Content

• How resources are allocated to a container
• How a container is created from an Image
• What happens behind the scenes when you build a Docker file

Prerequisites

• Should know basic docker commands
• Should know how to write Docker files
• Should have docker installed in your machine

How resources are allocated to a container?

Resources are allocated to a container by following two rules designed for Linux

• Namespacing: Isolating resources per process (or group of processes) like Hard drive, Network, Hostnames, Interprocess communication etc.
• Control Groups(cgroups): Limit amount of resources used per process like Memory, CPU usage, Hard drive I/O, Network Bandwidth etc.

These two rules belong to Linux so even if you are running container in you windows or mac machines it automatically creates a Linux based virtual machine on top of your OS and then apply these two rules.

How a container is created from an Image?

When you run docker pull, it downloads the image from docker hub into your machine and that downloaded image is used to create a container.

Image has two sections

• File system snapshot : which has all the files that come attached with the image

• Startup command : This commands runs when you start your container.

When you build a container from an image it takes a copy of FS snapshot from the image and place it into a small portion of your Hard Drive. Every image has a startup command in it which tells the container what command to run when you first start a container.

So, now we know how a container is created from an image, we'll move on to the next question which is what happens behind the scene when you build a Docker file.

What happens behind the scenes when you build a Docker file?

Whenever you build an image out of Docker file, it runs certain steps before giving you the final image. You might be having a question in your mind that what actually happens behind the scenes? Here is what happens when you build a Docker file, we'll take an example of a simple Docker file

Step-1 : FROM alpine

• It downloads the latest alpine image form docker hub if not present in your build cache.

Step-2 : RUN apk add --update redis

• It first looks back at step-1 and runs a temporary container out of the "alpine" image which we just downloaded in previous step.
• The command "apk add --update redis" is executed inside the running temporary container as the primary running process and installs redis in the container.
• Takes a snapshot of the file system with redis installed.
• Creates a new temporary image of the file system.
• Removes the temporary container.

Step-3 : CMD ["redis-server"]

• Now looks back at step-2, creates a new temporary container out of the image which we got in step-2 with redis installed.
• Sets the default/startup command of that container as CMD["redis-server"]
• Takes the snapshot of this container.
• Builds the final image out of the temporary container with CMD ["redis-server"] as the startup command and redis installed.
• Removes the temporary container.

Thanks for reading this blog! Any Suggestions and questions are welcomed in the comment section.