We have many guards making sure that there are always at least two instances of the kube-apiserver. If we ever reach a single kube-apiserver and it causes disruption for the clients, other alerts such as KubeAPIErrorBudgetBurn will fire.
KubeAPIDown is here to make sure that Prometheus and really any client can reach the kube-apiserver, which they can even when there is only one instance of kube-apiserver running. If they can't or that availability is disrupted, `KubeAPIErrorBudgetBurn` will fire.
Comment 23058588 by Marcel Härri at 2023-09-19T06:57:07.949+0000

Mar 12 18:52:24.937 - 58s E namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/firing severity/critical ALERTS
{alertname="KubeAPIErrorBudgetBurn", alertstate="firing", long="1h", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="critical", short="5m"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 5m10s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 5m10s, firing for 0s:
May 10 21:49:54.455 - 50s   I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
May 10 21:49:54.455 - 260s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 24m6s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 21m26s, firing for 2m40s:
May 10 14:47:15.593 - 118s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
May 10 14:47:15.593 - 1168s I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}
May 10 14:44:35.593 - 160s  E namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/firing severity/critical ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="firing", long="6h", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="critical", short="30m"}
# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above info
KubeAPIErrorBudgetBurn was at or above info for at least 2m40s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 21m26s, firing for 2m40s:

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 4s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 4s, firing for 0s:
May 09 00:14:45.337 - 4s    I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 3m32s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m32s, firing for 0s:
May 08 15:00:56.287 - 46s   I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
May 08 15:00:56.287 - 166s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 3m16s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m16s, firing for 0s:
May 07 21:27:27.693 - 196s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 1m4s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m4s, firing for 0s:
May 07 19:48:42.912 - 64s   I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 11m24s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 11m24s, firing for 0s:
May 07 00:42:28.967 - 208s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
May 07 00:42:28.967 - 418s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}
May 07 00:50:58.967 - 58s   I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 58s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 58s, firing for 0s:
May 03 11:42:36.828 - 58s   I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 3m36s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m36s, firing for 0s:
May 01 21:38:10.433 - 216s  I namespace/openshift-kube-apiserver alert/KubeAPIErrorBudgetBurn alertstate/pending severity/warning ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}