Docker Interview Questions and Answers for Beginners

1. What is Docker?
Answer: I will suggest you start with a small definition of Docker.
Docker is a containerization platform which packages your application and all its dependencies together in the form of containers so as to ensure that your application works seamlessly in any environment be it development or test or production. (Docker interview Questions)
Docker containers, wrap a piece of software in a complete filesystem that contains everything needed to run: code, runtime, system tools, system libraries, etc. anything that can be installed on a server.
This guarantees that the software will always run the same, regardless of its environment.
You can refer the diagram shown below, as you can see that containers run on a single machine share the same operating system kernel, they start instantly as only apps need to start as the kernel is already running and uses less RAM.
ting archive logs to disk. 

2. What is Dockerfile used for?
Answer: Dockerfile is nothing but a set of instructions that have to be passed on to Docker itself so that it can build images automatically reading these instructions from that specified Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession.

3. How far do Docker containers scale?
Answer: Best examples in the Web deployments like Google, Twitter and best examples in the Platform Providers like Heroku, dotCloud run on Docker which can scale from the ranges of hundreds of thousands to millions of containers running in parallel, given the condition that the OS and the memory doesn’t run out from the hosts which runs all these innumerable containers hosting your applications.

4. How many containers can run per host?
Answer: Depending on the environment where Docker is going to host the containers, there can be as many containers as the environment supports. The application size, available resources (like CPU, memory) will decide on the number of containers that can run on an environment. Though containers create newer CPU on their own they can definitely provide efficient ways of utilizing the resources. The containers themselves are super lightweight and only last as long as the process they are running.

5. What are the unique features of Docker over other containerization technology?
Answer: Here we list some of the most important and unique features of Docker that make it a top containerization technology unlike any other in the market today
You can run your Docker container either on your PC or your enterprise IT system
Along with the Docker Hub which is a repository of all containers you can deploy and download all your applications from a central location
You can even share your applications with the containers that you create. 

6. what is the single most important requirement for building a Docker container?
Answer: The most important requirement for building a container with Docker is the default image. This default image may vary depending on the code that you’re using. To find out (and access) the default image, you should go to the Docker Hub and search for the specific domain that you need. After you find the image, all that’s left to do is deal with the documentation and that it – you can create a Docker container!

7. How would you go about increasing the buffer cache hit ratio?
Answer: Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command. 

8. What do you understand by Docker?
Answer: Firstly, Docker is a type of tool that is used to make the application, deploy and run in an easier way. The Docker uses the container to make the applications run easier. It is a type of the new generation of virtualization which is used to make the complex software accessible and confined compared to the other types of computer programs.

9. What is Docker cloud?
Answer: 
Docker cloud is a type of service that is being provided by the Docker. In the Docker clouds you can just carry out the following operations:
Nodes
Cloud Repository
Continuous Integration
Application deployment
Continuous Deployment

10. If you are using the Docker container can you lose any data?
Answer: No, if you are using the Docker container you will not lose any of the data. The data will be preserved in the Docker container until and unless the container is deleted

11. What is the difference between Grant command and Revoke command?
Answer: A Grant command permits the End-User to perform certain activities onto the database whereas a Revoke command prevents the End-User from making any changes to the Database.

12. Explain Docker Container?
Answer: A container is the basic unit of software which holds the code and all its dependencies, in order to make the application run smoothly, quickly and reliably from one computing ecosystem to another. A Docker container may be created using a Docker image. It is an executable package of the software, which holds everything that is required to run an application, which are system tools, libraries, code, runtime, and settings. 

13. How to identify the status of a Docker Container
Answer: To identify the status of a Docker container, one should run the command
This command will furnish the list of all available Docker containers with the respective status on the host. From the list one can easily make out the intended container to check its status.

14. What are the Docker Image and Docker Run Command?
Answer: A Docker Image is a group of files and an amalgamation of parameters which allow the creation of instances that run in distinct containers as isolated processes. An image is basically built using the instructions for a complete and executable version of an application, which relies on the host OS kernel. The Docker run command can be used to create the instance called container which can be run using the Docker image. When the Docker user runs an image, it becomes one or multiple instances of that container.

15. Which is more suitable for Docker Container, Stateless or Stateful application?
Answer: Stateless applications should be preferred over a Stateful application for Docker Container. For instance, we can create one container from our application and take out the configurable state parameters from the app. Once it is one, we can run the same container with different parameters in production and other environments. Through the Stateless application, we can reuse the same image in distinct scenarios. Also, it is easier to scale a Stateless application that a Stateful application when it comes to Docker Containers.

