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 3m58s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m58s, firing for 0s:
May 10 07:35:27.296 - 238s  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 16m2s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 16m2s, firing for 0s:
May 08 07:43:53.552 - 394s  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"}
May 08 07:43:53.552 - 568s  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 11m36s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 11m36s, firing for 0s:
May 13 07:45:02.795 - 578s  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"}
May 13 07:45:32.795 - 118s  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 4m58s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 4m58s, firing for 0s:
May 03 07:41:51.264 - 298s  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 3m28s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m28s, firing for 0s:
May 06 07:37:10.575 - 208s  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 4m58s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 4m58s, firing for 0s:
May 07 07:42:52.043 - 298s  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 5m58s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"libvirt", Architecture:"s390x", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 5m58s, firing for 0s:
May 01 07:33:16.424 - 358s  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"}