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 4m54s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 4m54s, firing for 0s:
Apr 29 04:08:44.089 - 86s   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 29 04:11:42.089 - 208s  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 1m24s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m24s, firing for 0s:
Apr 29 11:31:51.412 - 84s   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 5m38s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 5m38s, firing for 0s:
Apr 23 21:44:27.049 - 338s  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 2m36s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m36s, firing for 0s:
Apr 23 16:10:23.707 - 156s  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 1h0m6s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1h0m6s, firing for 0s:
Apr 23 01:28:18.955 - 176s  I alert/KubeAPIErrorBudgetBurn ns/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="6h", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="critical", short="30m"}
Apr 23 01:28:18.955 - 800s  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 23 01:28:18.955 - 2630s 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 5m16s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 5m16s, firing for 0s:
Apr 23 10:00:14.241 - 316s  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 11m28s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 11m28s, firing for 0s:
Apr 21 01:06:57.731 - 688s  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 7m30s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 7m30s, firing for 0s:
Apr 20 11:54:41.602 - 450s  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 2m34s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m34s, firing for 0s:
Apr 19 05:26:47.236 - 154s  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 3m6s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 3m6s, firing for 0s:
Apr 18 11:59:32.857 - 186s  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"}