Docker architecture (client, daemon, registry) - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The Docker Client is the user interface that accepts commands from users and sends them to the Docker Daemon to execute. It acts like a remote control for Docker.

beginner

Explain the Docker Daemon and its function.

The Docker Daemon runs on the host machine and manages Docker objects like containers, images, networks, and volumes. It listens for commands from the Docker Client and performs the requested actions.

beginner

What is a Docker Registry and why is it important?

A Docker Registry is a storage and distribution system for Docker images. It allows users to share and download images. Docker Hub is a popular public registry.

intermediate

How do the Docker Client and Docker Daemon communicate?

They communicate through REST API calls over a socket or network interface. The client sends commands, and the daemon executes them.

intermediate

Describe the flow when you run 'docker run' command.

The Docker Client sends the 'run' command to the Docker Daemon. The daemon checks if the image exists locally or pulls it from the Registry. Then it creates and starts the container.

Which component in Docker architecture listens for commands and manages containers?

A Docker Daemon

B Docker Client

C Docker Registry

D Docker Compose

The Docker Daemon listens for commands from the client and manages containers and other Docker objects.

Where are Docker images stored for sharing and downloading?

A Dockerfile

B Docker Daemon

C Docker Client

D Docker Registry

Docker Registry stores Docker images so users can share and download them.

How does the Docker Client communicate with the Docker Daemon?

A Direct file access

B Through REST API calls

C Using SSH only

D Via Docker Registry

The Docker Client sends commands to the Docker Daemon using REST API calls over a socket or network.

What happens if the Docker Daemon does not find an image locally when running a container?

A It pulls the image from the Docker Registry

B It asks the Docker Client to build the image

C It throws an error and stops

D It creates a new image automatically

The Docker Daemon pulls the image from the Docker Registry if it is not found locally.

Which of these is NOT part of Docker architecture?

A Docker Client

B Docker Daemon

C Docker Kernel

D Docker Registry

Docker Kernel is not a component; Docker uses the host OS kernel but does not have its own kernel.

Describe the roles of Docker Client, Docker Daemon, and Docker Registry in Docker architecture.

Think about who talks, who does the work, and where images are kept.

Explain the process that happens when you run a Docker container using 'docker run'.

Follow the command from user input to container start.

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