What is 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.
What is Docker
image?
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 run. Images are stored in a Docker registry.
What is Docker
container?
Docker containers are basically runtime instances of
Docker images.
Docker
containers include the application and all of its dependencies,
but share the kernel with other containers, running as isolated processes in
user space on the host operating system. Docker containers are not tied to any
specific infrastructure: they run on any computer, on any infrastructure, and
in any cloud.
What
is Docker hub?
Docker hub is a cloud-based registry
service which allows you to link to code repositories, build your images and
test them, stores manually pushed images, and links to Docker cloud so you
can deploy images to your hosts. It provides a centralized resource for
container image discovery, distribution and change management, user and team
collaboration, and workflow automation throughout the development pipeline.
What is Dockerfile used for?
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.
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 filename to use for
docker-compose -f docker-compose.json up
What
are the various states that a Docker container can be in at any given point in
time?
There are four states that a Docker
container can be in, at any given point in time. Those states are as given as
follows:
§ Running
§ Paused
§ Restarting
§ Exited
Is
there a way to identify the status of a Docker container?
We can identify the status of a Docker
container by running the command
docker ps –a
which will in turn list down all the
available docker containers with its corresponding statuses on the host. From
there we can easily identify the container of interest to check its status
correspondingly.
What
are the most common instructions in Dockerfile?
Some of the common instructions in
Dockerfile are as follows:
§ FROM: We use FROM to
set the base image for subsequent instructions. In every valid Dockerfile, FROM
is the first instruction.
§ LABEL: We use LABEL to
organize our images as per project, module, licensing etc. We can also use
LABEL to help in automation. In LABEL we specify a key value pair that can be
later used for programmatically handling the Dockerfile.
§ RUN: We use RUN
command to execute any instructions in a new layer on top of the current image.
With each RUN command we add something on top of the image and use it in
subsequent steps in Dockerfile.
§ CMD: We use CMD
command to provide default values of an executing container. In a Dockerfile,
if we include multiple CMD commands, then only the last instruction is used.
What
type of applications - Stateless or Stateful are more suitable for Docker
Container?
It is preferable to create Stateless
application for Docker Container. We can create a container out of our
application and take out the configurable state parameters from application.
Now we can run same container in Production as well as QA environments with
different parameters. This helps in reusing the same Image in different
scenarios. Also a stateless application is much easier to scale with Docker
Containers than a stateful application.
Explain
basic Docker usage workflow
1. Everything starts
with the Dockerfile. The Dockerfile is the source code of the
Image.
2. Once the Dockerfile
is created, you build it to create the image of the container.
The image is just the "compiled version" of the "source
code" which is the Dockerfile.
3. Once you have the
image of the container, you should redistribute it using the registry.
The registry is like a git repository -- you can push and pull images.
4. Next, you can use
the image to run containers. A running container is very similar,
in many aspects, to a virtual machine (but without the hypervisor).
What is the difference between
the COPY and ADD commands in a Dockerfile?
COPY only supports
the basic copying of local files into the container, while ADD has some
features (like local-only tar extraction and remote URL support) that are not
immediately obvious. Consequently, the best use for ADD is local tar file
auto-extraction into the image, as in ADD rootfs.tar.xz /.
What
is the difference between Docker Image and Layer?
§ Image: A Docker image is
built up from a series of read-only layers
§ Layer: Each layer
represents an instruction in the image’s Dockerfile.
Difference between Docker Image and container?
Docker container is the runtime instance of docker image.
Docker Image does not have a
state and its state never changes as it is just set of files whereas docker
container has its execution state.
What
is an orphant volume and how to remove it?
An
orphant volume is a volume without any containers attached to it.
How
is Docker different from a virtual machine?
Docker
isn't a virtualization methodology. It relies on other tools that actually
implement container-based virtualization or operating
system level virtualization. For that, Docker was initially using LXC driver,
then moved to
libcontainer which is now
renamed asrunc. Docker primarily
focuses on automating the deployment of applications inside application
containers. Application containers are designed to package and run a single
service, whereas system containers are designed to run multiple processes, like
virtual machines. So, Docker is considered as a container management or
application deployment tool on containerized systems.
§ Unlike a virtual
machine, a container does not need to boot the operating system kernel, so
containers can be created in less than a second. This feature makes
container-based virtualization unique and desirable than other virtualization
approaches.
§ Since
container-based virtualization adds little or no overhead to the host machine,
container-based virtualization has near-native performance
§ For container-based
virtualization, no additional software is required, unlike other
virtualizations.
§ All containers on a
host machine share the scheduler of the host machine saving need of extra
resources.
§ Container states
(Docker or LXC images) are small in size compared to virtual machine images, so
container images are easy to distribute.
§ Resource management
in containers is achieved through cgroups. Cgroups does not allow containers to
consume more resources than allocated to them. However, as of now, all
resources of host machine are visible in virtual machines, but can't be used.
This can be realized by running top or htop on containers and host machine at
the same time. The output across all environments will look similar.
Can
you explain Dockerfile ONBUILD instruction?
The
ONBUILD instruction
adds to the image a trigger instruction to be executed at a later time, when
the image is used as the base for another build. This is useful if you are
building an image which will be used as a base to build other images, for
example an application build environment or a daemon which may be customized
with user-specific configuration.
Is
it good practice to run stateful applications on Docker? What are the scenarios
where Docker best fits in?
The problem with statefull
docker aplications is that they by default store their state (data) in the
containers filesystem. Once you update your software version or want to move to
another machine its hard to retrieve the data from there.
What you need to do is bind a volume to the container and store
any data in the volume.
if you run your
container with:
docker run -v hostFolder:/containerfolder any changes to /containerfolder will be persisted on the hostfolder. Something similar can be done with a nfs drive. Then you
can run you application on any host machine and the state will be saved in the
nfs drive.
What
are the differences between Docker and Hypervisors?
|
Features
|
Hypervisors
|
Docker
|
|
Default Security Support
|
To a great degree
|
To a slightly less degree
|
|
Memory on disk required
|
Complete OS plus apps
|
App requirement only
|
|
Time Taken to start up
|
Substantially longer as it requires boot of OS
plus app loading
|
Substantially shorter as apps only need to start
as the kernel is already running
|
|
Portability
|
Portable with proper preparation
|
Portable within image format; typically smaller
|
|
Operating System
|
Supports multiple OS
|
It uses the host OS
|
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