liveness and readiness

categories/devops/kubernetes.md

@@ -147,6 +147,59 @@ For storing keys, credentials and certificates. For instance: database token, 3r
 - Failed
     - the volume has failed its automatic reclamation
 
+### kubelet
+
+The `kubelet` is the primary “node agent” that runs on each node. It can register the node with the apiserver using one of: the hostname; a flag to override the hostname; or specific logic for a cloud provider. The `kubelet` works in terms of a PodSpec
+
+### Liveness vs Readiness
+
+#### Liveness
+
+The `kubelet` uses liveness probes to know when to restart a Container. For example, liveness probes could catch a deadlock, where an application is running, but unable to make progress. Restarting a Container in such a state can help to make the application more available despite bugs.
+
+The `kubelet` uses readiness probes to know when a Container is ready to start accepting traffic. A Pod is considered ready when all of its Containers are ready. One use of this signal is to control which Pods are used as backends for Services. When a Pod is not ready, it is removed from Service load balancers.
+
+The kubelet uses startup probes to know when a Container application has started. If such a probe is configured, it disables liveness and readiness checks until it succeeds, making sure those probes don’t interfere with the application startup. This can be used to adopt liveness checks on slow starting containers, avoiding them getting killed by the kubelet before they are up and running.
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: goproxy
+  labels:
+    app: goproxy
+spec:
+  containers:
+  - name: goproxy
+    image: k8s.gcr.io/goproxy:0.1
+    ports:
+    - containerPort: 8080
+    readinessProbe:
+      tcpSocket:
+        port: 8080
+      initialDelaySeconds: 5
+      periodSeconds: 10
+    livenessProbe:
+      tcpSocket:
+        port: 8080
+      initialDelaySeconds: 15
+      periodSeconds: 20
+```
+
+#### Readiness
+
+Sometimes, applications are temporarily unable to serve traffic. For example, an application might need to load large data or configuration files during startup, or depend on external services after startup. In such cases, you don’t want to kill the application, but you don’t want to send it requests either. Kubernetes provides readiness probes to detect and mitigate these situations. A pod with containers reporting that they are not ready does not receive traffic through Kubernetes Services.
+
+```yaml
+readinessProbe:
+  exec:
+    command:
+    - cat
+    - /tmp/healthy
+  initialDelaySeconds: 5
+  periodSeconds: 5
+```
+
 ## Several ways to access a service inside Kubernetes cluster
 
 * ClusterIP, Can only be accessed inside the cluster. You can access inside `kubectl proxy`