replicaset doc

kubernetes/workloads/replicaset.md

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+## Document: Kubernetes ReplicaSet YAML Explanation
+
+### Overview
+This document provides an explanation of a Kubernetes ReplicaSet YAML file and commands to manage the ReplicaSet. The YAML file defines the desired state for a ReplicaSet, which ensures a specified number of pod replicas are running at any given time.
+
+### YAML File Breakdown
+
+#### 1. Define the ReplicaSet
+The YAML file begins with the `apiVersion`, `kind`, and `metadata` fields, which specify the API version, the type of Kubernetes object, and metadata about the object, respectively.
+
+```yaml
+apiVersion: apps/v1
+kind: ReplicaSet
+metadata:
+  name: my-app
+  namespace: my-ns
+  labels:
+    app.kubernetes.io/name: my-app
+    app.kubernetes.io/env: development
+```
+- `apiVersion: apps/v1`: Specifies that this configuration uses the `apps/v1` API version.
+- `kind: ReplicaSet`: Defines the object as a ReplicaSet.
+- `metadata`: Provides metadata for the ReplicaSet, including:
+  - `name`: The name of the ReplicaSet (`my-app`).
+  - `namespace`: The namespace where the ReplicaSet will be created (`my-ns`).
+  - `labels`: Key-value pairs to categorize the ReplicaSet (`app.kubernetes.io/name: my-app` and `app.kubernetes.io/env: development`).
+
+#### 2. Define the Specification
+The `spec` section describes the desired state of the ReplicaSet.
+
+```yaml
+spec:
+  replicas: 3
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: my-app
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: my-app
+    spec:
+      containers:
+      - name: nginx
+        image: nginx
+```
+- `replicas: 3`: Specifies that three replicas of the pod should be running.
+- `selector`: Defines how to identify the pods managed by this ReplicaSet.
+  - `matchLabels`: Matches pods with the label `app.kubernetes.io/name: my-app`.
+- `template`: Provides the pod template used by the ReplicaSet to create new pods.
+  - `metadata`: Metadata for the pod template.
+    - `labels`: Labels applied to the pods (`app.kubernetes.io/name: my-app`).
+  - `spec`: Describes the pod specification.
+    - `containers`: Defines the containers within the pod.
+      - `name`: The name of the container (`nginx`).
+      - `image`: The container image to use (`nginx`).
+
+
+### Update the Container Image Version
+
+To update the container image version, edit the YAML manifest and change the `image` field:
+```yaml
+containers:
+- name: nginx
+  image: nginx:1.25
+```
+Apply the updated manifest to update the pods with the new image version.
+
+### Complete YAML File with Image Version Update
+
+Here is the complete YAML file with the container image version updated to `nginx:1.25`:
+
+```yaml
+apiVersion: apps/v1
+kind: ReplicaSet
+metadata:
+  name: my-app
+  namespace: my-ns
+  labels:
+    app.kubernetes.io/name: my-app
+    app.kubernetes.io/env: development
+spec:
+  replicas: 3
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: my-app
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: my-app
+    spec:
+      containers:
+      - name: nginx
+        image: nginx:1.25
+```