# Flow control

Flow controlAPI Priority and Fairness controls the behavior of the Kubernetes API server in an overload situation. You can find more information about it in the API Priority and Fairness documentation.DiagnosticsEvery HTTP response from an API server with the priority and fairness feature enabled has two extra headers: X-Kubernetes-PF-FlowSchema-UID and X-Kubernetes-PF-PriorityLevel-UID, noting the flow schema that matched the request and the priority level to which it was assigned, respectively. The API objects' names are not included in these headers (to avoid revealing details in case the requesting user does not have permission to view them). When debugging, you can use a command such as:kubectl get flowschemas -o custom-columns="uid:{metadata.uid},name:{metadata.name}" kubectl get prioritylevelconfigurations -o custom-columns="uid:{metadata.uid},name:{metadata.name}" to get a mapping of UIDs to names for both FlowSchemas and PriorityLevelConfigurations.Debug endpointsWith the APIPriorityAndFairness feature enabled, the kube-apiserver serves the following additional paths at its HTTP(S) ports.You need to ensure you have permissions to access these endpoints. You don't have to do anything if you are using admin. Permissions can be granted if needed following the RBAC doc to access /debug/api_priority_and_fairness/ by specifying nonResourceURLs./debug/api_priority_and_fairness/dump_priority_levels - a listing of all the priority levels and the current state of each. You can fetch like this:kubectl get --raw /debug/api_priority_and_fairness/dump_priority_levels The output will be in CSV and similar to this:PriorityLevelName, ActiveQueues, IsIdle, IsQuiescing, WaitingRequests, ExecutingRequests, DispatchedRequests, RejectedRequests, TimedoutRequests, CancelledRequests catch-all, 0, true, false, 0, 0, 1, 0, 0, 0 exempt, 0, true, false, 0, 0, 0, 0, 0, 0 global-default, 0, true, false, 0, 0, 46, 0, 0, 0 leader-election, 0, true, false, 0, 0, 4, 0, 0, 0 node-high, 0, true, false, 0, 0, 34, 0, 0, 0 system, 0, true, false, 0, 0, 48, 0, 0, 0 workload-high, 0, true, false, 0, 0, 500, 0, 0, 0 workload-low, 0, true, false, 0, 0, 0, 0, 0, 0 Explanation for selected column names:IsQuiescing indicates if this priority level will be removed when its queues have been drained./debug/api_priority_and_fairness/dump_queues - a listing of all the queues and their current state. You can fetch like this:kubectl get --raw /debug/api_priority_and_fairness/dump_queues The output will be in CSV and similar to this:PriorityLevelName, Index, PendingRequests, ExecutingRequests, SeatsInUse, NextDispatchR, InitialSeatsSum, MaxSeatsSum, TotalWorkSum workload-low, 14, 27, 0, 0, 77.64342019ss, 270, 270, 0.81000000ss workload-low, 74, 26, 0, 0, 76.95387841ss, 260, 260, 0.78000000ss ... leader-election, 0, 0, 0, 0, 5088.87053833ss, 0, 0, 0.00000000ss leader-election, 1, 0, 0, 0, 0.00000000ss, 0, 0, 0.00000000ss ... workload-high, 0, 0, 0, 0, 0.00000000ss, 0, 0, 0.00000000ss workload-high, 1, 0, 0, 0, 1119.44936475ss, 0, 0, 0.00000000ss Explanation for selected column names:NextDispatchR: The R progress meter reading, in units of seat-seconds, at which the next request will be dispatched.InitialSeatsSum: The sum of InitialSeats associated with all requests in a given queue.MaxSeatsSum: The sum of MaxSeats associated with all requests in a given queue.TotalWorkSum: The sum of total work, in units of seat-seconds, of all waiting requests in a given queue.Note: seat-second (abbreviate as ss) is a measure of work, in units of seat-seconds, in the APF world./debug/api_priority_and_fairness/dump_requests - a listing of all the requests including requests waiting in a queue and requests being executing. You can fetch like this:kubectl get --raw /debug/api_priority_and_fairness/dump_requests The output will be in CSV and similar to this:PriorityLevelName, FlowSchemaName, QueueIndex, RequestIndexInQueue, FlowDistingsher, ArriveTime, InitialSeats, FinalSeats, AdditionalLatency, StartTime exempt, exempt, -1, -1, , 2023-07-15T04:51:25.596404345Z, 1, 0, 0s, 2023-07-15T04:51:25.596404345Z workload-low, service-accounts, 14, 0, system:serviceaccount:default:loadtest, 2023-07-18T00:12:51.386556253Z, 10, 0, 0s, 0001-01-01T00:00:00Z workload-low, service-accounts, 14, 1, system:serviceaccount:default:loadtest, 2023-07-18T00:12:51.487092539Z, 10, 0, 0s, 0001-01-01T00:00:00Z You can get a more detailed listing with a command like this:kubectl get --raw '/debug/api_priority_and_fairness/dump_requests?includeRequestDetails=1' The output will be in CSV and similar to this:PriorityLevelName, FlowSchemaName, QueueIndex, RequestIndexInQueue, FlowDistingsher, ArriveTime, InitialSeats, FinalSeats, AdditionalLatency, StartTime, UserName, Verb, APIPath, Namespace, Name, APIVersion, Resource, SubResource exempt, exempt, -1, -1, , 2023-07-15T04:51:25.596404345Z, 1, 0, 0s, 2023-07-15T04:51:25.596404345Z, system:serviceaccount:system:admin, list, /api/v1/namespaces/kube-stress/configmaps, kube-stress, , v1, configmaps, workload-low, service-accounts, 14, 0, system:serviceaccount:default:loadtest, 2023-07-18T00:13:08.986534842Z, 10, 0, 0s, 0001-01-01T00:00:00Z, system:serviceaccount:default:loadtest, list, /api/v1/namespaces/kube-stress/configmaps, kube-stress, , v1, configmaps, workload-low, service-accounts, 14, 1, system:serviceaccount:default:loadtest, 2023-07-18T00:13:09.086476021Z, 10, 0, 0s, 0001-01-01T00:00:00Z, system:serviceaccount:default:loadtest, list, /api/v1/namespaces/kube-stress/configmaps, kube-stress, , v1, configmaps, Explanation for selected column names:QueueIndex: The index of the queue. It will be -1 for priority levels without queues.RequestIndexInQueue: The index in the queue for a given request. It will be -1 for executing requests.InitialSeats: The number of seats will be occupied during the initial (normal) stage of execution of the request.FinalSeats: The number of seats will be occupied during the final stage of request execution, accounting for the associated WATCH notifications.AdditionalLatency: The extra time taken during the final stage of request execution. FinalSeats will be occupied during this time period. It does not mean any latency that a user will observe.StartTime: The time a request starts to execute. It will be 0001-01-01T00:00:00Z for queued requests.Debug loggingAt -v=3 or more verbosity, the API server outputs an httplog line for every request in the API server log, and it includes the following attributes.apf_fs: the name of the flow schema to which the request was classified.apf_pl: the name of the priority level for that flow schema.apf_iseats: the number of seats determined for the initial (normal) stage of execution of the request.apf_fseats: the number of seats determined for the final stage of execution (accounting for the associated watch notifications) of the request.apf_additionalLatency: the duration of the final stage of execution of the request.At higher levels of verbosity there will be log lines exposing details of how APF handled the request, primarily for debugging purposes.Response headersAPF adds the following two headers to each HTTP response message. They won't appear in the audit log. They can be viewed from the client side. For client using klog, use verbosity -v=8 or higher to view these headers.X-Kubernetes-PF-FlowSchema-UID holds the UID of the FlowSchema object to which the corresponding request was classified.X-Kubernetes-PF-PriorityLevel-UID holds the UID of the PriorityLevelConfiguration object associated with that FlowSchema.What's nextFor background information on design details for API priority and fairness, see the enhancement proposal.