kubectl apply is used to manage objects in a Declarative way.- The
apply command takes into account the local definition file and the live object definition in Kubernetes.
- This is before making any configuration changes.
- Therefore when running the
apply command, if the object does not already exist, the object is then created.
- It firstly converts it into a
JSON format.
- This is stored as the
last applied configuration.
- Then in Kubernetes, the same definition file is created, similarly with what was generated locally.
- As mentioned previously, this one has additional fields to store the status of the object.
- For any updates, all three configurations (those being the
Local file, Last applied Configuration and Kubernetes Memory) are then checked.
- For example, the
nginx image in the local file is updated to version 1.19 (it would look like image: nginx:1.19) and then the kubectl apply -f nginx.yaml command is ran, the value of image is compared with the value in the live Kubernetes configuration. If there are any differences, the live configuration is updated with the new value. The Last Applied Configuration is also updated as well.
- If a field such as
type: *"front-end-service" is removed in the Local File, this then means it is removed from the Kubernetes memory as well. The Last Applied Configuration helps us to manage the changes.
- Where are the JSON files that have the
Last Applied Configuration?
- This is stored on the cluster itself. Under
metadata –> annotations
kubectl apply / kubectl replace commands do not store those values.- Must bear in mind to not mix
Imperative and Declarative approaches.