DCA Question #19: Real Exam Question with Answer & Explanation

Question

What behavior is expected when a service is created with the following command: 'docker service create --publish 8000:80 nginx'

Options

• AAll nodes in the cluster will listen on port 8080 and forward to port 80 in the container.
• BOnly a single node in the cluster will listen on port 8080 and forward to port 80 in the container.
• CAll nodes in the cluster will listen on port 80 and forward to port 8080 in the container.
• DOnly a single node in the cluster will listen on port 80 and forward to port 8080 in the container.

Explanation

In Docker Swarm, when a service publishes a port with host_port:container_port, all manager and worker nodes in the cluster listen on the specified host_port and route traffic to the container's container_port.

Common mistakes.

• B. The Docker Swarm routing mesh ensures that all nodes in the cluster listen on the published port, not just a single node, providing high availability and load balancing.
• C. The command --publish 8000:80 maps host port 8000 to container port 80, not the other way around.
• D. This option is incorrect because the mapping is reversed and the routing mesh involves all nodes, not just a single one.

Concept tested. Docker Swarm routing mesh and port publishing

Reference. https://docs.docker.com/engine/swarm/ingress/

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