Manager and worker nodes in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Role of Manager Node in Docker Swarm

In a Docker Swarm cluster, what is the primary responsibility of a manager node?

A Scheduling tasks and managing cluster state

B Only running containers without managing the cluster

C Storing container images locally

D Providing network access to containers

💡 Hint

Think about which node controls the cluster and decides where containers run.

The manager node controls the swarm, schedules tasks, and manages cluster state. Worker nodes only run tasks assigned by managers.

💻 Command Output

intermediate

Output of 'docker node ls' on Manager Node

What is the expected output of the command docker node ls when run on a manager node in a Docker Swarm?

Docker

docker node ls

A Lists all nodes in the swarm with their status and roles

B Shows only the current node's status

C Returns an error if run on a worker node

D Displays running containers on the node

💡 Hint

Consider what information a manager node can access about the swarm.

The docker node ls command lists all nodes in the swarm with their roles and status.

🔀 Workflow

advanced

Joining a Worker Node to a Docker Swarm

Which sequence of commands correctly adds a new worker node to an existing Docker Swarm managed by a manager node?

A 2,1,3,4

B 1,3,2,4

C 1,2,3,4

D 3,2,1,4

💡 Hint

Think about how you get the join token and then use it on the worker.

First, get the join token on the manager, then copy the full join command, run it on the worker, and finally verify the join on the manager.

❓ Troubleshoot

advanced

Error When Worker Node Fails to Join Swarm

A worker node fails to join the swarm and shows the error: Error response from daemon: rpc error: code = Unavailable desc = connection error: connection refused. What is the most likely cause?

A The worker node has an incorrect Docker version

B The worker node cannot reach the manager node on port 2377

C The join token is expired after 24 hours

D The worker node is already part of another swarm

💡 Hint

Consider network connectivity between nodes.

The error indicates the worker cannot connect to the manager's swarm port (2377). Network issues or firewall blocking this port are common causes.

✅ Best Practice

expert

Ensuring High Availability of Manager Nodes

What is the recommended minimum number of manager nodes in a Docker Swarm cluster to ensure high availability and fault tolerance?

A 1

B 2

C 5

D 3

💡 Hint

Think about quorum and consensus in distributed systems.

At least 3 manager nodes are recommended to maintain quorum and allow the swarm to tolerate one manager failure without losing cluster state.

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