It works on my machine

A developer says: "It works on my machine!" The tester says: "It does not work on mine."

The two computers are not the same. One has an old version of a tool. One has a missing file.

What if the program travelled with everything it needs, in one sealed box? This is the idea of a container.

Key words

Word	What it means	Example
Container	A program running in its own sealed box with everything it needs.
RAM	The fast memory a computer uses while a program runs.	A phone with 8 GB of RAM
Virtual machine	A whole pretend computer, running inside a real one.
Operating system	The main software of a computer, such as Windows or Linux.
Image	A saved package with a program and its files. You make many containers from one image.
Containerisation	Packing software into containers so it runs the same everywhere.
Registry	An online store of images that you can download from.
Orchestration	Software that starts, stops, moves and watches many containers for you.
Kubernetes	A popular platform that looks after containers. Its code is public.
Replica	One of several identical copies of a container.
Scaling	Running more copies when there is more work, and fewer when there is less.
Health check	A small test that shows a container is still working.

The cells students fill in are left blank in this preview.

What is a container?

A container holds a program and the files it needs. It runs apart from other containers.

You build a container from an image. One image can make 1 container or 100. They are all the same.

The best known tool is Docker. A file called a Dockerfile is the recipe that builds an image. It is a list of steps.