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 3m4s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m4s, firing for 0s:
Apr 20 19:15:29.056 - 156s  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"}
Apr 20 19:19:37.056 - 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 3m24s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m24s, firing for 0s:
Apr 18 14:32:07.502 - 204s  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 5m0s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 5m0s, firing for 0s:
Apr 18 12:15:41.866 - 242s  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"}
Apr 18 12:20:15.866 - 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 3m18s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m18s, firing for 0s:
Apr 17 15:30:01.955 - 110s  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"}
Apr 17 15:33:23.955 - 88s   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 10m58s on platformidentification.JobType{Release:"4.16", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 10m58s, firing for 0s:
Apr 12 11:41:02.957 - 194s  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"}
Apr 12 11:41:02.957 - 464s  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"}