Docker Compose Basics

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Docker-Compose is an essential tool in the Docker ecosystem that facilitates the orchestration and deployment of multi-container applications. It allows developers to define their application's architecture, dependencies, and configurations in a single file, making it easier to manage and scale the application across various environments consistently.

In this hands-on exercise, we will learn fundamentals of Docker-Compose by performing various operations. We will cover the following:

1.Create a simple Docker image for Flask app.

2.Define services and networks using Docker-Compose.

3.Build image and run containers with Docker-Compose

4.View logs for services running in Docker-Compose.

5.Use bind mounts to perform code changes without rebuilding the image.

6. Scale the deployment by running multiple service replicas

Why should we use Docker-Compose?

Docker-Compose provides numerous benefits, including:

Simplified container orchestration:

Define and manage complex applications with ease.

Environment consistency:

Ensure consistent environments across development, testing, and production.

Scalability:

Easily scale services up or down based on demand.

Portability:

Share the same Docker-Compose configuration across teams and environments.

Important Aspects of Docker-Compose

Service Definition

Docker-Compose allows you to define the services that make up your application. Each service represents a containerized component of your application, such as a web server, database, or backend service. You can specify various configurations for each service, including the base image, environment variables, ports, and more.

Networking

Docker-Compose enables you to create custom networks for your services. Containers within the same network can communicate with each other using their service names as hostnames. This simplifies the setup of communication between containers and allows you to isolate services on different networks as needed for security and organisational purposes.

Volumes

Managing data persistence and sharing between containers is crucial in any application. Docker-Compose allows you to define volumes that persist data beyond the lifespan of a container. This ensures that critical data, like databases or user uploads, is retained even if the container is restarted or replaced.

Environment Variables

Docker-Compose allows you to set environment variables for your services. This is especially useful for configuring application behaviour based on the environment it runs in, such as development, testing, or production environments. Environment variables can be easily managed in the Docker-Compose file.

Extensibility

You can extend Docker-Compose configurations by using external Compose files. These files can override or add to the services defined in the primary Docker-Compose file. This feature is valuable for managing environment-specific configurations or sharing common configurations across multiple projects.

Detached Mode

When running services with Docker-Compose, you can use detached mode (-d) to run the services in the background. This allows you to continue using the terminal for other tasks without the services' output cluttering your screen.

Logging

Docker-Compose provides easy access to service logs. You can view the logs of all the services running with docker-compose up or focus on logs for a specific service using docker-compose logs <service-name>. This feature aids in monitoring and troubleshooting the application.

Bind Mounts

Docker-Compose supports bind mounts, allowing you to mount a host directory directly into a container. This is especially useful during development, as it enables you to instantly reflect code changes made on the host machine in the running container, without the need to rebuild the image.

Resource Management

Docker-Compose allows you to control resource allocation for services. You can define resource constraints like CPU and memory limits to ensure fair distribution of resources and prevent one service from monopolising the host's resources.

By leveraging these powerful features of Docker-Compose, you can streamline the management of your multi-container applications, enhance application performance, ensure data integrity, and maintain consistency across various environments. Docker-Compose plays a significant role in enabling efficient container orchestration and deployment, making it an essential tool in the Docker ecosystem.

Exercise Manual

Step 1: In this exercise, we will cover basic operations and capabilities of Docker-Compose. Are you ready to get started?

Step 2: Our sandbox environment already has Docker and Docker compose installed. And, you can install it on your machine by following the provided documentation link.

Step 3: Let's move on to creating our Docker application setup using Docker Compose. We will create setup for a Flask app. Clone the repository by running `git clone https://github.com/web3coda/docker-basics` in your terminal.

Step 4: The link to the repository is also provided below. You can use it outside of sandbox environment as well.

Step 5: Open VS Code by clicking on the button. We will use VS Code to check the Docker Compose, Dockerfile and Flask app files.

Step 6: Check 'dockerfile-compose-tutorial' folder inside the cloned 'docker-basics' repository. This is the folder we will use for this exercise.

Step 7: Open Dockerfile by clicking on the file name in the left sidebar. This is the Dockerfile we will use for our Flask app. It is well documented so you can understand what each line does. Also, check app.py , requirements.txt and docker-compose.yml files. These files are used to define app, to list packages or dependencies and to define services for our Flask app.

Step 8: In the sandbox environment, you can use either 'docker compose' or 'docker-compose' command as per your wish.

Step 9: Important Note: docker compose commands only work when there is a docker-compose.yaml file present in the current working directoy. So, ensure you are running all commands from 'dockerfile-compose-tutorial' directory. Also, if the Docker compose file is not named docker compose then in that case, we have to specify the file with '-f file-name'.

Step 10: With everything set up, it's time to build and run our Docker-Compose file. Run 'docker-compose up' command to run the docker-compose.yaml file. It will first build the image and then create the network with redis. The complete setup will container one network and two containers (app container and redis).

Step 11: Check the page hosted by this webapp by clicking 'localhost:8000' button. By clicking the 'Go' button, you will see the count increasing.

Step 12: Switch back to 'terminal' tab and stop the running command with 'Ctrl + C'. Also, run 'docker compose down' to ensure every component of the setup is removed.

Step 13: In similar way, we can run Docker-Compose services in detached mode by appending '-d' argument to it.

Step 14: Check running docker conatiners. You will observe 2 containers, one for flask app and another one for redis.

Step 15: Check docker networks. You will be able to see 'docker-compose-tutorial_default' bridge created to connect these two containers.

Step 16: List the running docker-compose projects. As we are only running one project, you will only see that.

Step 17: List the running docker containers that are created by Docker Compose. Unlike 'docker ps' it won't show all containers but only the ones created with Docker Compose.

Step 18: In detached mode, Docker compose logs are not shown on terminal by default but we can check those with 'docker compose logs' command.

Step 19: Just like tail command, we can continuously follow the logs using '-f' argument with this command. And, stop it by using 'Ctrl + C'

Step 20: We can also view the logs for a specific service by running 'docker-compose logs [service-name]'. For example, in our case, we can check for 'web'service

Step 21: Stop the Docker-Compose services running in detached mode.

Step 22: If you want to start a specific service in detached mode. You can find the service name in the 'docker-compose.yml' file or use 'docker-compose ps' to view the service names.

Step 23: Like before, stop the Docker-Compose services running in detached mode.

Step 24: We can also mount files/directory using docker-compose. This allows us to persist the data across multiple container runs and change the application code without rebuilding the Docker image.

Step 24: Edit the 'docker-compose.yaml' file to add a bind mount using VS Code. Copy the code provided below and paste it in the docker-compose.yaml file

Step 25: Run Docker-Compose to apply the change

Step 26: Switch to VS Code tab and edit the 'app.py' file by replacing `Hello World` with `Happy Dockering`.

Step 27: Switch to Localhost tab and click 'Go', you should see the change.

Step 28: Docker compose also allow us to scale up and down the services as per need. In simple words, Scaling is the process of increasing or decreasing the number of containers running for a service assuming more containers can help in serving more clients. Lets scale our 'web' services to run 2 containers instead of 1.

Step 29: Verify the number of containers by running the container listing.

Step 30: Switch to localhost tab and press 'Go' button a few times, you should observe different hostname and counts. This is because now there are two containers serving our requests.

Step 31: Let's scale the service down.

Step 32: Great! You've successfully scaled your application. Now, stop the Docker-Compose services.

Step 33: Congratulations! You've successfully completed the lab and learned basics of Docker Compose.