Table of Contents
Introduction
Docker is one of the most popular containerization technologies. It is a simple-to-use, developer-friendly tool and has advantages over other similar technologies that make using it smooth and easy. Since its first open-source release in March 2013, Docker has gained attention from developers and ops engineers. According to Docker Inc., Docker users have downloaded over 105 billion containers and 'dockerized' 5.8 million containers on Docker Hub. The project has over 32K stars on GitHub.
Docker has since become mainstream. Over 100K 3rd-party projects use this technology, and developers with containerization skills are in increasing demand.
This article is the first of a two-part series. In this blog post, you will discover how to use Docker to containerize an application and then run it on development environments using Docker Compose. We are going to use a Python API as our main app. The second part will focus more on orchestration, more specifically, on how to deploy the same app to Kubernetes.
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Key Takeaways
- Docker is a popular containerization technology that has gained significant attention from developers and operations engineers since its open-source release in March 2013.
- Docker has become mainstream, with more than 100,000 third-party projects using it, and there is an increasing demand for developers with containerization skills.
- The article provides a two-part series on using Docker. The first part focuses on containerizing a Python API using Flask and running it in a development environment with Docker Compose.
- Setting up a development environment involves creating a Python virtual environment, installing Flask, and creating a simple API for displaying weather data.
- Docker Compose, an open-source tool for defining and running multi-container Docker applications, is introduced for use in development environments. It allows for auto-reloading containers when code changes occur, making development easier.
Setting Up a Development Environment
We will install some requirements before starting. We will use a mini Python API developed in Flask, a Python framework, to rapidly prototype an API. Our application will be developed using Flask. If you are not accustomed to Python, you can see the steps to create this API below.
We will start by creating a Python virtual environment to isolate our dependencies from the rest of the system dependencies. Before this, we will need PIP, a popular Python package manager.
The installation is quite easy - you need to execute the following two commands:
curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python get-pip.py
For your information, you should have Python 3 installed. You can verify this by typing:
python --version
After installing PIP, use the following command to install the virtual environment:
pip install virtualenv
You can follow the official guide to find other installation alternatives. Next, create a project for your folder in which you should create a virtual environment, then activate it. Also, create a folder for the app and a file called app.py.
mkdir app
cd app
python3 -m venv venv
. venv/bin/activate
mkdir code
cd code
touch app.py
We will build a simple API that shows the weather for a given city. For example, if we want to show the weather in London, we should request it using the route:
/london/uk
You need to install the Python dependencies called "flask" and "requests" using PIP. We are going to use them later on:
pip install flask requests
Don't forget to "freeze" your dependencies in a file called requirements.txt. This file will be used later to install our app dependencies in the container:
pip freeze > requirements.txt
This is what the requirements file looks like:
certifi==2019.9.11
chardet==3.0.4
Click==7.0
Flask==1.1.1
idna==2.8
itsdangerous==1.1.0
Jinja2==2.10.3
MarkupSafe==1.1.1
requests==2.22.0
urllib3==1.25.7
Werkzeug==0.16.0
This is the API initial code:
from flask import Flask
app = Flask(__name__)
@app.route('/')
def index():
return 'App Works!'
if __name__ == '__main__':
app.run(host="0.0.0.0", port=5000)
To test it, run python app.py and visit http://127.0.0.1:5000/. You should see "App Works!" on the web page. We will use data from openweathermap.org, so make sure to create an account on the same website and generate an API key.
Now, we need to add the convenient code to make the API show weather data about a given city:
@app.route('/<string:city>/<string:country>/')
def weather_by_city(country, city):
url = 'https://samples.openweathermap.org/data/2.5/weather'
params = dict(
q=city + "," + country,
appid= API_KEY,
)
response = requests.get(url=url, params=params)
data = response.json()
return data
The overall code looks like this:
from flask import Flask
import requests
app = Flask(__name__)
API_KEY = "b6907d289e10d714a6e88b30761fae22"
@app.route('/')
def index():
return 'App Works!'
