The Assembly Line (CI/CD)

The automated line from commit to production, and why “main is always deployable” is the whole point.

Before you start. A commit is one saved change to the code. An image is a frozen, runnable copy of the app and everything it needs, sealed once and never rebuilt - that sealing, from The Shipping Container (Docker), is what makes today’s story possible.

The idea in 30 seconds

A pipeline is a row of automatic checking stations; the first one, CI, runs on every single change with no human deciding whether to bother.
CD is the second half: moving a build that already passed CI through every environment towards real users, ending in one human click or none.
The image is built once and the same bytes are promoted everywhere; a tag is just a movable name for those bytes, and a digest is the bytes’ own fingerprint, which cannot be moved.
Cheap checks that catch a lot go first on the line; that ordering alone can save hours of machine time a day.
The last station isn’t one box. A canary gives the new version a small slice of real traffic before it gets all of it, and widens only while the numbers stay healthy.

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