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DevOps Interview Questions
* A Docker container is an open-source software development platform that stores the code and all of its dependencies and runs the application quickly and reliably from one computing environment to the other.

* Docker containers are not specified to any particular infrastructure; they can run on any infrastructure, on any computer, and in any cloud.

* A Docker container image is a standalone, lightweight, and executable package of software that has everything to run the application such as code, system tools, runtime, system libraries, and settings.

Docker Containers can be created with the Docker image using the following command :
docker run -t -i <image name> <command name>
This will create and start the container.
 
If you want to check the list of all running containers with status on the host, use the following command:
docker ps -a
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The auto-deployment feature is used for determining whether there are any new applications or changes in existing applications and dynamically deploy them.
 
It is enabled for servers that run in development mode.
 
To turn off the auto-deployment feature, follow one of the methods to place servers in production mode :
 
* In the Administration Console, click the name of the domain in the left pane and select the Production Mode checkbox in the right pane.
* At the command line, include the following argument when starting the domain’s Administration Server:
-Dweblogic.ProductionModeEnabled=true
* Production mode is set for all WebLogic Server instances in a given domain.
To enable startup sound
 
* Click control gear and then click on Startup Applications
* In the Startup Application Preferences window, click Add to add an entry
* Then fill the information in comment boxes like Name, Command, and Comment
/usr/bin/canberra-gtk-play—id= "desktop-login"—description= "play login sound"
* Logout and then login once you are done

You can also open it with shortcut key Ctrl+Alt+T.
When data changes you can update Memcached by
 
Clearing the Cache proactively : Clearing the cache when an insert or update is made

Resetting the Cache :
It is similar to the first method but rather than just deleting the keys and waiting for the next request for the data to refresh the cache, reset the values after the insert or update.
Dogpile effect is referred to the event when a cache expires, and websites are hit by the multiple requests made by the client at the same time. This effect can be prevented by using a semaphore lock. In this system when value expires, the first process acquires the lock and starts generating new value.
Blue/Green coloring pattern addresses the most important challenges faced during the automatic deployment process. In Blue/ Green Deployment approach, you need to ensure two identical production environment. However, only one among them is LIVE at any given point of time. The LIVE environment is called Blue environment.
 
When the team prepares the next release of their software, they conduct their final stage of testing in an environment which is known as Green environment. Once verified, the traffic is routed to the Green environment.
Git stash command is used to save the changes temporarily in the working directory. This gives developers a clean directory to work on. They can later merge the changes in the git workflow. If this command is used, the changes in the tracked files are merged in the working directory. Git stash command can be used many times in the git directory. It is used as git stash
By definition, kubectl is a command-line interface for running commands against Kubernetes clusters. Here, ‘ctl’ stands for ‘control’. This ‘kubectl’ command-line interface can be used to deploy applications, inspect and manage cluster resources, and view logs.
Following are the different components of Selenium :
 
Selenium Integrated Development Environment (IDE) : The Selenium IDE consists of a simple framework and comes with a Firefox plug-in that can be easily installed. This Selenium component should be used for prototyping.

Selenium Remote Control (RC) : It is a testing framework for developers and QA that supports coding in any programming language like Java, PHP, C#, Perl, etc. This helps automate the UI testing process of web applications against any HTTP website.

Selenium WebDriver : It has a better approach to automating the testing process of web-based applications and does not rely on JavaScript. This web framework allows cross-browser tests to be performed.

Selenium Grid : This proxy server works with Selenium RC and with the help of browsers, it is able to run parallel tests on different nodes or machines.
Below are the differences in multiple criteria that show why Docker has advantages over virtual machines.
 
Memory Space : In terms of memory, Docker occupies lesser space than a virtual machine.

Boot-up Time :
 Docker has a shorter boot-up time than a virtual machine.

Performance :
 Docker containers show better performance as they are hosted in a single Docker engine, whereas, performance is unstable if multiple virtual machines are run.

Scaling :
 Docker is easy to scale up compared to virtual machines.

Efficiency :
 The efficiency of docker is higher, which is an advantage over virtual machines.

Portability :
 Docker doesn’t have the same cross-platform compatibility issues with porting as virtual machines do.

Space Allocation :
 Data volumes can be shared and used repeatedly across multiple containers in Docker, unlike virtual machines that cannot share data volumes.