Why we need etcd in k8s

etcd is a distributed key-value store that is used as the backing store for all cluster data in Kubernetes. It plays a crucial role in Kubernetes' architecture by providing a consistent and highly available data store. Here are some of the key reasons why etcd is essential in Kubernetes:

Key Reasons for Using etcd in Kubernetes:

1. Configuration Management:

   • etcd stores the configuration data for the entire Kubernetes cluster. This includes information about nodes, pods, services, replication controllers, and other resources.
   • This configuration data can be queried and modified via the Kubernetes API server.

2. Service Discovery:

   • etcd helps with service discovery by maintaining information about the services and their associated endpoints.
   • Kubernetes uses etcd to store the cluster’s service registry, making it possible for different components to discover and communicate with each other.

3. State Management:

   • etcd maintains the desired and current state of the Kubernetes cluster.
   • It ensures that the actual state of the cluster matches the desired state as defined by the users through deployments, services, and other resources.
   • When a change is made to the desired state (e.g., scaling a deployment), Kubernetes uses etcd to persist these changes and reconcile the actual state to match the desired state.

4. High Availability and Consistency:

   • etcd is designed to be highly available and consistent, using the Raft consensus algorithm to ensure data integrity and consistency across distributed instances.
   • This high availability is crucial for Kubernetes clusters to maintain continuous operation even if some of the etcd nodes fail.

5. Leader Election and Coordination:

   • etcd supports leader election and coordination among distributed components.
   • Kubernetes components (like the controller manager) use etcd to elect a leader and coordinate their actions to avoid conflicts and ensure smooth cluster operations.

6. Storage of Secrets and Configuration Data:

   • Kubernetes uses etcd to store sensitive information such as secrets and configuration data (ConfigMaps).
   • This data is stored securely and can be accessed by various components and applications running within the cluster.

Example of How etcd Works in Kubernetes:

1. Storing Cluster State:

   • When you create a new deployment, the Kubernetes API server receives the request and stores the deployment object in etcd.
   • The scheduler and controller manager then read this information from etcd to take necessary actions, such as scheduling pods on nodes and ensuring the desired number of replicas.

2. Service Discovery:

   • When a new service is created, the API server stores service information in etcd.
   • Other components, such as kube-proxy, read this information to set up routing rules for the service.

3. Cluster Updates:

   • When a node is added or removed from the cluster, the node information is updated in etcd.
   • The scheduler reads this information to make decisions about where to place new pods.