16. If you were to exit the Docker Container, will you lose your data?
Answer: When a Docker Container is exited, no data loss occurs as all the data is written to the disk by the application for the sole purpose of preservation. This process is consistently repeated until and unless the container is unequivocally deleted. Moreover, the file system for the Docker container persists even after the Docker container is halted. 

17. What exactly do you mean by “Dockerized node”? Can this node be on-premises or in the cloud?
Answer: A Dockerized node is anything i.e a bare metal server, VM or public cloud instance that has the Docker Engine installed and running on it.
Docker can manage nodes that exist on-premises as well as in the cloud. Docker Datacenter is an on-premises solution that enterprises use to create, manage, deploy and scale their applications and comes with support from the Docker team. It can manage hosts that exist in your data center as well as in your virtual private cloud or public cloud provider (AWS, Azure, Digital Ocean, SoftLayer, etc.).

18. Briefly summarize containers and why they are innovative and useful for development?
Answer: Containers, at a basic level, are used to keep apps secluded from the entire host system that they will be running on. This is useful because developers will be able to package an application with everything it needs, from libraries to other files, and ship it as a single package. Containers make it substantially easier to provide consistency, speedy production, and replicability in a project.

19. What is an immutable image?
Answer: An immutable image is an image that includes that source code and has all the requirements needed to run through the application. Using immutable images is advantageous because they are more portable, make it easier to perform A/B testing and rollback, and can easily change runtime dependencies.

20. How can this process issue be solved
Answer: Use CMD’s JSON form plus ENTRYPOINT in the Dockerfile. Avoid using the string form because it forces Docker to run the processes with bash, which handles signals improperly. Modify the app that is running to add a SIGTERM signal handler. Set the stop_signal to a signal which the application knows how to handle. If modifying the app isn’t possible, wrap the application in a light init system that can handle SIGTERM correctly.

21. When comparing Docker and Kubernetes, what would you say are the biggest differences between them? Should both be used together?
Answer: Docker is particularly useful for building and deploying containers. Kubernetes is useful for managing containers. Docker Swarm does have a native container management tool that is helpful and gets the job done, but using Docker and Kubernetes together could have beneficial results as well. Docker Swarm runs at a much faster pace, provides documentation for every piece of information in the Swarm, allows for easy and quick configuration, ensures application isolation, and offers version control as well as component reusability. Unfortunately, Docker Swarm is very much dependant on the platform and doesn’t allow for storage options or decent monitoring. Kubernetes, on the other hand, provides great declarative configuration and has great storage volume. Using the two together (along with Swarm) could provide a perfect balance if the developers at hand know how to properly navigate them both as separate entities and as one solution. 

22. What, in your opinion, is the most exciting potential use for Docker?
Answer: The most exciting potential use of Docker that I can think of is its build pipeline. Most of the Docker professionals are seen using hyper-scaling with containers, and indeed get a lot of containers on the host that it actually runs on. These are also known to be blatantly fast. Most of the development – test build pipeline is completely automated using the Docker framework. (Company)

23. Tell us how you have used Docker in your past position?
Answer: 
 Explain how you have used Docker to help rapid deployment. Explain how you have scripted Docker and used Docker with other tools like Puppet, Chef or Jenkins.
If you have no past practical experience in Docker and have past experience with other tools in a similar space, be honest and explain the same. In this case, it makes sense if you can compare other tools to Docker in terms of functionality.

24. What is the use case for Docker?
Answer: Well, I think, docker is extremely useful in development environments. Especially for testing purposes. You can deploy and re-deploy apps in a blink of an eye.
Also, I believe there are use cases where you can use Docker in production. Imagine you have some Node.js application providing some services on the web. Do you really need to run the full OS for this
Eventually, if docker is good or not should be decided on an application basis. For some apps, it can be sufficient, for others not.

25. Why Do My Services Take 10 Seconds To Recreate Or Stop?
Answer: Compose stop attempts to stop a container by sending a SIGTERM. It then waits for a default timeout of 10 seconds. After the timeout, a SIGKILL is sent to the container to forcefully kill it. If you are waiting for this timeout, it means that your containers aren’t shutting down when they receive the SIGTERM signal.
There has already been a lot written about this problem of processes handling signals in containers.
Make sure you’re using the JSON form of CMD and ENTRYPOINT in your Dockerfile.
For example use [“program”, “arg1”, “arg2″] not” program arg1 arg2”. Using the string form causes Docker to run your process using bash which doesn’t handle signals properly. Compose always uses the JSON form, so don’t worry if you override the command or entry point in your Compose file.

If you are able, modify the application that you’re running to add an explicit signal handler for SIGTERM.
Set the stop_signal to a signal which the application knows how to handle:
web: build: stop_signal: SIGINT
If you can’t modify the application, wrap the application in a lightweight init system (like s6) or a signal proxy (like dumb-init or tini). Either of these wrappers takes care of handling SIGTERM properly. 

26. What are the components of Docker Architecture and explain?
Answer: This is the common laborer Interview queries asked in Associate in a Nursing interview. The Docker works on a client-server architecture. The laborer consumer establishes communication with the laborer Daemon. The laborer consumer and Daemon will run on an equivalent system. A Docket client can also be connected to a remote Docker daemon. The different varieties of laborer parts in a very laborer design AR

Docker Client: Performs Docker build pull and run operations to building communication with the Docker Host. The laborer command uses laborer API to decision the queries to be run.
Docker Host: Component contains Docker Daemon, Container, and its images. images will be the kind of metadata for the applications which are the containerized in the container. The Docker Daemon build a connection with Registry.
Registry: component will be storing the Docker images. The public registries Ar laborer Hub and laborer Cloud which may be s employed by anyone. 

27. What is the life-cycle of a Docker Container?
Answer: This is the most popular Docker Interview Questions asked in an interview.

The life cycle of the Docker container is as below:

  • Create a container.
  • Run the Docker container.
  • Pause the Container.
  • Unpause the Container.
  • Start the Container.
  • Stop the Container.
  • Restart the Container.
  • Kill the Container.
  • Destroy the Container.
  • Go ahead with docker training at onlineitguru with lifetime support

28. What’s the difference between up, run, and start?
Answer: Typically, you want to docker-compose up. Use up to start or restart all the services defined in a docker-compose.yml. In the default “attached” mode, you’ll see all the logs from all the containers. In “detached” mode (-d), Compose exits after starting the containers, but the containers continue to run in the background.
The docker-compose run command is for running “one-off” or “Adhoc” tasks. It requires the service name you want to run and only starts containers for services that the running service depends on. Use run to run tests or perform an administrative task such as removing or adding data to a data volume container. The run command acts like docker run -ti in that it opens an interactive terminal to the container and returns an exit status matching the exit status of the process in the container.
The docker-compose start command is useful only to restart containers that were previously created but were stopped. It never creates new containers.

29. Will Cloud Automation Overtake Containerization?
Answer: USER TABLES: Is a collection of tables created and maintained by the user. Contain USER information. DATA DICTIONARY: Is a collection of tables created and maintained by the Oracle Server. It contains database information. All data dictionary tables are owned by the SYS user.

30. Do You Already Have A Logging, Monitoring, Or Mature Deployment Solution?
Answer: Chances are that your application already has a framework for shipping logs and backing up data to the right places at the right times. To implement Docker, you not only need to replicate the logging behavior you expect in your virtual machine environment, but you also need to prepare your compliance or governance team for these changes.
New tools are entering the Docker space all the time, but many do not match the stability and maturity of existing solutions. Partial updates, rollbacks, and other common deployment tasks may need to be reengineered to accommodate a containerized deployment.
If it’s not broken, don’t fix it. If you’ve already invested the engineering time required to build a continuous integration/continuous delivery (CI/CD) pipeline, containerizing legacy apps may not be worth the time investment.

31. What is a Docker Registry?
Answer: A stevedore written record may be a place wherever all the stevedore pictures are going to behold on and stevedore Cloud and stevedore Hub area unit the general public registries wherever these pictures will be hosted upon. The stevedore hub is that the default storage for the stevedore pictures. An own written record also can come upon as per the necessity. Docker knowledge Center (DDC) also can be used which incorporates DTR (Docker trustworthy Registry). Docker store can give the feature of shopping for and merchandising the stevedore pictures.

32. How did you become involved with the Docker project?
Answer: I came across Docker not long after Solomon open-sourced it. I knew a bit about LXC and containers (a past life includes working on Solaris Zones and LPAR on IBM hardware too), and so I decided to try it out. I was blown away by how easy it was to use. My prior interactions with containers had left me with the feeling they were complex creatures that needed a lot of tuning and nurturing. Docker just worked out of the box. Once I saw that and then saw the CI/CD-centric workflow that Docker was building on top I was sold.

33. How Is Docker Different From Standard Virtualization?
Answer: Docker is operating system-level virtualization. Unlike hypervisor virtualization, where virtual machines run on physical hardware via an intermediation layer (“the hypervisor”), containers instead run userspace on top of an operating system’s kernel. That makes them very lightweight and very fast. 

