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 5m20s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 5m20s, firing for 0s:
Apr 30 22:38:08.039 - 320s  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 10m8s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 10m8s, firing for 0s:
Apr 30 15:09:17.506 - 608s  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 7m20s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 7m20s, firing for 0s:
Apr 25 02:29:30.454 - 440s  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 7m2s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 7m2s, firing for 0s:
Apr 23 10:11:37.657 - 422s  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 3m10s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m10s, firing for 0s:
Apr 23 07:32:40.755 - 72s   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"}
Apr 23 07:35:24.755 - 118s  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 2m18s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m18s, firing for 0s:
Apr 20 12:37:04.328 - 138s  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 3m58s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m58s, firing for 0s:
Apr 18 20:26:31.601 - 238s  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 7m48s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 7m48s, firing for 0s:
Apr 18 10:17:05.431 - 468s  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"}