Docker architecture (client, daemon, registry) - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Role of Docker Client in Docker Architecture

What is the primary role of the Docker Client in the Docker architecture?

A It acts as the user interface to send commands to the Docker Daemon.

B It manages and runs containers directly on the host system.

C It stores Docker images and makes them available for download.

D It builds Docker images from Dockerfiles automatically.

💡 Hint

Think about which part the user interacts with directly to control Docker.

The Docker Client is the command-line tool or API that users interact with. It sends commands to the Docker Daemon, which does the actual work of managing containers and images.

🧠 Conceptual

intermediate

Function of Docker Daemon

Which statement best describes the function of the Docker Daemon?

A It listens for Docker Client requests and manages Docker objects like images and containers.

B It stores Docker images in a cloud-based registry service.

C It provides a web interface for managing Docker containers remotely.

D It compiles Dockerfiles into executable binaries.

💡 Hint

Consider which component actually runs containers and handles Docker commands.

The Docker Daemon runs on the host machine and listens for commands from the Docker Client. It manages images, containers, networks, and storage volumes.

🧠 Conceptual

advanced

Purpose of Docker Registry

What is the main purpose of a Docker Registry in the Docker architecture?

A It manages network connections between Docker containers.

B It builds Docker images from source code automatically.

C It stores and distributes Docker images to clients and daemons.

D It monitors running containers and reports their status.

💡 Hint

Think about where Docker images are saved and shared from.

A Docker Registry is a storage and distribution system for Docker images. Public registries like Docker Hub or private registries allow users to push and pull images.

🔀 Workflow

advanced

Sequence of Docker Command Execution

Arrange the steps in the correct order when a user runs docker run command.

A 1,3,2,4

B 1,2,3,4

C 2,1,3,4

D 1,2,4,3

💡 Hint

Think about the logical order from command to container start.

First, the client sends the command. Then the daemon checks for the image locally. If missing, it pulls from the registry. Finally, it creates and starts the container.

❓ Troubleshoot

expert

Diagnosing Docker Client and Daemon Communication Issue

A user runs docker ps but gets the error: Cannot connect to the Docker daemon. What is the most likely cause?

A The container is stopped and cannot be listed.

B Docker Registry is down and cannot provide container images.

C Docker Client has an outdated version incompatible with the Daemon.

D Docker Daemon is not running or not reachable by the Docker Client.

💡 Hint

Consider what happens if the client cannot talk to the daemon.

The error means the Docker Client cannot reach the Docker Daemon. This usually happens if the daemon is stopped or there are permission/network issues.

Practice

1. Which component of Docker architecture is responsible for sending commands to manage containers?

easy

A. Docker Registry

B. Docker Daemon

C. Docker Client

D. Docker Image

Solution

1. Step 1: Understand the role of Docker Client

   The Docker Client is the tool users interact with to send commands like docker run or docker build.

2. Step 2: Differentiate from Daemon and Registry

   The Daemon executes commands, and the Registry stores images, but only the Client sends commands.

3. Final Answer:

   Docker Client -> Option C

4. Quick Check:

   Client sends commands = C [OK]

Hint: Client sends commands, Daemon does work, Registry stores images [OK]

Common Mistakes:

• Confusing Daemon with Client
• Thinking Registry sends commands
• Mixing up Image with Client

2. Which of the following commands correctly starts the Docker daemon on a Linux system?

easy

A. systemctl start docker

B. service docker-client start

C. docker daemon run

D. docker start daemon

Solution

1. Step 1: Identify the correct system command to start Docker daemon

   On Linux, the Docker daemon is managed by systemd and started with systemctl start docker.

2. Step 2: Eliminate incorrect commands

   docker start daemon and docker daemon run are invalid Docker commands. service docker-client start is incorrect because the daemon is called 'docker', not 'docker-client'.

3. Final Answer:

   systemctl start docker -> Option A

4. Quick Check:

   Use systemctl to start Docker daemon = D [OK]

Hint: Use systemctl to manage Docker daemon on Linux [OK]

Common Mistakes:

• Using docker commands to start daemon directly
• Confusing client and daemon service names
• Using service command with wrong service name

3. What will happen when you run docker pull nginx if the image is not present locally?

medium

A. The command fails because image is missing

B. Docker Client builds the image locally

C. Docker Registry sends the image to the Client directly

D. Docker Daemon downloads the image from Docker Registry

Solution

1. Step 1: Understand the pull command flow

   When docker pull nginx is run, the Client sends the request to the Daemon.

2. Step 2: Role of Daemon and Registry

   The Daemon downloads the image from the Registry and stores it locally. The Client does not build the image, and the Registry does not send images directly to Client.

3. Final Answer:

   Docker Daemon downloads the image from Docker Registry -> Option D

4. Quick Check:

   Pull downloads image via Daemon from Registry = A [OK]

Hint: Pull triggers daemon to fetch image from registry [OK]

Common Mistakes:

• Thinking Client builds image on pull
• Assuming Registry sends image directly to Client
• Believing pull fails if image missing locally

4. You run docker run hello-world but get an error saying the Docker daemon is not running. What is the most likely fix?

medium

A. Restart the Docker Client

B. Start the Docker Daemon service

C. Pull the image manually from the Registry

D. Reinstall Docker Registry

Solution

1. Step 1: Identify the error cause

   The error indicates the Docker Daemon is not running, so commands cannot be executed.

2. Step 2: Fix by starting the Daemon

   Starting the Docker Daemon service (e.g., systemctl start docker) will fix the issue. Restarting Client or pulling image manually won't help if Daemon is down.

3. Final Answer:

   Start the Docker Daemon service -> Option B

4. Quick Check:

   Daemon must run for commands to work = A [OK]

Hint: Daemon must be running to execute Docker commands [OK]

Common Mistakes:

• Restarting Client instead of Daemon
• Trying to pull image without Daemon running
• Reinstalling Registry unnecessarily

5. You want to push a custom image to a private Docker Registry. Which sequence correctly describes the interaction between Docker components?

hard

A. Client builds image -> Daemon pushes image -> Registry stores image

B. Daemon builds image -> Client pushes image -> Registry stores image

C. Registry builds image -> Client pushes image -> Daemon stores image

D. Client builds image -> Registry pushes image -> Daemon stores image

Solution

1. Step 1: Understand image build and push roles

   The Client sends build commands; the Daemon builds the image locally.

2. Step 2: Understand push process

   The Client commands the Daemon to push the image; the Daemon uploads it to the Registry, which stores it.

3. Step 3: Eliminate incorrect sequences

   Daemon does not push without Client command; Registry does not build or push images; Daemon stores images locally, not pushed images.

4. Final Answer:

   Client builds image -> Daemon pushes image -> Registry stores image -> Option A

5. Quick Check:

   Client commands, Daemon acts, Registry stores = B [OK]

Hint: Client commands, Daemon acts, Registry stores images [OK]

Common Mistakes:

• Thinking Registry builds or pushes images
• Mixing roles of Client and Daemon
• Assuming Daemon stores pushed images