How to use docker compose start command to start services

Introduction

In this lab, you will learn how to use the docker-compose start command to manage your multi-container Docker applications. We will begin by creating a simple docker-compose.yml file to define our services.

Following the creation of the compose file, you will practice starting all services defined within it using a single command. Finally, you will learn how to selectively start a specific service from your compose configuration. This hands-on experience will equip you with the fundamental skills to effectively manage your Docker Compose projects.

Skills Graph

Create a simple docker-compose.yml file

In this step, we will create a simple docker-compose.yml file. Docker Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application's services. Then, with a single command, you create and start all the services from your configuration.

Before we create the docker-compose.yml file, we need to install Docker Compose. Since it's not pre-installed in the LabEx environment, we will install it using pip.

First, let's update the package list and install pip if it's not already installed.

sudo apt update
sudo apt install -y python3-pip

Now, we can install Docker Compose using pip.

pip install docker-compose

After the installation is complete, you can verify the installation by checking the version of Docker Compose.

docker-compose --version

You should see output similar to docker-compose version 1.29.2, build 5becea4c.

Now that Docker Compose is installed, let's create a directory for our project and navigate into it. We will create the docker-compose.yml file inside this directory.

mkdir ~/project/my-compose-app
cd ~/project/my-compose-app

Next, we will create the docker-compose.yml file using the nano editor.

nano docker-compose.yml

Inside the nano editor, paste the following content. This docker-compose.yml file defines a single service named web that uses the nginx image and maps port 80 of the container to port 8080 on the host machine.

version: "3.8"
services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"

Let's break down this file:

• version: '3.8' specifies the Compose file format version.
• services: defines the different services that make up your application.
• web: is the name of our service. You can choose any name you like.
• image: nginx:latest specifies the Docker image to use for this service. In this case, we are using the latest version of the official Nginx image. We will pull this image later when we start the service.
• ports: maps ports between the host and the container.
• - "8080:80" maps port 80 inside the container (where Nginx runs by default) to port 8080 on your host machine. This means you can access the Nginx web server by visiting http://localhost:8080 in your web browser (or using curl from the terminal).

Save the file by pressing Ctrl + X, then Y, and then Enter.

You can verify the content of the file using the cat command.

cat docker-compose.yml

You should see the YAML content you just pasted.

Start all services defined in the compose file

In this step, we will start all the services defined in our docker-compose.yml file. We will use the docker-compose up command for this purpose.

First, make sure you are in the directory where you created the docker-compose.yml file.

cd ~/project/my-compose-app

Now, run the docker-compose up command. The -d flag runs the containers in detached mode, meaning they will run in the background and not block your terminal.

docker-compose up -d

When you run this command for the first time, Docker Compose will:

1. Look for the docker-compose.yml file in the current directory.
2. Read the service definitions.
3. For each service, it will check if the specified image exists locally. If not, it will pull the image from Docker Hub. In our case, it will pull the nginx:latest image.
4. Create and start the containers for each service.

You will see output indicating that the image is being pulled and the container is being created and started.

Pulling web (nginx:latest)...
latest: Pulling from library/nginx
... (output showing image layers being downloaded)
Status: Downloaded newer image for nginx:latest
Creating my-compose-app_web_1 ... done

After the command finishes, you can verify that the container is running using the docker ps command.

docker ps

You should see an entry for the web container, with the image nginx:latest and the status Up. The container name will be something like my-compose-app_web_1.

To further verify that the Nginx server is running and accessible, you can use the curl command to access the web server on port 8080 of the host machine.

curl http://localhost:8080

You should see the default Nginx welcome page HTML output in your terminal. This confirms that the container is running and the port mapping is working correctly.

Start a specific service defined in the compose file

In the previous step, we started all services defined in our docker-compose.yml file using docker-compose up. In this step, we will learn how to start a specific service if you have multiple services defined in your Compose file.

Although our current docker-compose.yml only has one service (web), the command to start a specific service is useful when you have a more complex application with multiple services (like a web server, a database, and a backend API).

First, let's stop the currently running service. We can do this using the docker-compose down command. This command stops and removes containers, networks, and volumes created by up.

Make sure you are in the ~/project/my-compose-app directory.

cd ~/project/my-compose-app

Now, run the docker-compose down command.

docker-compose down

You will see output indicating that the container and network are being stopped and removed.

Stopping my-compose-app_web_1 ... done
Removing my-compose-app_web_1 ... done
Removing network my-compose-app_default

You can verify that the container is stopped by running docker ps. There should be no running containers from our compose file.

docker ps

Now, let's demonstrate how to start a specific service. The command is docker-compose up <service_name>. In our case, the service name is web. We will again use the -d flag to run it in detached mode.

docker-compose up -d web

This command will only start the web service as defined in the docker-compose.yml file. Since the image is already pulled from the previous step, it will directly create and start the container.

You will see output similar to this:

Creating my-compose-app_web_1 ... done

Again, you can verify that the web container is running using docker ps.

docker ps

You should see the my-compose-app_web_1 container listed with a status of Up.

This demonstrates how to selectively start services defined in your docker-compose.yml file, which is helpful for managing more complex multi-service applications.

Summary

In this lab, we learned how to use the docker-compose start command. We began by creating a simple docker-compose.yml file. This involved installing Docker Compose using pip, verifying the installation, creating a project directory, and then creating the docker-compose.yml file itself. The created file defined a single service named web using the nginx image and mapping ports.

The subsequent steps, which were not fully detailed in the provided content, would involve using the docker-compose start command to either start all services defined in the compose file or to start a specific service. This demonstrates the core functionality of the docker-compose start command for managing the lifecycle of multi-container applications defined in a Compose file.