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 4m42s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 4m42s, firing for 0s:
Apr 26 06:23:08.371 - 282s  I alert/KubeAPIErrorBudgetBurn ns/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 9m2s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 9m2s, firing for 0s:
Apr 26 02:12:15.439 - 542s  I alert/KubeAPIErrorBudgetBurn ns/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 7m0s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 7m0s, firing for 0s:
Apr 23 01:19:06.350 - 420s  I alert/KubeAPIErrorBudgetBurn ns/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 1m20s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m20s, firing for 0s:
Apr 25 18:00:41.893 - 80s   I alert/KubeAPIErrorBudgetBurn ns/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 17m34s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 17m34s, firing for 0s:
Apr 21 01:06:07.504 - 62s   I alert/KubeAPIErrorBudgetBurn ns/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="1d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="2h"}
Apr 21 01:06:07.504 - 992s  I alert/KubeAPIErrorBudgetBurn ns/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 3m56s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m56s, firing for 0s:
Apr 20 20:18:40.629 - 236s  I alert/KubeAPIErrorBudgetBurn ns/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 5m42s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 5m42s, firing for 0s:
Apr 20 15:56:45.639 - 342s  I alert/KubeAPIErrorBudgetBurn ns/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:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 11m30s, firing for 0s:
Apr 19 12:51:53.508 - 690s  I alert/KubeAPIErrorBudgetBurn ns/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 3m54s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m54s, firing for 0s:
Apr 18 14:46:21.618 - 234s  I alert/KubeAPIErrorBudgetBurn ns/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 1m8s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m8s, firing for 0s:
Apr 18 03:20:26.173 - 68s   I alert/KubeAPIErrorBudgetBurn ns/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="3d", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="warning", short="6h"}