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 1m42s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m42s, firing for 0s:
Apr 28 11:47:57.711 - 102s  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 3m14s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m14s, firing for 0s:
Apr 23 10:02:48.354 - 194s  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:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m32s, firing for 0s:
Apr 22 20:30:11.369 - 64s   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 20:32:47.369 - 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 3m20s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m20s, firing for 0s:
Apr 22 16:17:37.452 - 200s  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 3m32s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m32s, firing for 0s:
Apr 20 16:36:26.092 - 212s  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 11m30s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 11m30s, firing for 0s:
Apr 19 12:59:33.596 - 210s  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 19 12:59:33.596 - 480s  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 7m48s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 7m48s, firing for 0s:
Apr 19 00:53:50.813 - 10s   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 19 00:53:50.813 - 400s  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 19 00:55:32.813 - 58s   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"}

# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above pending
KubeAPIErrorBudgetBurn was at or above pending for at least 6m24s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 6m24s, firing for 0s:
Apr 18 14:04:03.103 - 12s   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 18 14:04:03.103 - 372s  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 4m18s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 4m18s, firing for 0s:
Apr 18 07:25:12.491 - 258s  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 2m50s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m50s, firing for 0s:
Apr 18 10:30:03.843 - 170s  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:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 54s, firing for 0s:
Apr 15 21:01:03.818 - 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 21m42s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 21m42s, firing for 0s:
Apr 15 08:13:30.652 - 306s  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 08:13:30.652 - 996s  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"}