34. Why Is Docker The New Craze In Virtualization And Cloud Computing?
Answer: Compose uses the project name to create unique identifiers for all of a project’s containers and other resources. To run multiple copies of a project, set a custom project name using the -p command-line option or theCOMPOSE_PROJECT_NAME environment variable. 

35. What is Docker container?
Answer: A dock-walloper instrumentation could be a type of encapsulation to the appliance that holds all the dependencies that share the kernel with alternative containers within the period of running the isolated processes on the hosting package. Dock-Walloper instrumentation are often created by making a dock-walloper image. These dock-walloper pictures are often run at that time exploitation dock-walloper commands. dock-walloper containers area unit the instances of the dock-walloper pictures at the run time. dock-walloper pictures are often kept in any public hosts or personal hosts like a dock-walloper hub. dock-walloper Image could be a set of files which may be run in associate degree isolated method.

36. What are Docker Namespaces?
Answer: The Namespaces in loader could be a technology that provides isolated work-spaces referred to as the instrumentality. Namespaces give a layer of isolation for the loader containers.

37. Do I lose my data when the Docker container exits?
Answer: You can answer this by saying, no I won’t lose my data when Docker container exits, any data that your application writes to disk gets preserved in its container until you explicitly delete the container. The file system for the container persists even after the container halts.

38. What is Docker image?
Answer: I will suggest you go with the below mentioned flow:
Docker image is the source of Docker container. In other words, Docker images are used to create containers. Images are created with the build command, and they’ll produce a container when started with a run. Images are stored in a Docker registry such as registry.hub.docker.com because they can become quite large, images are designed to be composed of layers of other images, allowing a minimal amount of data to be sent when transferring images over the network.
Tip: Be aware of Dockerhub in order to answer questions on pre-available images.
running it or you can use Docker images that are present on the Dockerhub.

39. What is Docker Swarm?
Answer: Docker Swarm can be best understood as the native way of Clustering implementation for Docker itself. Docker Swarm turns a pool of Docker hosts into a single and virtual Docker host. It serves the standard Docker API or any other tool that can already communicate with a Docker daemon can make use of Docker Swarm to scale in a transparent way to multiple hosts. Following are the list of some of the supported tools that will be helpful in achieving what we have discussed just now.

  • Dokku
  • Docker Compose
  • Docker Machine
  • Jenkins

40. Will cloud automation overtake containerization any sooner?
Answer: Docker containers are gaining the popularity each passing day and definitely will be a quintessential part of any professional Continuous Integration / Continuous Development pipelines. Having said that there is equal responsibility on all the key stakeholders at each Organization to take up the challenge of weighing the risks and gains on adopting technologies that are budding up on a daily basis. In my humble opinion, Docker will be extremely effective in Organizations that appreciate the consequences of Containerization. 

41. What are the main factors that dictate the number of containers you can run?
Answer: There isn’t really a clearly-defined limit of containers that you run with Docker. However, that being said, the limitations come in when we start talking about hardware.
There are basically two factors that might limit the number of containers that you can run – the size of your app and your CPU strength. If your application isn’t ginormous and you have a never-ending supply of CPU power, you could probably run a huge amount of Docker container simultaneously.

42. What is the best way of viewing this course?
Answer: You have to just watch the course from beginning to end. Once you go through all the videos, try to answer the questions in your own words. Also, mark the questions that you could not answer by yourself. Then, in the second pass go through only the difficult questions. After going through this course 2-3 times, you will be well prepared to face a technical interview in Docker architecture.

43. Briefly explain the Docker Container lifestyle?
Answer: The lifecycle of a Docker Container is:

  • Creation of the container
  • Running the container
  • Pausing the container
  • Unpausing the container
  • Starting the container
  • Stopping the container
  • Restarting the container
  • Killing the container
  • Destroying the container

44. What is the difference between a registry and a repository?
Answer: The Docker Registry is a service for hosting and distributing images, whereas the Docker Hub is the default registry. On the other hand, the Docker Repository is the collection of Docker images which are related. That is, they have the same name but different tags.

45. Who do you believe Docker is developed for?
Answer: Docker as a tool benefits developers, system administrators, and operations staff.

46. What would cause services to lag or fully stop?
Answer:  Stop attempts are used to stop a container by releasing a SIGTERM (a.k.a. a blockable program termination signal). After ten seconds of the timeout, a SIGKILL (a.k.a. a fatal program termination signal) is deployed to force kill the container. If one experiences the timeout, it means that the containers are not shutting down after receiving the SIGTERM signal.

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