Create apiserver-dry-run-and-kubectl-diff (kubernetes#12135)

* Create apiserver-dry-run-and-kubectl-diff

* Update apiserver-dry-run-and-kubectl-diff

* Update apiserver-dry-run-and-kubectl-diff

* Rename apiserver-dry-run-and-kubectl-diff to apiserver-dry-run-and-kubectl-diff.md

* Update apiserver-dry-run-and-kubectl-diff.md

Author: apelisse

content/en/blog/_posts/apiserver-dry-run-and-kubectl-diff.md

@@ -0,0 +1,99 @@
+---
+layout: blog
+title: 'APIServer dry-run and kubectl diff'
+date: 2019-01-14
+---
+
+**Author**: Antoine Pelisse (Google Cloud, @apelisse)
+
+Declarative configuration management, also known as configuration-as-code, is
+one of the key strengths of Kubernetes. It allows users to commit the desired state of
+the cluster, and to keep track of the different versions, improve auditing and
+automation through CI/CD pipelines. The [Apply working-group](https://groups.google.com/forum/#!forum/kubernetes-wg-apply) 
+is working on fixing some of the gaps, and is happy to announce that Kubernetes
+1.13 promoted server-side dry-run and `kubectl diff` to beta. These
+two features are big improvements for the Kubernetes declarative model.
+
+## Challenges
+
+A few pieces are still missing in order to have a seamless declarative
+experience with Kubernetes, and we tried to address some of these:
+
+- While compilers and linters do a good job to detect errors in pull-requests
+  for code, a good validation is missing for Kubernetes configuration files.
+  The existing solution is to run `kubectl apply --dry-run`, but this runs a
+  *local* dry-run that doesn't talk to the server: it doesn't have server
+  validation and doesn't go through validating admission controllers. As an
+  example, Custom resource names are only validated on the server so a local
+  dry-run won't help.
+- It can be difficult to know how your object is going to be applied by the
+  server for multiple reasons:
+  - Defaulting will set some fields to potentially unexpected values,
+  - Mutating webhooks might set fields or clobber/change some values.
+  - Patch and merges can have surprising effects and result in unexpected
+    objects. For example, it can be hard to know how lists are going to be
+    ordered once merged.
+
+The working group has tried to address these problems.
+
+## APIServer dry-run
+
+[APIServer dry-run](https://kubernetes.io/docs/reference/using-api/api-concepts/#dry-run) was implemented to address these two problems:
+
+- it allows individual requests to the apiserver to be marked as "dry-run",
+- the apiserver guarantees that dry-run requests won't be persisted to storage,
+- the request is still processed as typical request: the fields are
+  defaulted, the object is validated, it goes through the validation admission
+  chain, and through the mutating admission chain, and then the final object is
+  returned to the user as it normally would, without being persisted.
+
+While dynamic admission controllers are not supposed to have side-effects on
+each request, dry-run requests are only processed if all admission controllers
+explicitly announce that they don't have any dry-run side-effects.
+
+### How to enable it
+
+Server-side dry-run is enabled through a feature-gate. Now that the feature is
+Beta in 1.13, it should be enabled by default, but still can be enabled/disabled
+using `kube-apiserver --feature-gates DryRun=true`.
+
+If you have dynamic admission controllers, you might have to fix them to:
+
+- Remove any side-effects when the dry-run parameter is specified on the webhook request,
+- Specify in the [`sideEffects`](https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.13/#webhook-v1beta1-admissionregistration) 
+field of the `admissionregistration.k8s.io/v1beta1.Webhook` object to indicate that the object doesn't
+have side-effects on dry-run (or at all).
+
+### How to use it
+
+You can trigger the feature from kubectl by using `kubectl apply
+--server-dry-run`, which will decorate the request with the dryRun flag
+and return the object as it would have been applied, or an error if it would
+have failed.
+
+## Kubectl diff
+
+APIServer dry-run is convenient because it lets you see how the object would be
+processed, but it can be hard to identify exactly what changed if the object is
+big. `kubectl diff` does exactly what you want by showing the differences between
+the current "live" object and the new "dry-run" object. It makes it very
+convenient to focus on only the changes that are made to the object, how the
+server has merged these and how the mutating webhooks affects the output.
+
+### How to use it
+
+`kubectl diff` is meant to be as similar as possible to `kubectl apply`:
+`kubectl diff -f some-resources.yaml` will show a diff for the resources in the yaml file. One can even use the diff program of their choice by using the KUBECTL_EXTERNAL_DIFF environment variable, for example:
+```
+KUBECTL_EXTERNAL_DIFF=meld kubectl diff -f some-resources.yaml
+```
+
+## What's next
+
+The working group is still busy trying to improve some of these things:
+
+- Server-side apply is trying to improve the apply scenario, by adding owner
+semantics to fields! It's also going to improve support for CRDs and unions!
+- Some kubectl apply features are missing from diff and could be useful, like the ability
+to filter by label, or to display pruned resources.
+- Eventually, kubectl diff will use server-side apply!