Docker Interview Questions

Checkout Vskills Interview questions with answers in DOCKER to prepare for your next job role. The questions are submitted by professionals to help you to prepare for the Interview.

Q.1 What is Docker?

Docker is a containerization platform that allows you to package applications and their dependencies into standardized units called containers, making it easy to deploy and run applications consistently across different environments.

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Q.2 What is a container in Docker?

A container in Docker is a lightweight, standalone, and executable package that includes everything needed to run a piece of software, including the code, runtime, system tools, libraries, and settings. Containers isolate applications from their host and other containers.

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Q.3 How does Docker differ from virtualization?

Docker uses containerization, which shares the host OS kernel, making it more lightweight and efficient than traditional virtualization, which runs multiple guest OSes on a hypervisor.

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Q.4 What is a Docker image?

A Docker image is a read-only template that contains all the instructions and dependencies needed to create a Docker container. Images are used to create containers and are stored in a registry.

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Q.5 What is a Docker container registry?

A Docker container registry is a repository for storing and distributing Docker images. Popular container registries include Docker Hub, Amazon ECR, Google Container Registry, and others.

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Q.6 How do you create a Docker container from an image?

To create a Docker container from an image, you use the docker run command followed by the image name. For example: docker run .

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Q.7 What is the Dockerfile?

A Dockerfile is a text file that contains instructions for building a Docker image. It specifies the base image, application code, dependencies, and configuration needed to create a container.

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Q.8 How do you build a Docker image from a Dockerfile?

To build a Docker image from a Dockerfile, you use the docker build command followed by the path to the Dockerfile. For example: docker build -t .

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Q.9 What is the purpose of the -t flag when building a Docker image?

The -t flag in the docker build command is used to tag the image with a user-friendly name and an optional version or tag, making it easier to reference when creating containers.

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Q.10 How do you list all running Docker containers?

To list all running Docker containers, you use the docker ps command. The -a flag can be added to show all containers, including stopped ones.

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Q.11 How do you stop a running Docker container?

To stop a running Docker container, you use the docker stop command followed by the container's name or ID. For example: docker stop .

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Q.12 How do you remove a Docker container?

To remove a Docker container, you use the docker rm command followed by the container's name or ID. For example: docker rm .

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Q.13 What is the purpose of the -f flag when removing a Docker container?

The -f flag in the docker rm command is used to force the removal of a running container. Use it with caution, as it will forcibly stop and remove the container.

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Q.14 How do you list all Docker images on your system?

To list all Docker images on your system, you use the docker images command.

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Q.15 How do you remove a Docker image?

To remove a Docker image, you use the docker rmi command followed by the image name or ID. For example: docker rmi .

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Q.16 What is Docker Compose?

Docker Compose is a tool for defining and running multi-container Docker applications. It allows you to define the services, networks, and volumes in a docker-compose.yml file and start them with a single command.

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Q.17 How do you start containers defined in a Docker Compose file?

To start containers defined in a Docker Compose file, you use the docker-compose up command in the same directory as the docker-compose.yml file.

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Q.18 How do you scale services in Docker Compose?

To scale services in Docker Compose, you can use the --scale flag followed by the service name and the desired number of replicas. For example: docker-compose up --scale =3.

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Q.19 What is Docker Swarm?

Docker Swarm is a native clustering and orchestration tool for Docker. It allows you to create and manage a swarm of Docker nodes to deploy and manage services across a cluster of machines.

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Q.20 How do you initialize a Docker Swarm?

To initialize a Docker Swarm, you use the docker swarm init command on a manager node. It generates a token that worker nodes can use to join the swarm.

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Q.21 How do you add worker nodes to a Docker Swarm?

To add worker nodes to a Docker Swarm, you use the docker swarm join command on each worker node, providing the token generated during swarm initialization.

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Q.22 How do you deploy a service in Docker Swarm?

To deploy a service in Docker Swarm, you use the docker service create command, specifying the desired image, replicas, and other configuration options.

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Q.23 What is the purpose of Docker Hub?

Docker Hub is a cloud-based container registry where you can find and share Docker images. It serves as a central repository for Docker images, making it easy to distribute and collaborate on containerized applications.

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Q.24 How do you log into Docker Hub from the command line?

To log into Docker Hub from the command line, you use the docker login command and provide your Docker Hub username and password.

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Q.25 How do you push a Docker image to Docker Hub?

To push a Docker image to Docker Hub, you first log in using docker login, then use the docker push command followed by the image name and tag. For example: docker push .

