Manager and worker nodes in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the role of a manager node in Docker Swarm?

A manager node controls the swarm, manages cluster state, and assigns tasks to worker nodes.

beginner

What does a worker node do in a Docker Swarm?

A worker node runs the containers assigned by the manager node but does not manage the swarm.

intermediate

How do manager nodes communicate with worker nodes in Docker Swarm?

Manager nodes send tasks to worker nodes using the swarm's internal network and keep track of their status.

intermediate

Can a node be both a manager and a worker in Docker Swarm?

Yes, a node can act as both manager and worker, handling management tasks and running containers.

intermediate

Why is it recommended to have multiple manager nodes in a Docker Swarm?

Multiple manager nodes provide high availability and fault tolerance for the swarm control plane.

What is the primary responsibility of a Docker Swarm manager node?

A Storing container images

B Running application containers only

C Assigning tasks to worker nodes

D Monitoring network traffic

Manager nodes assign tasks to worker nodes and manage the swarm state.

Which node type runs containers but does not manage the swarm?

A Load balancer node

B Manager node

C Registry node

D Worker node

Worker nodes run containers assigned by managers but do not manage the swarm.

Can a Docker Swarm node be both a manager and a worker?

A Yes, it can perform both roles

B No, roles are exclusive

C Only if it is a manager node

D Only if it is a worker node

A node can be both manager and worker, handling management and running containers.

Why should you have multiple manager nodes in a swarm?

A To increase container speed

B For high availability and fault tolerance

C To reduce network traffic

D To store more images

Multiple managers ensure the swarm control plane stays available if one fails.

How do manager nodes keep track of worker nodes?

A Using the swarm's internal network and status updates

B By monitoring their CPU usage only

C By checking container logs

D Through external monitoring tools only

Managers communicate with workers over the swarm network to track their status.

Explain the difference between manager and worker nodes in Docker Swarm.

Think about who controls and who executes.

Why is it important to have multiple manager nodes in a Docker Swarm cluster?

Consider what happens if one manager node fails.

Practice

1. What is the main role of a manager node in a Docker swarm?

easy

A. To monitor network traffic between containers

B. To run the containers and execute tasks

C. To control the swarm and assign tasks to worker nodes

D. To store container images locally

Solution

1. Step 1: Understand the role of manager nodes

   Manager nodes control the swarm and decide which tasks run where.

2. Step 2: Compare with worker nodes

   Worker nodes only run tasks assigned by managers, they don't control the swarm.

3. Final Answer:

   To control the swarm and assign tasks to worker nodes -> Option C

4. Quick Check:

   Manager node role = control and assign tasks [OK]

Hint: Managers control, workers execute tasks [OK]

Common Mistakes:

• Confusing worker nodes as controllers
• Thinking managers run containers directly
• Assuming managers store images only
• Mixing network monitoring with node roles

2. Which of the following commands correctly initializes a Docker swarm as a manager node?

easy

A. docker node create manager

B. docker swarm init

C. docker swarm start manager

D. docker node init

Solution

1. Step 1: Recall the command to start a swarm

   The command docker swarm init initializes the current node as a manager.

2. Step 2: Check other options for correctness

   Other commands like docker node create or docker swarm start do not exist or are invalid.

3. Final Answer:

   docker swarm init -> Option B

4. Quick Check:

   Initialize swarm = docker swarm init [OK]

Hint: Use 'docker swarm init' to start manager node [OK]

Common Mistakes:

• Using non-existent commands
• Confusing node creation with swarm init
• Trying 'docker node init' which is invalid
• Assuming 'docker swarm start' works

3. Given the following commands run on a new machine:

docker swarm init
docker node ls

What will be the output of docker node ls?

medium

A. A list showing one manager node with status 'Ready' and availability 'Active'

B. An error saying no nodes found

C. A list showing one worker node with status 'Down'

D. Empty output with no nodes listed

Solution

1. Step 1: Understand 'docker swarm init' effect

   This command makes the current machine a manager node in the swarm.

2. Step 2: Check 'docker node ls' output

   It lists all nodes in the swarm. Since only one node exists (the manager), it shows that node as 'Ready' and 'Active'.

3. Final Answer:

   A list showing one manager node with status 'Ready' and availability 'Active' -> Option A

4. Quick Check:

   One manager node listed after init = A list showing one manager node with status 'Ready' and availability 'Active' [OK]

Hint: After init, node list shows one active manager [OK]

Common Mistakes:

• Expecting worker nodes without joining
• Thinking output is empty or error
• Confusing node status terms
• Assuming node is down immediately

4. You ran docker swarm join --token SWMTKN-1-xyz 192.168.1.10:2377 on a node but it failed with "Error response from daemon: This node is already part of a swarm." What is the likely cause?

medium

A. The node is already a member of another swarm

B. The token is invalid or expired

C. The manager IP address is incorrect

D. Docker daemon is not running on the node

Solution

1. Step 1: Analyze the error message

   The error says the node is already part of a swarm, so it cannot join another without leaving first.

2. Step 2: Check other options

   Invalid token or wrong IP would cause different errors. Docker daemon not running would prevent any command execution.

3. Final Answer:

   The node is already a member of another swarm -> Option A

4. Quick Check:

   Node already in swarm = join fails with this error [OK]

Hint: Node must leave swarm before joining another [OK]

Common Mistakes:

• Assuming token or IP is always the cause
• Ignoring existing swarm membership
• Not checking Docker daemon status
• Trying to join without leaving first

5. You want to add a new worker node to an existing swarm. The manager node IP is 10.0.0.5 and the join token is SWMTKN-1-abc123. Which sequence of commands correctly adds the worker node and verifies it joined?

hard

A. On worker: docker node add --token SWMTKN-1-abc123 10.0.0.5On manager: docker swarm ls

B. On manager: docker swarm join --token SWMTKN-1-abc123 10.0.0.5:2377On worker: docker node ls

C. On manager: docker node join --token SWMTKN-1-abc123 10.0.0.5On worker: docker swarm ls

D. On worker: docker swarm join --token SWMTKN-1-abc123 10.0.0.5:2377On manager: docker node ls

Solution

1. Step 1: Join the worker node to the swarm

   The worker node uses docker swarm join --token TOKEN MANAGER_IP:2377 to join the swarm.

2. Step 2: Verify the node joined on the manager

   On the manager node, docker node ls lists all nodes including the new worker.

3. Final Answer:

   On worker: docker swarm join --token SWMTKN-1-abc123 10.0.0.5:2377; On manager: docker node ls -> Option D

4. Quick Check:

   Join on worker, list on manager = correct process [OK]

Hint: Join on worker, verify on manager with 'docker node ls' [OK]

Common Mistakes:

• Running join on manager instead of worker
• Using wrong commands like 'docker node add'
• Trying to list nodes on worker node
• Omitting port 2377 in join command