@app.route('/<string:city>/<string:country>/')
def weather_by_city(country, city):
url = 'https://samples.openweathermap.org/data/2.5/weather'
params = dict(
q=city + "," + country,
appid= API_KEY,
)
response = requests.get(url=url, params=params)
data = response.json()
return data
if __name__ == '__main__':
app.run(host="0.0.0.0", port=5000)
Now if you visit 127.0.0.1:5000/london/uk, you should be able to see a JSON similar to the following one:
{
"base": "stations",
"clouds": {
"all": 90
},
"cod": 200,
"coord": {
"lat": 51.51,
"lon": -0.13
},
...
Our mini API is working. Let's containerize it using Docker.
Using Docker to Create a Container for our App
Let's create a container for our API. The first step is creating a Dockerfile. A Dockerfile is an instructive text file containing the steps and instructions that the Docker Daemon should follow to build an image. After building the image, we will be able to run the container.
A Dockerfile always starts with the FROM instructions:
FROM python:3
ENV PYTHONUNBUFFERED 1
RUN mkdir /app
WORKDIR /app
COPY requirements.txt /app
RUN pip install --upgrade pip
RUN pip install -r requirements.txt
COPY . /app
EXPOSE 5000
CMD [ "python", "app.py" ]
In the above file, we did the following things:
- We are using a base image called "python:3".
- We also set PYTHONUNBUFFERED to 1. Setting PYTHONUNBUFFERED to 1 allows log messages to be dumped to the stream instead of being buffered.
- We also created the folder /app, and we set it as a workdir.
- We copied the requirements and then used them to install all the dependencies.
- We copied all of the files composing our application, namely the app.py file to the workdir.
- We finally exposed port 5000, since our app will use this port, and we launched the python command with our app.py as an argument. This will start the API when the container starts.
After creating the Dockerfile, we must build it using an image name and a tag. In our case, we will use "weather" as a name and "v1" as a tag:
docker build -t weather:v1 .
Ensure you are building from inside the folder containing the Dockerfile and the app.py file.
After building the container, you can run it using:
docker run -dit --rm -p 5000:5000 --name weather weather:v1
The container will run in the background since we use the -d option. The container is called "weather" (--name weather). It's also reachable on port 5000 since we mapped the host port 5000 to the exposed container port 5000.
If we want to confirm the creation of the container, we can use:
docker ps
You should be able to see a very similar output to the following one:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
0e659e41d475 weather:v1 "python app.py" About a minute ago Up About a minute 0.0.0.0:5000->5000/tcp weather
You should be able to query the API now. Let's test it using CURL.
curl http://0.0.0.0:5000/london/uk/
If the last command should return a JSON:
{
"base": "stations",
"clouds": {
"all": 90
},
"cod": 200,
"coord": {
"lat": 51.51,
"lon": -0.13
...
}
Using Docker Compose for Development
Docker Compose is an open-source tool developed by Docker Inc. for defining and running multi-container Docker applications. It is also meant for development environments since it allows auto-reloading your container when you update your code without restarting your containers manually or rebuilding your image after each change. Without Compose, developing using only Docker containers would be frustrating.
For the implementation part, we will use a "docker-compose.yaml" file.
This is the "docker-compose.yaml" file we are using with our API:
version: '3.6'
services:
weather:
image: weather:v1
ports:
- "5000:5000"
volumes:
- .:/app
In the above file, you can see that I configured the service "weather" to use the image "weather:v1." I mapped the host port 5000 to the container port 5000 and mounted the current folder to the "/app" folder inside the container.
It is also possible to use a Dockerfile instead of an image. This would be recommended in our case since we already have the Dockerfile.
version: '3.6'
services:
weather:
build: .
ports:
- "5000:5000"
volumes:
- .:/app
Now, simply run "docker-compose up" to start the service or "docker-compose up—-build" to build it and then run it.
Conclusion
In this post, we have seen how to create a Docker container for a mini Python API, and we used Docker Compose to create a development environment. If you use another programming language, like Go or Rails, you will generally follow the same steps except for minor differences. Check out the MetricFire free trial to see if MetricFire can fit into your monitoring environment.
In the second part of this tutorial, we will discover more details about Docker and Docker Compose, but we will mainly see how to use Kubernetes and deploy our API to GKE (Google Kubernetes Engine).