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Q.26 How do you pull a Docker image from Docker Hub?

To pull a Docker image from Docker Hub, you use the docker pull command followed by the image name and tag. For example: docker pull .

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Q.27 What is a Docker volume?

A Docker volume is a mechanism for persisting data in Docker containers, separate from the container's file system. Volumes are used to share and manage data between containers and can be mounted to specific paths.

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Q.28 How do you create a Docker volume?

To create a Docker volume, you use the docker volume create command followed by the volume name. For example: docker volume create .

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Q.29 How do you mount a Docker volume to a container?

To mount a Docker volume to a container, you use the -v or --volume flag with the docker run command, specifying the volume name and the container path where the volume should be mounted.

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Q.30 What is Docker network mode "bridge"?

The "bridge" network mode is the default network mode in Docker. It creates an isolated network on the host machine, allowing containers to communicate with each other using private IP addresses.

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Q.31 How do you create a custom Docker network?

To create a custom Docker network, you use the docker network create command followed by the network name. For example: docker network create .

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Q.32 How do you attach a container to a Docker network?

To attach a container to a Docker network, you use the --network flag with the docker run command, specifying the network name. For example: docker run --network .

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Q.33 What is Docker container orchestration?

Docker container orchestration is the automated management and scaling of containerized applications across multiple hosts. It ensures high availability, load balancing, and fault tolerance. Docker Swarm and Kubernetes are popular orchestration tools.

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Q.34 What is Kubernetes?

Kubernetes is an open-source container orchestration platform for automating the deployment, scaling, and management of containerized applications. It provides features for container scheduling, load balancing, and more.

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Q.35 What is the difference between Docker Swarm and Kubernetes?

Docker Swarm is a simpler, built-in orchestration tool for Docker, while Kubernetes is a more complex, feature-rich, and widely adopted orchestration platform. Kubernetes offers more advanced scheduling and management capabilities.

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Q.36 What are Docker containers best suited for?

Docker containers are best suited for microservices architecture, CI/CD pipelines, scaling applications, and isolating dependencies. They are ideal for modern, cloud-native development and deployment practices.

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Q.37 What are some common challenges when using Docker?

Common challenges with Docker include container security, managing container sprawl, monitoring and logging, storage management, and learning curve for orchestration tools like Kubernetes.

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Q.38 What is Docker Compose used for?

Docker Compose is used for defining and running multi-container applications. It simplifies the management of interconnected containers by specifying their configuration in a single file.

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Q.39 How can you secure Docker containers?

You can secure Docker containers by regularly updating images and packages, limiting container privileges, using image vulnerability scanning tools, implementing network segmentation, and following security best practices.

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Q.40 What is the Docker Swarm "manager" node?

In a Docker Swarm cluster, the "manager" node is responsible for orchestrating and managing the swarm. It can schedule containers and handle tasks such as service discovery and load balancing.

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Q.41 How does Docker handle application scaling?

Docker can handle application scaling by creating multiple instances (replicas) of a service across nodes in a swarm or cluster. Load balancing ensures that incoming traffic is distributed among the replicas.

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Q.42 What is the purpose of a Docker Compose YAML file?

A Docker Compose YAML file is used to define a multi-container application's services, networks, and volumes, as well as their configurations. It simplifies the deployment of complex applications with interconnected containers.

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Q.43 How do you remove all Docker containers and images?

To remove all Docker containers and images, you can use a combination of commands. First, remove all containers using docker rm -f $(docker ps -aq), and then remove all images using docker rmi $(docker images -q).

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Q.44 What is Docker Desktop?

Docker Desktop is a tool for Windows and macOS that provides an easy way to run Docker containers on desktop machines. It includes the Docker Engine, Compose, and Kubernetes support for local development.

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Q.45 How can you troubleshoot issues in Docker containers?

Troubleshooting Docker containers involves checking container logs, examining the container's environment, running diagnostic commands, and using monitoring and logging tools to identify and resolve issues.

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Q.46 What is Docker Swarm "service" mode?

Docker Swarm "service" mode is used to define long-running, scalable services in a Docker Swarm cluster. Services can be configured with replicas, network settings, and other options for scaling and high availability.

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Q.47 What is Docker's "overlay" network driver?

Docker's "overlay" network driver is used for multi-host networking in Docker Swarm. It enables containers on different hosts to communicate over a virtual network and provides built-in load balancing and DNS resolution.

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Q.48 How do you create a secret in Docker Swarm?

