually in my CI/CD pipeline.

In a two-part series, I will cover the development and pipeline creation steps. In this post, I will cover the first part: developing and testing with Docker Compose.

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Requirements for This Tutorial

This tutorial requires you to have a few items before you can get started.

The todo app here is essentially a stand-in, and you could replace it with your own application. Some of the setup here is specific for this application, and the needs of your application may not be covered, but it should be a good starting point for you to get the concepts needed to Dockerize your own applications.

Once you have everything set up, you can move on to the next section.

Creating the Dockerfile

At the foundation of any Dockerized application, you will find a Dockerfile . The Dockerfile contains all of the instructions used to build out the application image. You can set this up by installing NodeJS and all of its dependencies; however the Docker ecosystem has an image repository (the Docker Store) with a NodeJS image already created and ready to use.

In the root directory of the application, create a new Dockerfile .

/> touch Dockerfile

Open the newly created Dockerfile in your favorite editor. The first instruction, FROM , will tell Docker to use the prebuilt NodeJS image. There are several choices, but this project uses the node:7.7.2-alpine image. For more details about why I’m using alpine here over the other options, you can read this post.

FROM node:7.7.2-alpine

If you run docker build . , you will see something similar to the following:

Sending build context to Docker daemon 249.3 kB Step 1/1 : FROM node:7.7.2-alpine 7.7.2-alpine: Pulling from library/node 709515475419: Pull complete 1a7746e437f7: Pull complete 662ac7b95f9d: Pull complete Digest: sha256:6dcd183eaf2852dd8c1079642c04cc2d1f777e4b34f2a534cc0ad328a98d7f73 Status: Downloaded newer image for node:7.7.2-alpine ---> 95b4a6de40c3 Successfully built 95b4a6de40c3

With only one instruction in the Dockerfile, this doesn’t do too much, but it does show you the build process without too much happening. At this point, you now have an image created, and running docker images will show you the images you have available:

REPOSITORY TAG IMAGE ID CREATED SIZE node 7.7.2-alpine 95b4a6de40c3 6 weeks ago 59.2 MB

The Dockerfile needs more instructions to build out the application. Currently it’s only creating an image with NodeJS installed, but we still need our application code to run inside the container. Let’s add some more instructions to do this and build this image again.

This particular Docker file uses RUN , COPY , and WORKDIR . You can read more about those on Docker’s reference page to get a deeper understanding.

Let’s add the instructions to the Dockerfile now:

FROM node:7.7.2-alpine WORKDIR /usr/app COPY package.json . RUN npm install --quiet COPY . .

Here is what is happening:

Set the working directory to /usr/app

Copy the package.json file to /usr/app

file to Install node_modules

Copy all the files from the project’s root to /usr/app

You can now run docker build . again and see the results:

Sending build context to Docker daemon 249.3 kB Step 1/5 : FROM node:7.7.2-alpine ---> 95b4a6de40c3 Step 2/5 : WORKDIR /usr/app ---> e215b737ca38 Removing intermediate container 3b0bb16a8721 Step 3/5 : COPY package.json . ---> 930082a35f18 Removing intermediate container ac3ab0693f61 Step 4/5 : RUN npm install --quiet ---> Running in 46a7dcbba114 ### NPM MODULES INSTALLED ### ---> 525f662aeacf ---> dd46e9316b4d Removing intermediate container 46a7dcbba114 Step 5/5 : COPY . . ---> 1493455bcf6b Removing intermediate container 6d75df0498f9 Successfully built 1493455bcf6b

You have now successfully created the application image using Docker. Currently, however, our app won’t do much since we still need a database, and we want to connect everything together. This is where Docker Compose will help us out.

Docker Compose Services

Now that you know how to create an image with a Dockerfile , let’s create an application as a service and connect it to a database. Then we can run some setup commands and be on our way to creating that new todo list.

Create the file docker-compose.yml :

/> touch docker-compose.yml

The Docker Compose file will define and run the containers based on a configuration file. We are using compose file version 2 syntax, and you can read up on it on Docker’s site.

An important concept to understand is that Docker Compose spans “buildtime” and “runtime.” Up until now, we have been building images using docker build . , which is “buildtime.” This is when our containers are actually built. We can think of “runtime” as what happens once our containers are built and being used.

Compose triggers “buildtime” — instructing our images and containers to build — but it also populates data used at “runtime,” such as env vars and volumes. This is important to be clear on. For instance, when we add things like volumes and command , they will override the same things that may have been set up via the Dockerfile at “buildtime.”

Open your docker-compose.yml file in your editor and copy/paste the following lines:

version: '2' services: web: build: . command: npm run dev volumes: - .:/usr/app/ - /usr/app/node_modules ports: - "3000:3000" depends_on: - postgres environment: DATABASE_URL: postgres://todoapp@postgres/todos postgres: image: postgres:9.6.2-alpine environment: POSTGRES_USER: todoapp POSTGRES_DB: todos

This will take a bit to unpack, but let’s break it down by service.

