Docker Interview Questions for Freshers/Docker Interview Questions and Answers for Freshers & Experienced

Can you lose data when the container exits?

No, any data that your application writes to disk get stored in container. The file system for the contain persists even after the container halts.

Can I use JSON instead of YAML for my compose file in Docker?

You can use JSON instead of YAML for your compose file, to use JSON file with compose, specify the JSON filename to use, for eg:

$ docker-compose -f docker-compose.json up

Does Docker offer support for IPV6?

Yes, Docker provides support IPv6. IPv6 networking is supported only on Docker daemons runs on Linux hosts. However, if you want to enable IPv6 support in the Docker daemon, you need to modify /etc/docker/daemon.json and set the ipv6 key to true.

How is Docker different from other containerization methods?

By using docker-compose, you can run multiple containers using a single service. All docker-compose files uses yaml language.

What are the steps for the Docker container life cycle?

Below are the steps for Docker life cycle:

Build
Pull
Run

List some of the more advanced Docker commands and what they do.

Some advanced commands include:

<> Docker info. Displays system-wide information regarding the Docker installation
<> Docker pull. Downloads an image
<> Docker stats. Provides you with container information
<> Docker images. Lists downloaded images

What is the method for creating a Docker container?

You can use any of the specific Docker images for creating a Docker container using the below command.

docker run -t -i command name

This command not only creates the container but also start it for you.

What is the functionality of a hypervisor?

A hypervisor, or virtual machine monitor, is software that helps us create and run virtual machines. It enables us to use a single host computer to support multiple guest virtual machines. It does this by dividing the system resources of the host and allocating them to the installed guest environments. Multiple operating systems can be installed on a single host operating system. There are two kinds of hypervisors:

>> Native: Native hypervisors, or bare-metal hypervisors, run directly on the underlying host system. It gives us direct access to the hardware of the host system and doesn’t require a base server operating system.

>> Hosted: Hosted hypervisors use the underlying host operating system.

Explain the process of scaling your Docker containers

The Docker containers can be scaled to any level starting from a few hundred to even thousands or millions of containers. The only condition for this is that the containers need the memory and the OS at all times, and there should not be a constraint when the Docker is getting scaled.

How to include code with copy/add or volumes?

In docker file, we need to use COPY or ADD directive. This is useful to relocate code. However, we should use a volume if we want to make changes.

Explain Implementation method of Continuous Integration(CI) and Continues Development (CD) in Docker?

You need to do the following things:

* Runs Jenkins on docker
* You can run integration tests in Jenkins using docker-compose

How does communication happen between Docker client and Docker Daemon?

You can communicate between Docker client and Docker Daemon with the combination of Rest API, socket.IO, and TCP.

How to build a Dockerfile?

Once you’ve written a Dockerfile, you need to build it to create an image with those specifications. Use the following command to build a Dockerfile:

$ docker build <path to docker file>

The next question would be when do you use “.dockerfile_name” and when to use the entire path?

Use “.dockerfile_name” when the dockerfile exits in the same file directory and you use the entire path if it lives somewhere else.

What is the best way of deleting a container?

We need to follow the following two steps for deleting a container:
- docker stop <container_id>
- docker rm <container_id>

What is the way to establish communication between docker host and Linux host?

This can be done using networking by identifying the “ipconfig” on the docker host. This command ensures that an ethernet adapter is created as long as the docker is present in the host.

How do you create a docker container from an image?

Pull an image from docker repository with the above command and run it to create a container. Use the following command:

$ docker run -it -d <image_name>

Most probably the next question would be, what does the ‘-d’ flag mean in the command?

-d means the container needs to start in the detached mode. Explain a little about the detach mode. Have a look at this blog to get a better understanding of different docker commands.

If you wish to use a base image and make modifications or personalize it, how do you do that?

You pull an image from docker hub onto your local system

It’s one simple command to pull an image from docker hub:

$ docker pull <image_name>

Can you differentiate between Daemon Logging and Container Logging?

In docker, logging is supported at 2 levels and they are logging at the Daemon level or logging at the Container level.
Daemon Level: This kind of logging has four levels- Debug, Info, Error, and Fatal.
- Debug has all the data that happened during the execution of the daemon process.
- Info carries all the information along with the error information during the execution of the daemon process.
- Errors have those errors that occurred during the execution of the daemon process.
- Fatal has the fatal errors that occurred during the execution.
Container Level:
- Container level logging can be done using the command: sudo docker run –it <container_name> /bin/bash
- In order to check for the container level logs, we can run the command: sudo docker logs <container_id>

How can you monitor the docker in production environments?