To create a secret in Docker Swarm, you use the docker secret create command followed by the secret name and a file or literal value. For example: docker secret create my_secret my_secret.txt.

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Q.49 What is the purpose of Docker's "healthcheck" feature?

Docker's "healthcheck" feature allows you to define a health check command for a container. Docker periodically runs the command and updates the container's health status based on the command's exit code and output.

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Q.50 How can you remove all Docker containers with a specific label?

You can remove all Docker containers with a specific label by using a combination of commands. First, list the containers with the label using docker ps -q -f "label=", and then remove them using docker rm -f $(docker ps -q -f "label=").

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Q.51 What is the Docker Hub "Official Images" program?

The Docker Hub "Official Images" program is a curated collection of Docker images maintained by Docker and trusted partners. These images are considered reliable and undergo regular security updates.

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Q.52 What are some security considerations when using Docker images from the internet?

When using Docker images from the internet, consider image source trustworthiness, image scanning for vulnerabilities, minimizing the attack surface, and following security best practices during image development.

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Q.53 How do you remove all Docker volumes?

To remove all Docker volumes, you can use a combination of commands. First, list all volumes using docker volume ls -q, and then remove them using docker volume rm $(docker volume ls -q).

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Q.54 What is the purpose of Docker's "build context" in a Dockerfile?

The build context in a Dockerfile is the set of files and directories that are sent to the Docker daemon during the image build process. It includes the Dockerfile itself and any additional files needed for the build.

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Q.55 How do you create a custom bridge network in Docker?

To create a custom bridge network in Docker, you use the docker network create command followed by the --driver bridge option and the network name. For example: docker network create --driver bridge my_bridge_network.

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Q.56 What is container orchestration and why is it important?

Container orchestration is the automated management of containerized applications. It is important for ensuring scalability, high availability, load balancing, fault tolerance, and efficient resource utilization in containerized environments.

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Q.57 What is the purpose of Docker's "scratch" image?

The "scratch" image in Docker is an empty image that serves as a minimal base for creating custom images. It has no filesystem layers and is often used for building minimal, statically linked executables.

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Q.58 How do you pass environment variables to a Docker container?

You can pass environment variables to a Docker container using the -e or --env flag with the docker run command, specifying the variable name and value. For example: docker run -e MY_VAR=value .

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Q.59 What is Docker's "swarm mode"?

Docker's "swarm mode" is a built-in feature for orchestrating containers in a cluster of Docker nodes. It enables you to create and manage a swarm of nodes, deploy services, and handle tasks such as load balancing and scaling.

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Q.60 How do you configure a Docker container to restart automatically?

To configure a Docker container to restart automatically, you can use the --restart option with the docker run command and specify policies like "always" or "on-failure." For example: docker run --restart=always .

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Q.61 What is the purpose of a Docker Compose override file?

A Docker Compose override file is used to define configuration overrides for services defined in the main docker-compose.yml file. It allows you to customize settings for specific environments or deployments.

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Q.62 How can you access logs from a Docker container?

You can access logs from a Docker container using the docker logs command followed by the container's name or ID. For example: docker logs .

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Q.63 What is Docker's "user" directive in a Dockerfile?

The "user" directive in a Dockerfile is used to specify the user and optionally the group that a container process should run as when the container is started. It helps improve security by reducing the permissions of the container process.

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Q.64 How do you create a custom Docker network with a specific subnet and gateway?

To create a custom Docker network with a specific subnet and gateway, you use the docker network create command and specify the --subnet and --gateway options along with the network name.

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Q.65 What is Docker's "attach" command?

The docker attach command is used to attach the standard input, output, and error streams of a running container to the current terminal, allowing you to interact with the container's console.

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Q.66 What is Docker's "entrypoint" in a Dockerfile?

The "entrypoint" in a Dockerfile is used to specify the default command that should be executed when a container starts. It can be overridden by providing a command when running the container.

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Q.67 What is the difference between a Docker image and a Docker container?

A Docker image is a read-only template that contains the application code and its dependencies. A Docker container is a running instance of an image, including the application process and a writable filesystem layer for runtime changes.

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Q.68 What are some ways to optimize Docker image size?

You can optimize Docker image size by using multi-stage builds, minimizing layers, cleaning up unnecessary files, and using a smaller base image. Alpine Linux is often used as a minimal base image for size optimization.

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Q.69 What is Docker's "volume mode"?

Docker's "volume mode" is a feature that allows you to specify how volumes are mounted on Windows containers. It can be set to "default," "bind," or "mix," depending on the desired behavior.

