Dockerfile for microservices - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile line to specify the base image for a Node.js microservice.

Microservices

FROM [1]

Drag options to blanks, or click blank then click option'

Amysql:8

Bpython:3.12

Cubuntu:latest

Dnode:18-alpine

Attempts:

3 left

💡 Hint

Common Mistakes

Using a base image for a different language or service like Python or MySQL.

Using a heavy base image when a lightweight one is preferred.

The FROM instruction sets the base image. For a Node.js microservice, node:18-alpine is a lightweight and common choice.

2fill in blank

medium

Complete the Dockerfile command to copy the application source code into the container.

Microservices

COPY [1] /app

Drag options to blanks, or click blank then click option'

Apackage.json

B.

Cnode_modules

DDockerfile

Attempts:

3 left

💡 Hint

Common Mistakes

Copying only package.json without source files.

Copying node_modules which should be installed inside the container.

The COPY . /app command copies all files from the current directory into the container's /app folder.

3fill in blank

hard

Fix the error in the Dockerfile command to install dependencies using npm.

Microservices

RUN npm [1]

Drag options to blanks, or click blank then click option'

Ainstall

Bstart

Cbuild

Dtest

Attempts:

3 left

💡 Hint

Common Mistakes

Using npm start which runs the app, not installs packages.

Using npm build which is not a standard npm command.

The RUN npm install command installs all dependencies listed in package.json.

4fill in blank

hard

Fill both blanks to expose the correct port and set the container's working directory.

Microservices

WORKDIR [1]
EXPOSE [2]

Drag options to blanks, or click blank then click option'

A/app

B3000

C/usr/src/app

D8080

Attempts:

3 left

💡 Hint

Common Mistakes

Setting a working directory that doesn't match the copied files.

Exposing the wrong port that the app does not listen on.

WORKDIR /app sets the working directory inside the container. EXPOSE 3000 tells Docker the app listens on port 3000.

5fill in blank

hard

Fill the blanks to define the command that starts the Node.js microservice.

Microservices

CMD ["[1]", "[2]"]

Drag options to blanks, or click blank then click option'

Anode

Bapp.js

Cserver.js

Dnpm

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'npm' as the first command in this JSON array format.

Using 'app.js' when the main file is 'server.js' or vice versa.

The CMD instruction defines the command to run the app. Usually, node server.js starts the service. Using npm here is incorrect for this format.

Practice

1. What is the main purpose of a Dockerfile in a microservices project?

easy

A. To monitor the performance of the microservice

B. To write the microservice's business logic code

C. To define how to build a container image for the microservice

D. To deploy the microservice to the cloud

Solution

1. Step 1: Understand the role of Dockerfile

   A Dockerfile contains instructions to build a container image, including base image, dependencies, and commands.

2. Step 2: Differentiate from other tasks

   Writing code, monitoring, and deployment are separate tasks outside the Dockerfile's scope.

3. Final Answer:

   To define how to build a container image for the microservice -> Option C

4. Quick Check:

   Dockerfile = build container image [OK]

Hint: Dockerfile builds images, not code or deployment [OK]

Common Mistakes:

• Confusing Dockerfile with source code files
• Thinking Dockerfile handles deployment
• Assuming Dockerfile monitors services

2. Which of the following is the correct syntax to specify the base image in a Dockerfile?

easy

A. BASE python:3.12-slim

B. START python:3.12-slim

C. IMAGE python:3.12-slim

D. FROM python:3.12-slim

Solution

1. Step 1: Recall Dockerfile base image syntax

   The Dockerfile uses the FROM keyword to specify the base image.

2. Step 2: Verify other options

   BASE, IMAGE, and START are not valid Dockerfile instructions.

3. Final Answer:

   FROM python:3.12-slim -> Option D

4. Quick Check:

   Base image starts with FROM [OK]

Hint: Base image always starts with FROM in Dockerfile [OK]

Common Mistakes:

• Using incorrect keywords like BASE or IMAGE
• Forgetting the colon between image name and tag
• Writing lowercase FROM

3. Given this Dockerfile snippet:

FROM node:18-alpine
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
CMD ["node", "server.js"]

What happens when you build and run this container?

medium

A. The container fails because WORKDIR is missing

B. The container runs the server.js file using Node.js

C. The container installs Python dependencies

D. The container runs npm start automatically

Solution

1. Step 1: Analyze Dockerfile commands

   The base image is Node.js 18 Alpine. It sets working directory to /app, copies package.json, runs npm install, copies all files, then runs node server.js.

2. Step 2: Understand container behavior

   On running, the container executes node server.js, starting the Node.js app. No Python involved. WORKDIR is present, so no failure.

3. Final Answer:

   The container runs the server.js file using Node.js -> Option B

4. Quick Check:

   CMD runs node server.js [OK]

Hint: CMD runs the specified command when container starts [OK]

Common Mistakes:

• Assuming Python dependencies install
• Thinking WORKDIR is missing
• Confusing CMD with npm start

4. Identify the error in this Dockerfile snippet for a Python microservice:

FROM python:3.12
COPY requirements.txt /app/
RUN pip install -r requirements.txt
WORKDIR /app
COPY . .
CMD ["python", "app.py"]

medium

A. The WORKDIR should be set before copying requirements.txt

B. The pip install command is missing the --user flag

C. The CMD syntax is incorrect

D. The base image version is invalid

Solution

1. Step 1: Check file paths and working directory order

   The requirements.txt is copied to /app/, but WORKDIR is set after. So pip install runs in root, not /app, causing file not found error.

2. Step 2: Correct order for Dockerfile commands

   Set WORKDIR /app before copying files and running commands to ensure correct paths.

3. Final Answer:

   The WORKDIR should be set before copying requirements.txt -> Option A

4. Quick Check:

   Set WORKDIR before file operations [OK]

Hint: Set WORKDIR before copying files and running commands [OK]

Common Mistakes:

• Running pip install before setting WORKDIR
• Misunderstanding CMD JSON syntax
• Assuming base image version is wrong

5. You want to optimize a Dockerfile for a Java microservice to reduce build time and image size. Which change is best to achieve this?

FROM openjdk:17
COPY . /app
WORKDIR /app
RUN ./gradlew build
CMD ["java", "-jar", "build/libs/app.jar"]

hard

A. Copy only build.gradle and settings.gradle first, run gradlew build, then copy the rest

B. Remove the WORKDIR instruction

C. Use CMD java -jar build/libs/app.jar without JSON array

D. Change base image to openjdk:8

Solution

1. Step 1: Understand Docker layer caching

   Docker caches layers. Copying only build files first and running build caches dependencies, so changes in source code don't rebuild dependencies.

2. Step 2: Apply multi-step copy for optimization

   Copy build.gradle and settings.gradle first, run gradlew build, then copy source files. This reduces rebuild time and image size.

3. Final Answer:

   Copy only build.gradle and settings.gradle first, run gradlew build, then copy the rest -> Option A

4. Quick Check:

   Optimize Dockerfile with layered caching [OK]

Hint: Copy build files first to leverage Docker cache [OK]

Common Mistakes:

• Removing WORKDIR breaks path context
• Using shell form CMD can cause signal issues
• Downgrading base image unnecessarily