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 28s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 28s, firing for 0s:
May 13 20:52:28.758 - 28s   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 1m18s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m18s, firing for 0s:
May 08 12:40:10.029 - 78s   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 2m16s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m16s, firing for 0s:
May 08 09:38:56.063 - 136s  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 2m48s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m48s, firing for 0s:
May 11 17:11:20.602 - 168s  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 2m46s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m46s, firing for 0s:
May 07 03:32:14.891 - 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 2m44s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m44s, firing for 0s:
May 06 12:24:03.255 - 164s  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 24m8s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 24m8s, firing for 0s:
May 04 03:20:48.132 - 94s   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 04 03:20:48.132 - 1354s 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 1m22s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m22s, firing for 0s:
May 03 17:05:02.282 - 82s   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 4m18s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 4m18s, firing for 0s:
May 02 08:43:53.685 - 258s  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 28s on platformidentification.JobType{Release:"4.15", FromRelease:"4.14", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 28s, firing for 0s:
May 02 13:52:39.585 - 28s   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"}