Liveness and readiness probes are a way for Kubernetes to constantly monitor your application by performing restarts or holding off traffic as needed. From the Kubernetes documentation:
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 Kubernetes documentation does a good job of explaining how to configure these probes, and we have implemented examples as well.
The parameters are defined in values.yaml.
In the aladdin-demo container in server/deploy.yaml, we make an http request to the ping endpoint of the aladdin-demo-server.
# Readiness probe stops traffic to this pod if it is not ready, wait until it is ready
readinessProbe:
httpGet:
path: /ping
port: {{ .Values.app.uwsgi.port }}
initialDelaySeconds: {{ .Values.app.readiness.initalDelay }}
periodSeconds: {{ .Values.app.readiness.period }}
# Liveness probe terminates and restarts the pod if unhealthy
livenessProbe:
httpGet:
path: /ping
port: {{ .Values.app.uwsgi.port }}
initialDelaySeconds: {{ .Values.app.liveness.initalDelay }}
periodSeconds: {{ .Values.app.readiness.period }}For the redis container in redis/deploy.yaml, and the elasticsearch container in aladdin-demo-elasticsearch.statefulset.yaml, we try to open a TCP socket at the appropriate ports.
readinessProbe:
tcpSocket:
port: {{ .Values.redis.port }}
initialDelaySeconds: {{ .Values.redis.readiness.initalDelay }}
periodSeconds: {{ .Values.redis.readiness.period }}
livenessProbe:
tcpSocket:
port: {{ .Values.redis.port }}
initialDelaySeconds: {{ .Values.redis.liveness.initalDelay }}
periodSeconds: {{ .Values.redis.liveness.period }}