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 15m10s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 15m10s, firing for 0s:
Apr 28 12:54:46.396 - 910s  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 3m34s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m34s, firing for 0s:
Apr 28 17:23:05.182 - 214s  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 26m34s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 26m34s, firing for 0s:
Apr 27 04:58:06.060 - 1594s 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 56s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 56s, firing for 0s:
Apr 26 21:32:45.570 - 56s   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 16m42s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 16m42s, firing for 0s:
Apr 23 15:15:56.343 - 1002s 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 1m44s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m44s, firing for 0s:
Apr 23 13:25:10.149 - 104s  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 58s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 58s, firing for 0s:
Apr 22 20:38:56.989 - 58s   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 2m44s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m44s, firing for 0s:
Apr 22 19:51:46.153 - 164s  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 38s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 38s, firing for 0s:
Apr 21 14:41:24.612 - 38s   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 45m54s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 45m54s, firing for 0s:
Apr 21 12:36:02.954 - 2754s 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 1m30s on platformidentification.JobType{Release:"4.14", FromRelease:"4.13", Platform:"aws", Architecture:"amd64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m30s, firing for 0s:
Apr 15 21:05:09.444 - 90s   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"}