Overlay networks in Swarm in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of an overlay network in Docker Swarm?

Overlay networks in Docker Swarm allow containers to communicate across different hosts. What is the main reason to use an overlay network?

A To enable communication between containers on different Docker hosts within the same Swarm cluster

B To isolate containers on the same host from each other

C To speed up container startup time on a single host

D To provide persistent storage for containers

💡 Hint

Think about how containers on different machines can talk to each other.

Overlay networks connect containers running on different Docker hosts in a Swarm, enabling them to communicate as if they were on the same network.

💻 Command Output

intermediate

Output of creating an overlay network in Docker Swarm

What is the expected output after running the command to create an overlay network named mynet in Docker Swarm?

Docker

docker network create --driver overlay mynet

A Network mynet created successfully

B Error: network driver not found

C a long JSON output describing the network

D mynet

💡 Hint

Docker usually returns the network ID or name after creation.

The command returns the network ID or name (here, the name 'mynet') to confirm creation.

❓ Configuration

advanced

Correct Docker Compose snippet for using overlay network in Swarm

Which Docker Compose snippet correctly defines a service that uses an overlay network named mynet in a Swarm stack?

Docker

version: '3.8'
services:
  web:
    image: nginx
    networks:
      - mynet
networks:
  mynet:
    driver: overlay

A

version: '3.8'
services:
  web:
    image: nginx
    networks:
      - mynet
networks:
  mynet:
    driver: overlay

B

version: '3.8'
services:
  web:
    image: nginx
    networks:
      - mynet
networks:
  mynet:
    driver: bridge

C

version: '3.8'
services:
  web:
    image: nginx
    network_mode: overlay
networks:
  mynet:
    driver: overlay

D

version: '3.8'
services:
  web:
    image: nginx
    networks:
      - mynet
networks:
  mynet:
    driver: host

💡 Hint

Overlay networks require the driver to be set to 'overlay' under networks.

Option A correctly defines the overlay network and attaches the service to it. Other options use wrong drivers or incorrect keys.

❓ Troubleshoot

advanced

Why does a container fail to communicate over an overlay network?

You created an overlay network and deployed services in Docker Swarm, but containers on different nodes cannot communicate. What is a likely cause?

A The overlay network was created with the bridge driver

B The containers are using the host network driver instead of overlay

C The Docker Swarm nodes are not connected via a VPN or have blocked required ports

D The containers are running on the same node

💡 Hint

Overlay networks require certain ports open between nodes.

Overlay networks need nodes to communicate over specific ports (like 4789 UDP). If firewalls block these, communication fails.

🔀 Workflow

expert

Correct sequence to deploy a multi-service app using overlay network in Swarm

Arrange the steps in the correct order to deploy a multi-service application using an overlay network in Docker Swarm.

A 2,1,4,3

B 1,4,2,3

C 1,2,4,3

D 4,1,2,3

💡 Hint

Think about setting up the cluster before creating networks and deploying services.

First initialize Swarm, then add workers, create overlay network, then deploy services using it.