ADD userguide/managingApp/configuration-drift-detection (#6411)

* ADD managingApp/config-drift-detection

Signed-off-by: rahulshendre <rahulshendre789@gmail.com>

* wording change

Signed-off-by: rahulshendre <rahulshendre789@gmail.com>

* Update configuration-drift-detection.md

Signed-off-by: Eeshaan Sawant <100678386+eeshaanSA@users.noreply.github.com>

* fixed formatting

Signed-off-by: Rahul Shendre <144231863+rahulshendre@users.noreply.github.com>

---------

Signed-off-by: rahulshendre <rahulshendre789@gmail.com>
Signed-off-by: Eeshaan Sawant <100678386+eeshaanSA@users.noreply.github.com>
Signed-off-by: Rahul Shendre <144231863+rahulshendre@users.noreply.github.com>
Co-authored-by: Eeshaan Sawant <100678386+eeshaanSA@users.noreply.github.com>

Author: rahulshendre

docs/content/en/docs-v1.0.x/user-guide/managing-application/configuration-drift-detection.md

@@ -0,0 +1,102 @@
+---
+title: "Configuration drift detection"
+linkTitle: "Configuration drift detection"
+weight: 8
+description: >
+  Detect and resolve differences between running resources and Git definitions.
+---
+
+Configuration drift occurs when running resources diverge from their definitions in Git, typically due to manual ad-hoc changes or direct updates to the cluster.
+
+Since PipeCD uses Git as the single source of truth, all application and infrastructure changes should go through pull requests. When drift occurs, developers need to be notified so they can reconcile the differences.
+PipeCD's **configuration drift detection** feature helps you identify these discrepancies. It periodically compares your running resources against the definitions in Git and:
+
+- Displays the comparison results on the application details page
+- Sends notifications to developers when drift is detected
+
+## Enabling Configuration drift detection
+
+Configuration drift detection is enabled by default for all applications. You can adjust the interval for how frequently PipeCD compares running resources against Git definitions in your `Piped` configuration. To customize notifications for drift events, see [Configuring notifications](../../managing-piped/configuring-notifications/).
+
+## Ignoring drift detection for specific fields
+
+> **Note:** This feature is currently supported only for Kubernetes Applications.
+
+You can also ignore drift detection for specified fields in your application manifests. In other words, even if the selected fields have different values between live state and Git, the application status will not be set to `Out of Sync`.
+
+Suppose you have the application manifest:
+
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: simple
+spec:
+  replicas: 2
+  template:
+    spec:
+      containers:
+        - args:
+            - hi
+            - hello
+          image: gcr.io/pipecd/helloworld:v1.0.0
+          name: helloworld
+```
+
+And you want to ignore the drift detection for these two fields:
+- pod's replicas
+- `helloworld` container's args
+
+You can add the following statements to `app.pipecd.yaml`:
+
+```yaml
+spec:
+  ...
+  driftDetection:
+    ignoreFields:
+      - apps/v1:Deployment:default:simple#spec.replicas
+      - apps/v1:Deployment:default:simple#spec.template.spec.containers.0.args
+```
+
+>**Note:** `ignoreFields` is in the format `apiVersion:kind:namespace:name#yamlFieldPath`
+
+## Detection results
+
+Drift detection reports one of three statuses: `SYNCED`, `OUT_OF_SYNC`, or `DEPLOYING`.
+
+### SYNCED
+
+No drift detected. All running resources match their Git definitions. The application details page displays a green "Synced" mark.
+
+![A screenshot of displaying a 'SYNCED' state](/images/application-synced.png)
+<p style="text-align: center;">
+Application in SYNCED state
+</p>
+
+### OUT_OF_SYNC
+
+Drift detected. An application enters this status when any of the following is true:
+
+- A resource is defined in Git but not running in the cluster
+- A resource is running in the cluster but not defined in Git
+- A resource exists in both but with differing configurations
+
+The application details page displays a red "Out of Sync" mark.
+
+![Screenshot showing "OUT OF SYNC" resources configuration state](/images/application-out-of-sync.png)
+<p style="text-align: center;">
+Application in OUT_OF_SYNC state
+</p>
+
+Click **Show Details** to see what caused the drift. In the example below, a Deployment's replica count is `300` in Git but `3` in the cluster.
+
+![Application out of sync](/images/application-out-of-sync-details.png)
+<p style="text-align: center;">
+Details showing why the application is OUT_OF_SYNC
+</p>
+
+### DEPLOYING
+
+Deployment in progress. PipeCD pauses drift detection while a deployment is running and resumes it once the deployment completes.
+
+For more information, see the [configuration reference](./configuration-reference/#driftdetection).