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Q.70 What is the purpose of Docker's "network-alias" in a Docker Compose file?

The "network-alias" in a Docker Compose file is used to define additional network aliases for a service. These aliases can be used to access the service by different hostnames within the same network.

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Q.71 How can you list all Docker networks on your system?

To list all Docker networks on your system, you can use the docker network ls command.

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Q.72 How do you specify the network mode for a service in a Docker Compose file?

You can specify the network mode for a service in a Docker Compose file by using the network_mode key under the service's configuration, followed by the desired mode (e.g., "bridge," "host," or a custom network name).

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Q.73 What is the purpose of Docker's "buildkit" feature?

Docker's "buildkit" is an advanced build subsystem that improves the efficiency of image builds. It offers features like parallelization, cache management, and build secrets for enhanced security.

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Q.74 How can you pass a secret to a Docker container?

You can pass a secret to a Docker container by creating a secret in Docker Swarm using docker secret create and then referencing the secret in a service's configuration or as an environment variable in the docker run command.

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Q.75 What is the purpose of Docker's "external" network in Docker Compose?

The "external" network in Docker Compose is used to connect services in the Compose file to an existing Docker network created externally. It allows you to reuse and share networks between multiple Compose projects or stacks.

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Q.76 How do you specify the startup order of services in Docker Compose?

You can specify the startup order of services in Docker Compose using the depends_on key in a service's configuration. This ensures that services start in the specified order, waiting for dependent services to become healthy.

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Q.77 What is Docker's "health" directive in a Dockerfile?

The "health" directive in a Dockerfile is used to define a custom health check command for a container. It allows you to specify how Docker should determine the health status of the container.

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Q.78 How do you check the health status of a Docker service in Docker Swarm?

You can check the health status of a Docker service in Docker Swarm using the docker service inspect command followed by the service name. The "Health" section of the output provides information about the service's health.

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Q.79 How do you remove a Docker Compose stack?

To remove a Docker Compose stack, you use the docker stack rm command followed by the stack name. For example: docker stack rm .

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Q.80 What is the purpose of Docker's "secret" feature in Swarm mode?

Docker's "secret" feature in Swarm mode allows you to securely manage sensitive data such as passwords and API keys by storing them as secrets and making them available to services in a secure and controlled manner.

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Q.81 How can you update a Docker service in Swarm mode?

You can update a Docker service in Swarm mode using the docker service update command, specifying the service name and desired configuration changes such as image updates, environment variables, and replicas.

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Q.82 What is Docker's "Config" feature in Swarm mode?

Docker's "Config" feature in Swarm mode allows you to manage configuration data as named objects, making it easier to maintain and update application configuration across multiple services and replicas.

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Q.83 How do you create a Docker Config in Swarm mode?

To create a Docker Config in Swarm mode, you use the docker config create command followed by the config name and a file or literal value. For example: docker config create my_config my_config.txt.

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Q.84 How can you use a Docker Config in a service in Swarm mode?

You can use a Docker Config in a service in Swarm mode by referencing the config in the service's configuration, typically as a file or environment variable. This allows you to inject configuration data into containers securely.

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Q.85 What is Docker's "stack" concept in Swarm mode?

In Docker Swarm mode, a "stack" is a collection of services, networks, and volumes defined in a single Compose file. Stacks simplify the deployment and management of multi-service applications on a Swarm cluster.

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Q.86 How do you deploy a Docker stack in Swarm mode?

To deploy a Docker stack in Swarm mode, you use the docker stack deploy command followed by the stack name and the Compose file. For example: docker stack deploy -c .

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Q.87 What is Docker's "secret ref" feature in Swarm mode?

Docker's "secret ref" feature in Swarm mode allows you to reference a secret in a service's configuration by name, making it easier to manage and update secrets across services without exposing their values.

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Q.88 How do you update a Docker stack in Swarm mode?

To update a Docker stack in Swarm mode, you modify the Compose file used for deployment and then use the docker stack deploy command again with the updated Compose file. Docker Swarm will apply the changes to the existing stack.

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Q.89 What is Docker's "config ref" feature in Swarm mode?

Docker's "config ref" feature in Swarm mode allows you to reference a config in a service's configuration by name, making it simpler to manage and update configuration data across services.

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Q.90 How do you list all Docker services in Swarm mode?

To list all Docker services in Swarm mode, you use the docker service ls command. It provides information about the services, including their names, replicas, and other details.

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Q.91 What is Docker's "network alias" feature in Swarm mode?

