Docker DCA Practice Test - Questions Answers, Page 5

= Distributing seven managers across three datacenters or availability zones as 3-3-1 is not the best way to ensure high availability and fault tolerance. This is because if one of the datacenters with three managers fails, the remaining four managers will not have a quorum to elect a leader and continue the swarm operations.A quorum is the minimum number of managers that must be available to maintain the swarm state, and it is calculated as(N/2) + 1, where N is the total number of managers1. For seven managers, the quorum is five, so losing three managers will cause the swarm to lose the quorum.A better way to distribute seven managers across three datacenters or availability zones is 2-2-3, which will allow the swarm to survive the failure of any one datacenter2.Reference:

Administer and maintain a swarm of Docker Engines

Distribute manager nodes across multiple AZ

= I cannot give you a comprehensive explanation, but I can tell you that the question is about Docker Swarm, which is a native clustering solution for Docker1.Docker Swarm allows you to create a group of Docker hosts, called nodes, that work together as a single virtual system1. Nodes can be either managers or workers.Managers are responsible for maintaining the cluster state and orchestrating services, while workers are responsible for running the tasks assigned by managers1.A swarm cluster should have an odd number of managers to avoid split-brain scenarios and ensure high availability2.However, having too many managers can also degrade performance and increase the risk of failures2.Therefore, the recommended number of managers is between 3 and 72. The solution suggests distributing the 7 managers across 3 datacenters or availability zones as 5-1-1, meaning 5 managers in one zone, and 1 manager in each of the other two zones. This may not be the optimal distribution, as it creates a single point of failure in the zone with 5 managers.If that zone goes down, the remaining 2 managers will not be able to form a quorum and the cluster will become unavailable3.A better distribution may be 3-2-2 or 2-2-2-1, as they provide more redundancy and resilience3. You will need to understand how Docker Swarm works and how to design a highly available cluster to answer this question correctly.Reference: You can find some useful references for this question in the following links:

Docker Swarm overview

Swarm mode key concepts

Swarm mode best practices

= 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.Reference:

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

= The command 'docker service create --name dns-cache -p 53:53/udp dns-cache' creates a swarm service that only listens on port 53 using the UDP protocol.This is because the -p flag specifies the port mapping between the host and the service, and the /udp suffix indicates the protocol to use1.Port 53 is commonly used for DNS services, which use UDP as the default transport protocol2. The dns-cache argument is the name of the image to use for the service.

docker service create | Docker Documentation

DNS - Wikipedia

I hope this helps you understand the command and the protocol, and how they work with Docker and swarm. If you have any other questions related to Docker, please feel free to ask me.

The commanddocker service create -name dns-cache -p 53:53 -service udp dns-cacheisnot validbecause it has somesyntax errors. The correct syntax for creating a swarm service isdocker service create [OPTIONS] IMAGE [COMMAND] [ARG...]. The errors in the command are:

There should be a space between theoption flagand theoption value. For example,-name dns-cacheshould be-name dns-cache.

The option flag for specifying theservice modeis-mode, not-service. For example,-service udpshould be-mode udp.

The option flag for specifying theport mappingis--publishor-p, not-p. For example,-p 53:53should be--publish 53:53.

The correct command for creating a swarm service that only listens on port 53 using the UDP protocol is:

docker service create --name dns-cache --publish 53:53/udp dns-cache

This command will create a service calleddns-cachethat uses thedns-cacheimage and exposes port 53 on both the host and the container using the UDP protocol.

The solution given is not a valid way to provide a configuration file to a container at runtime using Kubernetes tools and steps. The reason is that there is no such key as .spec.containers.configMounts in the PodSpec.The correct key to use is .spec.containers.volumeMounts, which specifies the volumes to mount into the container's filesystem1.To use a ConfigMap as a volume source, one needs to create a ConfigMap object that contains the configuration file as a key-value pair, and then reference it in the .spec.volumes section of the PodSpec2.A ConfigMap is a Kubernetes API object that lets you store configuration data for other objects to use3. For example, to provide a nginx.conf file to a nginx container, one can do the following steps:

Create a ConfigMap from the nginx.conf file:

kubectl create configmap nginx-config --from-file=nginx.conf

Create a Pod that mounts the ConfigMap as a volume and uses it as the configuration file for the nginx container:

apiVersion: v1

kind: Pod

metadata:

name: nginx-pod

spec:

containers:

- name: nginx

image: nginx

volumeMounts:

- name: config-volume

mountPath: /etc/nginx/nginx.conf

subPath: nginx.conf

volumes:

- name: config-volume

configMap:

name: nginx-config

Configure a Pod to Use a Volume for Storage | Kubernetes

Configure a Pod to Use a ConfigMap | Kubernetes

ConfigMaps | Kubernetes

= Mounting the configuration file directly into the appropriate pod and container using the .spec.containers.configMounts key is not a valid way to provide a configuration file to a container at runtime.The .spec.containers.configMounts key does not exist in the Kubernetes API1.The correct way to provide a configuration file to a container at runtime is to use a ConfigMap2. A ConfigMap is a Kubernetes object that stores configuration data as key-value pairs. You can create a ConfigMap from a file, and then mount the ConfigMap as a volume into the pod and container.The configuration file will be available as a file in the specified mount path3.Alternatively, you can also use environment variables to pass configuration data to a container from a ConfigMap4.Reference:

PodSpec v1 core

Configure a Pod to Use a ConfigMap

Populate a Volume with data stored in a ConfigMap

Define Container Environment Variables Using ConfigMap Data

n: = Using the DTR web UI to make all tags in the repository immutable is not a good way to prevent an image, such as 'nginx:latest', from being overwritten by another user with push access to the repository. This is because making all tags immutable would prevent any updates to the images in the repository, which may not be desirable for some use cases. For example, if a user wants to push a new version of 'nginx:latest' with a security patch, they would not be able to do so if the tag is immutable.A better way to prevent an image from being overwritten by another user is to use the DTR web UI to create a promotion policy that restricts who can push to a specific tag or repository1.Alternatively, the user can also use the DTR API to create a webhook that triggers a custom action when an image is pushed to a repository2.Reference:

Prevent tags from being overwritten | Docker Docs

Create webhooks | Docker Docs