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Probes in Kubernetes

According to Kube’s docs:

  • A probe is a diagnostic performed periodically by the kubelet on a container. To perform a diagnostic, the kubelet either executes code within the container or makes a network request [docs].

Types of Probes 

Currently, Kubernetes provides support for three types of probes, namely Liveness, Readiness, and Startup. The latter was introduced as an alpha feature gate in v1.16 and recently promoted to GA in v1.20.

livenessProbe

This probe indicates whether the container is running. If the liveness probe fails, the kubelet kills the container, and the container is subjected to its restart policy [docs].

Note:

  • LivenessProbes are only executed once StartupProbes (if defined) completes successfully. On the other hand, ReadinessProbes and LivenessProbes are executed independently, and the latter can restart the container if the check mechanism fails.
Lifecycle

In order to design a robust recovery strategy using livenessProbes, it is important to deeply understand its entire lifecycle first. The high-level state machine depicted below illustrates the various stages involved.

lifecycle-livenessprobe
Figure 1: State Machine of the LivenessProbe Lifecycle.

readinessProbe

This probe indicates whether the container is ready to respond to traffic requests. If the readiness probe fails, the endpoints controller removes the Pod's IP address from the endpoints of all Services that match the Pod [docs].

Note:

  • ReadinessProbes are also executed once StartupProbes (if defined) completes successfully. Unlike LivenessProbe, the ReadinessProbe does not terminate the container, it only stops the traffic to the pod by removing its IP from the service endpoints. However, both are executed independently.
Lifecycle

ReadinessProbes also provide a strong foundation for recovery when applications fail. Below, we also provide a high-level state machine describing the lifecycle of this probe.

lifecycle-readinessprobe
Figure 2: State Machine of the ReadinessProbe Lifecycle.

startupProbe

This probe indicates whether the application within the container is started. If the startup probe fails, the kubelet kills the container, and the container is subjected to its restart policy [docs].

Note:

  • When defined, the StartupProbes disable all other probes (i.e. LivenessProbes, and ReadinessProbes) until successful completion.
Lifecycle

In contrast to previously described probes, the startupProbe is specifically designed for applications that have slow (or uncertain) startup times. Below we present a high-level state machine that outlines the various stages of the StartupProbe's lifecycle.

lifecycle-startupprobe
Figure 3: State Machine of the StartupProbe Lifecycle.

Probes API Spec

First recommendation when planning the configuration of any probe (or any other Kubernetes resource in general) is to inspect what’s available in the API spec for that object.

-> kubectl explain pod.spec.containers.{liveness,readiness,startup}Probe --recursive

KIND: Pod
VERSION: v1

RESOURCE: {liveness,readiness,startup}Probe <Object>

DESCRIPTION:
... REDACTED ...

FIELDS:
exec <Object>
command <[]string>
failureThreshold <integer> # <- configuration field
grpc <Object>
port <integer>
service <string>
httpGet <Object>
host <string>
httpHeaders <[]Object>
name <string>
value <string>
path <string>
port <string>
scheme <string>
initialDelaySeconds <integer> # <- configuration field
periodSeconds <integer> # <- configuration field
successThreshold <integer> # <- configuration field
tcpSocket <Object>
host <string>
port <string>
terminationGracePeriodSeconds <integer> # <- configuration field
timeoutSeconds <integer> # <- configuration field

As shown above, the spec has defined four check mechanisms, namely exec, grpc, httpGet, and tcpSocket. Apart from those, there are only six fields available (denoted as "configuration field" above) to configure any Kubernetes probe [docs].

  • initialDelaySeconds: Number of seconds after the container has started before startup, liveness, or readiness probes are initiated. Its default and minimum value is 0.
  • periodSeconds: How often (in seconds) to perform the probe. It defaults to 10 seconds, but its minimum value is 1.
  • successThreshold: Minimum consecutive successes for a probe to be considered successful after having failed. Its default and minimum value is 1. Must be 1 for liveness and startup probes.
  • timeoutSeconds: Number of seconds after which the probe times out. Its default and minimum value is 1.
  • failureThreshold: Minimum consecutive failures for a probe to be considered failed after having succeeded. It defaults to 3, but its minimum value is 1.
  • terminationGracePeriodSeconds (tGPS): Optional field that requires its feature gate activation prior to v1.27. When defined, Kubelet overrides the pod-level with the probe-level tGPS. If omitted, tGPS at pod-level will be used. It defaults to 30 seconds, but its minimum value is 1.

Check Mechanisms

As previously commented, probes can use four different methods to diagnose the container they are monitoring, specifically exec, httpGet, tcpSocket, and grpc.

