Add a health check to a Docker container running a simple Node.js application.

Question

Add a health check to a Docker container running a simple Node.js application. The health check should verify that the application is running and accessible.

Sample Healthcheck API in node.js,

app.js

// app.js
const express = require('express');
const app = express();
 
// A simple route to return the status of the application
app.get('/health', (req, res) => {
    res.status(200).send('OK');
});
 
// Example main route
app.get('/', (req, res) => {
    res.send('Hello, Docker!');
});
 
// Start the server on port 3000
const port = 3000;
app.listen(port, () => {
    console.log(`App is running on http://localhost:${port}`);
});

Solution

Step 1: Create a Dockerfile

Create a Dockefile to containerize the node app. It also should include a health check command. https://docs.docker.com/reference/dockerfile/#healthcheck

# Use an official Node.js runtime as a parent image. 
# We are using slim version, so that i can install curl.
FROM node:14-slim

# Install curl
RUN apt-get update && apt-get install -y curl

# Set the working directory in the container
WORKDIR /app

# Copy the current directory contents into the container
COPY package*.json ./
COPY app.js ./

# Install any needed packages
RUN npm install

# Make port 3000 available to the world outside this container
EXPOSE 3000

# Run the application
CMD [ "node", "app.js" ]

# Add a HEALTHCHECK instruction
HEALTHCHECK --interval=30s --timeout=5s --start-period=5s --retries=3 \
  CMD curl --fail http://localhost:3000/health || exit 1

• HEALTHCHECK: The health check is added at the end of the Dockerfile. It checks the /health endpoint of your application every 30 seconds.

• --interval=30s: Check the health every 30 seconds.

• --timeout=5s: If the health check takes longer than 5 seconds, it’s considered a failure.

• --start-period=5s: Wait 5 seconds before starting the first health check.

• --retries=3: If the health check fails 3 consecutive times, the container is considered unhealthy.

Step 2: Build and Run the Docker Image

First, build the Docker image,

docker build -t node-app .

Then, run the container,

docker run -d --name my-node-app -p 3000:3000 node-app

Step 3: Verify the Health Check

To check the status of the container’s health, run

docker ps

In the output, you should see a STATUS column that will change from starting to healthy once the health check passes.