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 1m52s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m52s, firing for 0s:
May 14 21:51:25.485 - 112s  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 1m52s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 1m52s, firing for 0s:
May 12 13:36:50.251 - 24s   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 12 13:38:46.251 - 88s   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 3m14s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m14s, firing for 0s:
May 11 12:04:45.820 - 194s  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 11m12s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 11m12s, firing for 0s:
May 10 00:41:29.050 - 34s   I alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="pending", long="6h", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="critical", short="30m"}
May 10 00:41:29.050 - 184s  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 10 00:41:29.050 - 454s  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 4m6s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 4m6s, firing for 0s:
May 08 19:13:00.454 - 246s  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 3m2s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 3m2s, firing for 0s:
May 09 00:07:18.407 - 182s  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 5m6s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 5m6s, firing for 0s:
May 07 22:30:13.236 - 306s  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 2m24s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 2m24s, firing for 0s:
May 07 00:53:32.383 - 144s  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 11m44s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 11m4s, firing for 40s:
May 04 15:24:13.082 - 182s  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 04 15:24:13.082 - 482s  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 04 15:23:33.082 - 40s   E alert/KubeAPIErrorBudgetBurn namespace/openshift-kube-apiserver ALERTS{alertname="KubeAPIErrorBudgetBurn", alertstate="firing", long="6h", namespace="openshift-kube-apiserver", prometheus="openshift-monitoring/k8s", severity="critical", short="30m"}
# [bz-kube-apiserver][invariant] alert/KubeAPIErrorBudgetBurn should not be at or above info
KubeAPIErrorBudgetBurn was at or above info for at least 40s on platformidentification.JobType{Release:"4.14", FromRelease:"", Platform:"aws", Architecture:"amd64", Network:"sdn", Topology:"ha"} (maxAllowed=0s): pending for 11m4s, firing for 40s: