Docker Updated DCA Exam Questions and Answers by rueben

= The command ‘docker service create -name dns-cache -p 53:53/udp dns-cache’ is not correct and will not create a swarm service that only listens on port 53 using the UDP protocol. There are two errors in the command:

The option -name should be --name with two dashes, otherwise it will be interpreted as a short option -n followed by an argument ame1.

The option -p or --publish will publish the service port to the host port, which means the service will be reachable from outside the swarm2. To create a service that only listens on the internal network, you need to use the --publish-add option with the mode=ingress flag3.

The correct command should be:

docker service create --name dns-cache --publish-add mode=ingress,target=53,published=53,protocol=udp dns-cache

docker service create | Docker Docs

Publish ports on the host | Docker Docs

Publish a port for a service | Docker Docs

 Environment variables cannot be used to schedule containers to meet the security policy requirements. Environment variables are used to pass configuration data to the containers, not to control where they run1. To schedule containers to run on separate nodes in a Swarm cluster, you need to use node labels and service constraints23. Node labels are key-value pairs that you can assign to nodes to organize them into groups4. Service constraints are expressions that you can use to limit the nodes where a service can run based on the node labels. For example, you can label some nodes as env=dev and others as env=prod, and then use the constraint --constraint node.labels.env==dev or --constraint node.labels.env==prod when creating a service to ensure that it runs only on the nodes with the matching label. References:

1: Environment variables in Compose | Docker Docs

2: Deploy services to a swarm | Docker Docs

3: How to use Docker Swarm labels to deploy containers on specific nodes

4: Manage nodes in a swarm | Docker Docs

[5]: Swarm mode routing mesh | Docker Docs

[6]: Docker Swarm - How to set environment variables for tasks on various nodes

= Distributing seven managers across three datacenters or availability zones as 3-2-2 is not a good way to ensure high availability and fault tolerance. This is because a swarm cluster requires a majority of managers (more than half) to be available and able to communicate with each other in order to maintain the swarm state and avoid a split-brain scenario1. If one of the datacenters or availability zones with three managers goes down, the remaining four managers will not have a quorum and the swarm will stop functioning. A better way to distribute seven managers across three datacenters or availability zones is 3-3-1 or 3-2-1-1, which will allow the swarm to survive the loss of one or two datacenters or availability zones, respectively2. References:

Administer and maintain a swarm of Docker Engines | Docker Docs

How to Create a Cluster of Docker Containers with Docker Swarm and DigitalOcean on Ubuntu 16.04 | DigitalOcean

A Kubernetes node is allocated a /26 CIDR block (64 unique IPs) for its address space. This means that the node can assign up to 64 IP addresses to its resources, such as pods and containers. If every pod on this node has exactly two containers in it, then each pod will need two IP addresses, one for each container. Therefore, the node can support up to 32 pods, since 64 / 2 = 32. The other options are incorrect because they either exceed the available IP addresses or do not account for the number of containers per pod. References:

•CIDR Blocks and Container Engine for Kubernetes - Oracle

•How kubernetes assigns podCIDR for nodes? - Stack Overflow