Docker states and Docker Events are used to monitoring docker in the production environment.

How to login into docker repository?

You can use the following command to login into hub.docker.com:

$ docker login

You’ll be prompted for your username and password, insert those and congratulations, you’re logged in.

What are the basic requirements for the docker to run on any system?

Docker can run on both Windows and Linux platforms.

<> For the Windows platform, docker atleast needs Windows 10 64bit with 2GB RAM space. For the lower versions, docker can be installed by taking help of the toolbox. Docker can be downloaded from https://docs.docker.com/docker-for-windows/ website.

<> For Linux platforms, Docker can run on various Linux flavors such as Ubuntu >=12.04, Fedora >=19, RHEL >=6.5, CentOS >=6 etc

Show how you would create a container from an image.

To create a container, you pull an image from the Docker repository and run it using the following command: $ docker run -it -d <image_name>

What are Docker object labels?

Docker object labels are key-value pairs that are stored as strings. They enable us to add metadata to Docker objects such as containers, networks, local daemons, images, Swarm nodes, and services.

If you vaguely remember the command and you’d like to confirm it, how will you get help on that particular command?

The following command is very useful as it gives you help on how to use a command, the syntax, etc.

$ docker --help

The above command lists all Docker commands. If you need help with one specific command, you can use the following syntax:

$ docker <command> --help

Docker Basic Commands

Once you’ve aced the basic conceptual questions, the interviewer will increase the difficulty level. So let’s move on to the next section of this Docker Interview Questions article. This section talks about the commands that are very common amongst docker users.

What is Docker Machine?

Docker machine is a tool that lets you install Docker Engine on virtual hosts. These hosts can now be managed using the docker-machine commands. Docker machine also lets you provision Docker Swarm Clusters.

Tell us something about Docker Compose.

Docker Compose is a YAML file which contains details about the services, networks, and volumes for setting up the Docker application. So, you can use Docker Compose to create separate containers, host them and get them to communicate with each other. Each container will expose a port for communicating with other containers.

What is Docker Hub?

Docker images create docker containers. There has to be a registry where these docker images live. This registry is Docker Hub. Users can pick up images from Docker Hub and use them to create customized images and containers. Currently, the Docker Hub is the world’s largest public repository of image containers.

What are the major components of Docker Architecture?

The four major components of Docker are daemon, Client, Host, and Registry

>> Docker daemon: It is also referred to as ‘dockerd’ and it accepts Docker API requests and manages Docker objects such as images, containers, networks, and volumes. It can also communicate with other daemons to manage Docker services.

>> Docker Client: It is the predominant way that enables Docker users to interact with Docker. It sends the docker commands to docked, which actually executes them using Docker API. The Docker client can communicate with more than one daemon.

>> Docker Registry: It hosts the Docker images and is used to pull and push the docker images from the configured registry. Docker Hub is the public registry that anyone can use, and Docker is configured to look for images on Docker Hub by default. However, it is always recommended for organizations to use own private registry.

>> Docker Host: It is the physical host (VM) on which Docker Daemon is running and docker images and containers are created.

Why use Docker?

Following are the reasons why one should use Docker:

<> It allows the use of system resources more efficiently
<> Software delivery cycles are faster with it
<> Application portability is possible and easy
<> It is great for the microservices architecture

What is the use of a Dockerfile?

A Dockerfile is a set of specific instructions that we need to pass on to Docker so that the images can be built. We can think of the Dockerfile as a text document which has all the commands that are needed for creating a Docker image. We can create an automated build that lets us execute multiple command lines one after the other.

What is a Docker Hub?

We can think of Docker Hub as a cloud registry that lets us link the code repositories, create the images, and test them. We can also store our pushed images, or we can link to the Docker Cloud, so that the images can be deployed to the host. We have a centralized container image discovery resource that can be used for the collaboration of our teams, automating the workflow and distribution, and changing management by creating the development pipeline.

Last simple question…Describe a Docker container’s lifecycle.

Although there are several different ways of describing the steps in a Docker container’s lifecycle, the following is the most common:

1. Create container
2. Run container
3. Pause container
4. Unpause container
5. Start container
6. Stop container
7. Restart container
8. Kill container
9. Destroy container

We will next look at the intermediate-level docker interview questions and answers.

Explain the Docker components.

The three architectural components include Docker Client, Host, and Registry.

<> Docker Client: This component executes build and run operations to communicate with the Docker Host.

<> Docker Host: This component holds the Docker Daemon, Docker images, and Docker containers. The daemon sets up a connection to the Docker Registry.

