Ship it: the sync

The harbour’s plans live in one ledger, and the bottom of the board says what that buys you: rollback is a git revert. This lesson puts the dock app from the last lesson on a cluster that way, ships a bad change, and takes it back by writing in the ledger.

Before you start. Ship it: the line built the app, dock, into a small image, a sealed copy of the program that any machine can run. One cargo plan, three ports gave you the chart, a Kubernetes cargo plan with blanks that a short values file fills in, and showed that Kubernetes swaps in a new version one copy at a time, keeping the old copies running until the new ones answer. Infrastructure as Code & GitOps gave you the idea this lesson runs for real: write down what should be running, and let a machine keep the world matching the page.

The idea in 30 seconds

Argo CD runs inside a cluster, reads a git repository every few minutes, and changes the cluster to match what the repository says.
An Argo CD Application names the repository, the folder holding the plan, and the part of the cluster to keep matching.
A sync is Argo CD comparing the cluster with git and making them match; “Synced” means they match, “Healthy” means what runs actually works.
With self-heal on, a change made on the cluster by hand is undone within seconds, so the only lasting fix is a change in git.
Rolling back is a git revert: a new commit that cancels a bad one, which Argo CD then applies like any other change.
Kubernetes replaces copies one at a time, so a new version that cannot start stalls the rollout while the old copies keep serving.

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