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CMSC398W

Docker Application Day

Three debugging tasks, each involving a broken Docker setup. Read the error, find the cause, fix it.

Repository: https://github.com/cmsc398w/DockerApplicationDay

Submit on Gradescope.

Part 1: Debug a Broken Docker Build

A FastAPI app has a Dockerfile that fails to build. Find the problem and fix it.

Terminal window
cd assignment/part1
docker build -t fastapi-app .

Read the error. Look at the Dockerfile and the app/ directory. Think about build context and working directory.

After fixing it:

Terminal window
docker build -t fastapi-app .
docker run -d -p 8000:8000 fastapi-app
curl http://localhost:8000

Submit: The corrected Dockerfile and 1-2 sentences on what was wrong and how you fixed it.

Part 2: Debug a Container Networking Issue

A FastAPI app and a Redis cache are running via Docker Compose, but the API cannot reach Redis.

Terminal window
cd assignment/part2
docker-compose up -d
curl http://localhost:8000/health
# {"status":"unhealthy","redis":"disconnected",...}

Get into the container and investigate:

Terminal window
docker exec -it part2-api-1 bash


ss -tulnp          # what's listening on this container's localhost?
nc -zv localhost 6379   # can we reach port 6379 here?
ping -c 2 redis         # does the "redis" hostname resolve?
dig localhost +short
dig redis +short         # are these the same IP?
nc -zv redis 6379        # can we reach redis:6379?

Look at api/app.py. What host is the code using to connect to Redis? Fix it.

Terminal window
docker-compose down && docker-compose up
curl http://localhost:8000/health
# {"status":"healthy","redis":"connected"}

Submit: A brief write-up on the change you made and why.

Part 3: Write a Dockerfile from Scratch

You have a Python HTTP server. Write a Dockerfile that packages it.

The Dockerfile must:

  • Use an appropriate Python base image
  • Set a working directory
  • Copy the application files
  • Expose port 8080
  • Define the command to run the server
Terminal window
docker build -t simple-server .
docker run -p 8080:8080 simple-server


curl http://localhost:8080
# {"message":"Simple Python HTTP Server","status":"running"}

Submit: Your Dockerfile and a brief explanation of why you chose each instruction.