Explaining Docker Image

The aim of this page📝 is to explain the concept of docker image.

Pavol Kutaj

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Oct 5, 2022

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DEFINITION

• software object
• contained within it is a “kind of” snapshot of a full file system
• including everything necessary to launch a viable virtual server
• and the image itself (not container!), no matter what the structure, is immutable (should never change)
• in a lifecycle of a docker application lifecycle, it belongs to the build_time

APP:   dockerfile  →   image         →   container
-----------------------------------------------------
TIME:   code        →   build_time    →   run_time

STRUCTURE

• Docker image consists of 3 layers, starting from the bottom:

1. WRITABLE record of ongoing activity is saved 🠉
2. APPS: dependencies, DBs, your programs 🠉
3. OS: just a basic OS such as a minimalistic Alpine Linux

PURPOSE

BENEFIT#1: SPEED

• image layers outlined above (os/apps/writeable) are modular, i.e. function independently from each other
• from that it follows that components of each of the three layers can be shared among multiple images that are hosted by the same/shared instance of a Docker engine
• this means that pulling/building new images can sometimes mean adding very little new data (you share OS, share DB and you may just pull several dependencies)
• …the Docker engine adds only those resources that are not already present on a Docker host

BENEFIT#2: PREDICTABILITY/CONSISTENCY

• launched containers provide identical environments
• …no matter a) where they are hosted
• …no matter b) how many times they were already launched
• resonant with the Devops Handbook principle asserting to Make Infrastructure Easier to Rebuild than to Repair < The DevOps Handbook < The Technical Practices of Flow
• great for prod / remote collaboration
• all the code, and dependencies are there exactly the way you created them
• much better than painstakingly documented instructions on how to set up the environment
• much better working when snapshots of virtual machines
• docker image is the playbook
• it will reliably deliver exactly what you want it