  • exec: Executes a specified command inside the container. The diagnostic is considered successful if the command exits with a status code of 0.
  • httpGet: Performs an HTTP GET request against the Pod's IP address. The diagnostic is considered successful if the response has a status code greater than or equal to 200 and less than 400.
  • tcpSocket: Performs a TCP check against the Pod's IP address. The diagnostic is considered successful if the port is open.
  • grpc: Performs a remote procedure call using gRPC. The diagnostic is considered successful if the status of the response is SERVING.

Note:

  • In terms of CPU usage, the exec mechanism requires more CPU cycles when running the command inside the container (see below Section). The exact amount of CPU usage may vary depending on the specific use case and workload.

Probes Outcomes

Depending on the result obtained by the check mechanism, the probes have one of three possible outcomes, namely Success, Failure, or Unknown.

  • Success: The container successfully passed the diagnostic.
  • Failure: The container failed the diagnostic.
  • Unknown: The diagnostic failed (no action should be taken, and the kubelet will make further checks).

Design Considerations

Kubernetes documentation provides a short (but great!) explanation of when we should use liveness, readiness, and startup probes. Below, we expand on the reasoning behind those considerations, as well as further expand on new configuration samples and design patterns.

Decide on check mechanism

As shown in the API Spec in the above subsection, the three types of probes may use the same check mechanisms to perform diagnostics. Therefore, this decision mainly depends on whether your application supports the check mechanisms configured in the probe.

Usually, front-end applications can be monitored with httpGet and tcpSocket mechanisms. For some databases, the newly available grpc mechanism can be used. Ultimately, for some back-end deployments, the exec mechanism can provide a reliable diagnostic when none of the other checks is a feasible solution.

However, if your application supports any of the available check mechanisms, then the decision might narrow down to the check mechanism with less resource consumption for the platform where your application is running.

CPU usage

In order to determine which check mechanism consumes fewer CPU resources, we have deployed a simple demo application and measured the CPU usage using Prometheus. For the measurements, we have used four livenessProbes, each one configured with a different check mechanism.

As shown in the below figure, the livenessProbe using the exec mechanism drastically consumes more CPU cycles than the rest of the available mechanisms. Followed by the httpGet mechanism which consumes way less than the exec process. It has been observed that this behavior greatly worsens in cluster nodes running on rt-kernel. Finally, with a negligible difference, in terms of CPU usage, are the grpc and tcpSocket mechanisms.

The Prometheus query used to measure the CPU usage of the probes can be found below.

sum(rate(container_cpu_usage_seconds_total{namespace="probes-study",pod~="liveness-.*",container=""})) by (pod)

cpu-check-mechanism
Figure 4: CPU usage per check mechanism for a LivenessProbe.

Important Note:

  • We strongly recommend NOT TO USE probes with the exec mechanism in resource constrained environments due to the high CPU usage of this type of probe. Whenever possible, try to use alternative check mechanisms (i.e. httpGet, tcpSocket, or grpc).

Decide on probe configurations

This section by no means provides all the possible patterns for recovering an application using probes, since issues may vary depending on the specific use case and workload. However, we do provide the main guidelines for properly configuring probes in Kubernetes.

Protect against slow starting containers

Kubernetes documentation provides clear guidelines on how to configure a startupProbe properly to protect against slow starting containers [docs].

  • A slow starting container is typically a legacy application that requires additional time, which can be unpredictable, during its initialization processes.

Essentially, we need to compute the worst-case startup time and then set the failureThreshold and periodSeconds fields accordingly. Note that the initialDelaySeconds field should NOT BE USED for this purpose, but instead, the formula presented below.

worstCaseStartupTime > failureThreshold * periodSeconds

Recommendations for Telco workloads

When dealing with resource-constrained environments (e.g. Telco or low-latency workloads) in Kubernetes, it is highly recommended NOT to use probes with the exec mechanism configured, unless there are no other feasible alternatives such as tcpSocket, grpc, or httpGet.

In scenarios with nodes using a real-time kernel, the exec probes on a pinned application might be scheduled in the same CPU(s) the pod uses. Particularly, when multiple probes are scheduled to be executed on the same CPU(s) where the rt polling thread is running, it may affect the overall cluster's performance, and ultimately provokes the cluster nodes to hang unexpectedly.

Mind that kubelet usually requires more effort to perform exec probes (i.e. involving diverse hops within code like kubelet, cri-o, conmon, runc, and finally the probe) while other types of probes just happen directly inline in the kubelet process.

It is worth highlighting that probes from low-priority services could be also omitted, particularly where the exec command would need to be relatively expensive to accurately determine healthiness.

If exec-based diagnostics are indeed needed, it is advisable to limit the number of probes to a maximum of 10 per SNO (Single Node OpenShift) cluster and set the periodSeconds field to be no less than 10 seconds.

Note:

  • It is important to review your probes periodically. With updates, optimizations, and potential regressions in your application, your probes' performance and definition of a healthy state may change. Therefore, set up a reminder to regularly assess your probes. This practice will make sure that your probes accurately reflect the state of your applications and help you avoid any unexpected downtime.

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