Docker's "network alias" feature in Swarm mode allows you to define additional network aliases for a service. These aliases can be used to access the service by different hostnames within the same network.

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Q.92 How do you remove a Docker Config in Swarm mode?

To remove a Docker Config in Swarm mode, you use the docker config rm command followed by the config name. For example: docker config rm .

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Q.93 What is Docker's "network attachable" mode for user-defined networks?

Docker's "network attachable" mode for user-defined networks allows you to create networks that are not limited to a single stack. These networks can be shared and used across multiple stacks, making it easier to connect services.

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Q.94 How do you create an attachable network in Docker Swarm mode?

To create an attachable network in Docker Swarm mode, you use the --attachable option when creating the network with the docker network create command.

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Q.95 How do you set environment variables for a Docker service in Swarm mode?

You can set environment variables for a Docker service in Swarm mode by specifying them in the service's configuration using the environment key or by using the --env option with the docker service create command.

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Q.96 What is the purpose of Docker's "placement preferences" in Swarm mode?

Docker's "placement preferences" in Swarm mode allow you to influence the scheduling decisions for service replicas by specifying constraints based on node labels, such as preferring nodes with specific attributes or hardware capabilities.

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Q.97 What is Docker's "shutdown signal" feature in Swarm mode?

Docker's "shutdown signal" feature in Swarm mode allows you to specify a custom signal that a service uses when stopping containers. It can be useful for applications that require a specific signal for graceful shutdown.

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Q.98 How do you remove a Docker service in Swarm mode?

To remove a Docker service in Swarm mode, you use the docker service rm command followed by the service name. For example: docker service rm .

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Q.99 How do you list all Docker Configs in Swarm mode?

To list all Docker Configs in Swarm mode, you use the docker config ls command. It provides information about the configs, including their names and IDs.

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Q.100 What is Docker's "rolling update" feature in Swarm mode?

Docker's "rolling update" feature in Swarm mode allows you to update a service with minimal downtime by gradually replacing old containers with new ones, based on the specified update parameters and health checks.

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Q.101 How do you inspect the details of a Docker Config in Swarm mode?

You can inspect the details of a Docker Config in Swarm mode using the docker config inspect command followed by the config name. For example: docker config inspect .

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Q.102 What is Docker's "container name pattern" in Swarm mode?

Docker's "container name pattern" in Swarm mode allows you to specify a custom pattern for naming containers when deploying services. It can include placeholders for values like service name, task ID, and more.

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Q.103 How do you use Docker Compose in Swarm mode?

To use Docker Compose in Swarm mode, you add the --orchestrator swarm option when running Compose commands. This allows you to deploy and manage Compose stacks on a Docker Swarm cluster.

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Q.104 What is Docker's "endpoint mode" for services in Swarm mode?

Docker's "endpoint mode" for services in Swarm mode allows you to specify how the service's published ports should behave, such as "vip" (Virtual IP), "dnsrr" (DNS round-robin), or "host" (host mode).

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Q.105 How do you specify resource limits for a Docker service in Swarm mode?

You can specify resource limits for a Docker service in Swarm mode by using the --limit-cpu and --limit-memory options when creating or updating the service. These options define the maximum CPU and memory resources the service can use.

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Q.106 What is the purpose of Docker's "node constraint" in Swarm mode?

Docker's "node constraint" in Swarm mode allows you to control the placement of service tasks on nodes based on labels and constraints. It ensures that tasks are scheduled on nodes that meet specified criteria.

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Q.107 How do you specify service replicas in a Docker Compose file for Swarm mode?

To specify service replicas in a Docker Compose file for Swarm mode, you use the replicas key under the service's configuration, specifying the desired number of replicas. For example: replicas: 3.

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Q.108 What is Docker's "endpoint mode" for published ports in Swarm mode?

Docker's "endpoint mode" for published ports in Swarm mode determines how incoming traffic to a service is distributed. "vip" mode uses a virtual IP address, "dnsrr" uses DNS round-robin, and "host" uses the host's IP and port.

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Q.109 How do you create a custom network with a specific driver in Docker Swarm mode?

To create a custom network with a specific driver in Docker Swarm mode, you use the docker network create command and specify the --driver option along with the network name.

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Q.110 What is Docker's "rollback" feature for service updates in Swarm mode?

Docker's "rollback" feature for service updates in Swarm mode allows you to revert to the previous service definition if an update fails. You can specify rollback conditions and thresholds to trigger automatic rollbacks.

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