Create 03-ETCD-in-Kubernetes.md

docs/02-Core-Concepts/03-ETCD-in-Kubernetes.md

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+# ETCD in Kubernetes
+  - Take me to [Video Tutorial](https://kodekloud.com/courses/539883/lectures/9808160)
+
+In this section, we will take a look at ETCD role in kubernetes
+
+## ETCD Datastore
+- The ETCD Datastore stores information regarding the cluster such as **`Nodes`**, **`PODS`**, **`Configs`**, **`Secrets`**, **`Accounts`**, **`Roles`**, **`Bindings`** and **`Others`**.
+- Every information you see when you run the **`kubectl get`** command is from the **`ETCD Server`**.
+
+## Setup - Manual
+- If you setup your cluster from scratch then you deploy **`ETCD`** by downloading ETCD Binaries yourself
+- Installing Binaries and Configuring ETCD as a service in your master node yourself.
+
+  ![etcd](../../images/etcd.PNG)
+
+## Setup - Kubeadm
+- If you setup your cluster using **`kubeadm`** then kubeadm will deploy etcd server for you as a pod in **`kube-system`** namespace.
+  ```
+  $ kubectl get pods -n kube-system
+  ```
+  ![etcd1](../../images/etcd1.PNG)
+
+## Explore ETCD
+- To list all keys stored by kubernetes, run the below command
+  ```
+  $ kubectl exec etcd-master -n kube-system etcdctl get / --prefix -key
+  ```
+- Kubernetes Stores data in a specific directory structure, the root directory is the **`registry`** and under that you have varies kubernetes constructs such as **`minions`**, **`nodes`**, **`pods`**, **`replicasets`**, **`deployments`**, **`roles`**, **`secrets`** and **`Others`**.
+
+  ![etcdctl1](../../images/etcdctl1.PNG)
+
+## ETCD in HA Environment
+   - In a high availability environment, you will have multiple master nodes in your cluster that will have multiple ETCD Instances spread across the master nodes.
+   - Make sure etcd instances know each other by setting the right parameter in the **`etcd.service`** configuration. The **`--initial-cluster`** option where you need to specify the different instances of the etcd service.
+     ![etcd-ha](../../images/etcd-ha.PNG)