The web service

The first directive in the web service is to build the image based on our Dockerfile . This will recreate the image we used before, but it will now be named according to the project we are in, nodejsexpresstodoapp . After that, we are giving the service some specific instructions on how it should operate:

command: npm run dev – Once the image is built, and the container is running, the npm run dev command will start the application.

– Once the image is built, and the container is running, the command will start the application. volumes: – This section will mount paths between the host and the container.

– This section will mount paths between the host and the container. .:/usr/app/ – This will mount the root directory to our working directory in the container.

– This will mount the root directory to our working directory in the container. /usr/app/node_modules – This will mount the node_modules directory to the host machine using the buildtime directory.

– This will mount the directory to the host machine using the buildtime directory. environment: – The application itself expects the environment variable DATABASE_URL to run. This is set in db.js .

– The application itself expects the environment variable to run. This is set in . ports: – This will publish the container’s port, in this case 3000 , to the host as port 3000 .

The DATABASE_URL is the connection string. postgres://todoapp@postgres/todos connects using the todoapp user, on the host postgres , using the database todos .

The Postgres service

Like the NodeJS image we used, the Docker Store has a prebuilt image for PostgreSQL. Instead of using a build directive, we can use the name of the image, and Docker will grab that image for us and use it. In this case, we are using postgres:9.6.2-alpine . We could leave it like that, but it has environment variables to let us customize it a bit.

environment: – This particular image accepts a couple environment variables so we can customize things to our needs. POSTGRES_USER: todoapp – This creates the user todoapp as the default user for PostgreSQL. POSTGRES_DB: todos – This will create the default database as todos .

Running The Application

Now that we have our services defined, we can build the application using docker-compose up . This will show the images being built and eventually starting. After the initial build, you will see the names of the containers being created:

Pulling postgres (postgres:9.6.2-alpine)... 9.6.2-alpine: Pulling from library/postgres 627beaf3eaaf: Pull complete e351d01eba53: Pull complete cbc11f1629f1: Pull complete 2931b310bc1e: Pull complete 2996796a1321: Pull complete ebdf8bbd1a35: Pull complete 47255f8e1bca: Pull complete 4945582dcf7d: Pull complete 92139846ff88: Pull complete Digest: sha256:7f3a59bc91a4c80c9a3ff0430ec012f7ce82f906ab0a2d7176fcbbf24ea9f893 Status: Downloaded newer image for postgres:9.6.2-alpine Building web ... Creating nodejsexpresstodoapp_postgres_1 Creating nodejsexpresstodoapp_web_1 ... web_1 | Your app is running on port 3000

At this point, the application is running, and you will see log output in the console. You can also run the services as a background process, using docker-compose up -d . During development, I prefer to run without -d and create a second terminal window to run other commands. If you want to run it as a background process and view the logs, you can run docker-compose logs .

At a new command prompt, you can run docker-compose ps to view your running containers. You should see something like the following:

Name Command State Ports ------------------------------------------------------------------------------------------------ nodejsexpresstodoapp_postgres_1 docker-entrypoint.sh postgres Up 5432/tcp nodejsexpresstodoapp_web_1 npm run dev Up 0.0.0.0:3000->3000/tcp

This will tell you the name of the services, the command used to start it, its current state, and the ports. Notice nodejsexpresstodoapp_web_1 has listed the port as 0.0.0.0:3000->3000/tcp . This tells us that you can access the application using localhost:3000/todos on the host machine.

/> curl localhost:3000/todos []

The package.json file has a script to automatically build the code and migrate the schema to PostgreSQL. The schema and all of the data in the container will persist as long as the postgres:9.6.2-alpine image is not removed.

Eventually, however, it would be good to check how your app will build with a clean setup. You can run docker-compose down , which will clear things that are built and let you see what is happening with a fresh start.

Feel free to check out the source code, play around a bit, and see how things go for you.

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Testing the Application

The application itself includes some integration tests built using jest . There are various ways to go about testing, including creating something like Dockerfile.test and docker-compose.test.yml files specific for the test environment. That’s a bit beyond the current scope of this article, but I want to show you how to run the tests using the current setup.

The current containers are running using the project name nodejsexpresstodoapp . This is a default from the directory name. If we attempt to run commands, it will use the same project, and containers will restart. This is what we don’t want.

Instead, we will use a different project name to run the application, isolating the tests into their own environment. Since containers are ephemeral (short-lived), running your tests in a separate set of containers makes certain that your app is behaving exactly as it should in a clean environment.

In your terminal, run the following command:

/> docker-compose -p tests run -p 3000 --rm web npm run watch-tests

You should see jest run through integration tests and wait for changes.

The docker-compose command accepts several options, followed by a command. In this case, you are using -p tests to run the services under the tests project name. The command being used is run , which will execute a one-time command against a service.

Since the docker-compose.yml file specifies a port, we use -p 3000 to create a random port to prevent port collision. The --rm option will remove the containers when we stop the containers. Finally, we are running in the web service npm run watch-tests .

Conclusion

At this point, you should have a solid start using Docker Compose for local app development. In the next part of this series about using Docker Compose for NodeJS development, I will cover integration and deployments of this application using Codeship.

Is your team using Docker in its development workflow? If so, I would love to hear about what you are doing and what benefits you see as a result.

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