What is benefit of readiness probe and healthCheck?

Question

I am working on an application that, as I can see is doing multiple health checks?

1. DB readiness probe
2. Another API dependency readiness probe

When I look at cluster logs, I realize that my service, when it fails a DB-check, just throws 500 and goes down. What I am failing to understand here is that if DB was down or another API was down and IF I do not have a readiness probe then my container is going down anyway. Also, I will see that my application did throw some 500 because DB or another service was off.

What is the benefit of the readiness probe of my container was going down anyway? Another question I have is that is Healthcheck something that I should consider only if I am deploying my service to a cluster? If it was not a cluster microservice environment, would it increase/decrease benefits of performing healtheck?

Answer 1

There are three types of probes that Kubernetes uses to check the health of a Pod:

• Liveness: Tells Kubernetes that something went wrong inside the container, and it's better to restart it to see if Kubernetes can resolve the error.
• Readiness: Tells Kubernetes that the Pod is ready to receive traffic. Sometimes something happens that doesn't wholly incapacitate the Pod but makes it impossible to fulfill the client's request. For example: losing connection to a database or a failure on a third party service. In this case, we don't want Kubernetes to reset the Pod, but we also don't wish for it to send it traffic that it can't fulfill. When a Readiness probe fails, Kubernetes removes the Pod from the service and stops communication with the Pod. Once the error is resolved, Kubernetes can add it back.
• Startup: Tells Kubernetes when a Pod has started and is ready to receive traffic. These probes are especially useful on applications that take a while to begin. While the Pod initiates, Kubernetes doesn't send Liveness or Readiness probes. If it did, they might interfere with the app startup. You can get more information about how probes work on this link:

https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/

Answer 2

Readiness probes are used in a few places. A big one is that non-ready pods are removed from all Services that reference them. They also matter for rolling updates on Deployments/StatefulSets as the roll won't continue until the new pods reach a ready state. In general the checks used for readiness probes should only be checking the current service. So it shouldn't be reaching out to a database. Sometimes that's hard to implement and does indeed make them less useful. But check per-pod stuff like the web server is listening on the port and can return HTTP responses.