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 6m36s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 6m36s, firing for 0s:
Apr 28 11:54:14.162 - 396s  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 2m28s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m28s, firing for 0s:
Apr 22 20:31:51.583 - 148s  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 4m46s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 4m46s, firing for 0s:
Apr 22 11:50:04.051 - 168s  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"}
Apr 22 11:55:54.051 - 118s  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 6m50s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 6m50s, firing for 0s:
Apr 19 12:53:13.586 - 410s  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 2m54s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m54s, firing for 0s:
Apr 19 00:56:18.651 - 174s  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 4m56s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 4m56s, firing for 0s:
Apr 18 10:36:30.623 - 296s  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 28s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 28s, firing for 0s:
Apr 15 21:03:48.700 - 28s   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 8m28s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 8m28s, firing for 0s:
Apr 15 04:54:37.824 - 508s  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 56m38s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 56m0s, firing for 38s:
Apr 15 08:18:31.179 - 330s  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 15 08:18:31.179 - 3030s 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"}
Apr 15 08:17:53.179 - 38s   E alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver 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 38s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 56m0s, firing for 38s: