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 3m42s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m42s, firing for 0s:
Apr 28 18:56:27.370 - 222s  I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 2m42s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m42s, firing for 0s:
Apr 27 20:09:26.567 - 162s  I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 54s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 54s, firing for 0s:
Apr 24 16:38:38.923 - 54s   I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 1m32s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m32s, firing for 0s:
Apr 25 18:54:50.536 - 92s   I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 2m0s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m0s, firing for 0s:
Apr 22 16:34:27.041 - 120s  I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 15m12s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 15m12s, firing for 0s:
Apr 21 10:42:49.135 - 20s   I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
Apr 21 10:42:49.135 - 892s  I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 3m26s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m26s, firing for 0s:
Apr 15 19:54:55.414 - 206s  I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}