<> Docker Registry: This component stores Docker images. It can be a public registry, such as Docker Hub or Docker Cloud, or a private registry.

What is Docker Swarm?

Docker Swarm is a container orchestration tool that allows us to manage multiple containers across different host machines. With Swarm, we can turn multiple Docker hosts into a single host for easy monitoring and management.

Difference between virtualization and containerization

Once you’ve explained containerization and virtualization, the next expected question would be differences. The question could either be differences between virtualization and containerization or differences between virtual machines and containers. Either way, this is how you respond.

Containers provide an isolated environment for running the application. The entire user space is explicitly dedicated to the application. Any changes made inside the container is never reflected on the host or even other containers running on the same host. Containers are an abstraction of the application layer. Each container is a different application.

Whereas in Virtualization, hypervisors provide an entire virtual machine to the guest(including Kernal). Virtual machines are an abstraction of the hardware layer. Each VM is a physical machine.

What is containerization?

Let me explain this is with an example. Usually, in the software development process, code developed on one machine might not work perfectly fine on any other machine because of the dependencies. This problem was solved by the containerization concept. So basically, an application that is being developed and deployed is bundled and wrapped together with all its configuration files and dependencies. This bundle is called a container. Now when you wish to run the application on another system, the container is deployed which will give a bug-free environment as all the dependencies and libraries are wrapped together. Most famous containerization environments are Docker and Kubernetes.

Name and explain the various Docker components.

The three main Docker components are:

1. Docker Client. Performs Docker build pull and run operations to open up communication with the Docker Host. The Docker command then employs Docker API to call any queries to run.

2. Docker Host. Contains Docker daemon, containers, and associated images. The Docker daemon establishes a connection with the Registry. The stored images are the type of metadata dedicated to containerized applications.

3. Registry. This is where Docker images are stored. There are two of them, a public registry and a private one. Docker Hub and Docker Cloud are two public registries available for use by anyone.

When a docker container exits, will your data discard?

No, exiting a docker container will not discard your data. All the data written to the container gets automatically preserved on disk. It will get deleted only when you intentionally delete the container.

How do you build a Docker file?

After writing a docker file, you will now need to build it so as to create the image with the specifications given. Use the given command to build a docker file:

$ docker build <path to docker file>

What is memory-swap flag?

Memory-swap is a modified flag that only has meaning if- memory is also set. Swap allows the container to write express memory requirements to disk when the container has exhausted all the RAM which is available to it.

Why should anyone use Docker? What does it offer?

Docker gives users many incentives for adoption, such as:

<> An efficient and easy initial set up experience
<> The means to describe an application lifecycle in detail
<> Simple configuration and smooth interaction with Docker Compose
<> Complete and well-detailed documentation
<> Ability to run on a PC or enterprise IT system with equal ease

How can you stop, start or kill a container?

To stop a container, use the following command:
$ docker stop <container_id>
To start a docker container, use the following command:
$ docker start <container_id>
Kill a container by using the given command:
$ docker kill <container_id>

Explain Registries

There are two types of registry is

<> Public Registry
<> Private Registry

Docker’s public registry is called Docker hub, which allows you to store images privately. In Docker hub, you can store millions of images.

What is Docker Engine?

Docker daemon or Docker engine represents the server. The docker daemon and the clients should be run on the same or remote host, which can communicate through command-line client binary and full RESTful API.

What is Docker image?

The Docker image help to create Docker containers. You can create the Docker image with the build command. Due to this, it creates a container that starts when it begins to run. Every docker images are stored in the Docker registry.

What is virtualization?

Virtualization is the process of creating a software-based, virtual version of something(compute storage, servers, application, etc.). These virtual versions or environments are created from a single physical hardware system. Virtualization lets you split one system into many different sections which act like separate, distinct individual systems. A software called Hypervisor makes this kind of splitting possible. The virtual environment created by the hypervisor is called Virtual Machine.

What are the important features of Docker?

Here are the essential features of Docker:

* Easy Modeling
* Version control
* Placement/Affinity
* Application Agility
* Developer Productivity
* Operational Efficiencies

What are the advantages of using Docker container?

Here, are a major advantage of using Docker.

* Offers an efficient and easy initial set up
* Allows you to describe your application lifecycle in detail
* Simple configuration and interacts with Docker Compose.
* Documentation provides every bit of information.

What is a Container?

Docker is an open-source lightweight containerization technology. It has gained widespread popularity in the cloud and application packaging world. It allows you to automate the deployment of applications in lightweight and portable containers.

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