How desired state is maintained.
WHAT A CONTROLLER IS
A loop comparing desired state against actual, and acting on the difference.
WHY THAT MODEL MATTERS
It handles failures and deployments through the same mechanism.
WHAT THAT MEANS PRACTICALLY
You declare what you want; you do not issue instructions.
WHAT A DEPLOYMENT CONTROLLER MANAGES
A set of identical pods, and how they are replaced.
WHAT IT CREATES UNDERNEATH
A replica set per version, which is how rollback is instant.
WHY THAT DETAIL IS USEFUL
It explains why old replica sets persist, and why history is retained.
WHAT TO SET
How many old versions to keep.
WHAT HAPPENS WHEN YOU DELETE A POD
The controller creates another.
WHY THAT SURPRISES PEOPLE
Deleting something does not remove it while its controller exists.
WHAT TO DELETE INSTEAD
The controlling object.
WHAT CUSTOM CONTROLLERS DO
Extend the same pattern to your own concepts.
WHAT AN OPERATOR IS
A controller encoding operational knowledge about a specific application.
WHERE THAT HELPS
Databases and clustered software, where failover and backups need judgement.
WHAT IT COSTS
Another component to trust and upgrade.
WHAT TO ASSESS BEFORE ADOPTING ONE
Who maintains it, and how mature it is.
WHAT TO PREFER
Fewer moving parts you do not understand.
