Proposal for porcelain commands to make working with the last-applied-config easier

contributors/design-proposals/sig-cli/kubectl_apply_getsetdiff_last_applied_config.md

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+# Kubectl apply subcommands for last-config
+
+## Abstract
+
+`kubectl apply` uses the `last-applied-config` annotation to compute
+the removal of fields from local object configuration files and then
+send patches to delete those fields from the live object.  Reading or
+updating the `last-applied-config` is complex as it requires parsing
+out and writing to the annotation.  Here we propose a set of porcelain
+commands for users to better understand what is going on in the system
+and make updates.
+
+## Motivation
+
+What is going on behind the scenes with `kubectl apply` is opaque.  Users
+have to interact directly with annotations on the object to view
+and make changes.  In order to stop having `apply` manage a field on
+an object, it must be manually removed from the annotation and then be removed
+from the local object configuration.  Users should be able to simply edit
+the local object configuration and set it as the last-applied-config
+to be used for the next diff base.  Storing the last-applied-config
+in an annotation adds black magic to `kubectl apply`, and it would
+help users learn and understand if the value was exposed in a discoverable
+manner.
+
+## Use Cases
+
+1. As a user, I want to be able to diff the last-applied-configuration
+   against the current local configuration to see which changes the command is seeing
+2. As a user, I want to remove fields from being managed by the local
+   object configuration by removing them from the local object configuration
+   and setting the last-applied-configuration to match.
+3. As a user, I want to be able to view the last-applied-configuration
+   on the live object that will be used to calculate the diff patch
+   to update the live object from the configuration file.
+
+## Naming and Format possibilities
+
+### Naming
+
+1. *cmd*-last-applied
+
+Rejected alternatives:
+
+2. ~~last-config~~
+3. ~~last-applied-config~~
+4. ~~last-configuration~~
+5. ~~last-applied-configuration~~
+6. ~~last~~
+
+### Formats
+
+1. Apply subcommands
+  - `kubectl apply set-last-applied/view-last-applied/diff-last-applied
+  - a little bit odd to have 2 verbs in a row
+  - improves discoverability to have these as subcommands so they are tied to apply
+
+Rejected alternatives:
+
+2. ~~Set/View subcommands~~
+  - `kubectl set/view/diff last-applied
+  - consistent with other set/view commands
+  - clutters discoverability of set/view commands since these are only for apply
+  - clutters discoverability for last-applied commands since they are for apply
+3. ~~Apply flags~~
+  - `kubectl apply [--set-last-applied | --view-last-applied | --diff-last-applied]
+  - Not a fan of these
+
+## view last-applied
+
+Porcelain command that retrieves the object and prints the annotation value as yaml or json.
+
+Prints an error message if the object is not managed by `apply`.
+
+1. Get the last-applied by type/name
+
+```sh
+kubectl apply view-last-applied deployment/nginx
+```
+
+```yaml
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: nginx
+spec:
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        run: nginx
+    spec:
+      containers:
+      - image: nginx
+        name: nginx
+```
+
+2. Get the last-applied by file, print as json
+
+```sh
+kubectl apply view-last-applied -f deployment_nginx.yaml -o json
+```
+
+Same as above, but in json
+
+## diff last-applied
+
+Porcelain command that retrieves the object and displays a diff against
+the local configuration
+
+1. Diff the last-applied
+
+```sh
+kubectl apply diff-last-applied -f deployment_nginx.yaml
+```
+
+Opens up a 2-way diff in the default diff viewer.  This should
+follow the same semantics as `git diff`.  It should accept either a
+flag `--diff-viewer=meld` or check the environment variable
+`KUBECTL_EXTERNAL_DIFF=meld`.  If neither is specified, the `diff`
+command should be used.
+
+This is meant to show the user what they changed in the configuration,
+since it was last applied, but not show what has changed in the server.
+
+The supported output formats should be `yaml` and `json`, as specified
+by the `-o` flag.
+
+A future goal is to provide a 3-way diff with `kubectl apply diff -f deployment_nginx.yaml`.
+Together these tools would give the user the ability to see what is going
+on and compare changes made to the configuration file vs other
+changes made to the server independent of the configuration file.
+
+## set last-applied
+
+Porcelain command that sets the last-applied-config annotation to as
+if the local configuration file had just been applied.
+
+1. Set the last-applied-config
+
+```sh
+kubectl apply set-last-applied -f deployment_nginx.yaml
+```
+
+Sends a Patch request to set the last-applied-config as if
+the configuration had just been applied.
+
+## edit last-applied
+
+1. Open the last-applied-config in an editor
+
+```sh
+kubectl apply edit-last-applied -f deployment_nginx.yaml
+```
+
+Since the last-applied-configuration annotation exists only
+on the live object, this command can alternatively take the
+kind/name.
+
+```sh
+kubectl apply edit-last-applied deployment/nginx
+```
+
+Sends a Patch request to set the last-applied-config to
+the value saved in the editor.
+
+## Example workflow to stop managing a field with apply - using get/set
+
+As a user, I want to have the replicas on a Deployment managed by an autoscaler
+instead of by the configuration.
+
+1. Check to make sure the live object is up-to-date
+  - `kubectl apply diff-last-applied -f deployment_nginx.yaml`
+  - Expect no changes
+2. Update the deployment_nginx.yaml by removing the replicas field
+3. Diff the last-applied-config to make sure the only change is the removal of the replicas field
+4. Remove the replicas field from the last-applied-config so it doesn't get deleted next apply
+  - `kubectl apply set-last-applied -f deployment_nginx.yaml`
+5. Verify the last-applied-config has been updated
+  - `kubectl apply view-last-applied -f deployment_nginx.yaml`
+
+## Example workflow to stop managing a field with apply - using edit
+
+1. Check to make sure the live object is up-to-date
+  - `kubectl apply diff-last-applied -f deployment_nginx.yaml`
+  - Expect no changes
+2. Update the deployment_nginx.yaml by removing the replicas field
+3. Edit the last-applied-config and remove the replicas field
+  - `kubectl apply edit-last-applied deployment/nginx`
+4. Verify the last-applied-config has been updated
+  - `kubectl apply view-last-applied -f deployment_nginx.yaml`
+
+
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