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 9m4s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 9m4s, firing for 0s:
Apr 24 11:44:03.326 - 122s  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 24 11:44:03.326 - 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 2m16s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m16s, firing for 0s:
Apr 24 03:26:50.144 - 136s  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 2m4s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m4s, firing for 0s:
Apr 24 00:29:32.910 - 124s  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 2m0s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m0s, firing for 0s:
Apr 23 18:58:31.820 - 120s  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 2m10s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 2m10s, firing for 0s:
Apr 23 01:28:49.123 - 130s  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 1m54s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m54s, firing for 0s:
Apr 20 17:26:38.838 - 114s  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 1m46s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m46s, firing for 0s:
Apr 20 20:20:00.078 - 106s  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 25m36s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 25m36s, firing for 0s:
Apr 19 12:46:16.965 - 86s   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 19 12:46:16.965 - 200s  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 19 12:46:16.965 - 1250s 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 1m52s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m52s, firing for 0s:
Apr 18 20:20:45.092 - 112s  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 1m4s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m4s, firing for 0s:
Apr 18 03:17:35.672 - 64s   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 1m28s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 1m28s, firing for 0s:
Apr 15 21:02:52.305 - 88s   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 10m6s on platformidentification.JobType{Release:"4.13", FromRelease:"", Platform:"aws", Architecture:"arm64", Network:"ovn", Topology:"ha"} (maxAllowed=0s): pending for 10m6s, firing for 0s:
Apr 15 08:37:28.192 - 108s  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 15 08:37:28.192 - 498s  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"}