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vendor/k8s.io/kubernetes/docs/design/scheduler_extender.md

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+# Scheduler extender
+
+There are three ways to add new scheduling rules (predicates and priority
+functions) to Kubernetes: (1) by adding these rules to the scheduler and
+recompiling (described here:
+https://github.com/kubernetes/kubernetes/blob/master/docs/devel/scheduler.md),
+(2) implementing your own scheduler process that runs instead of, or alongside
+of, the standard Kubernetes scheduler, (3) implementing a "scheduler extender"
+process that the standard Kubernetes scheduler calls out to as a final pass when
+making scheduling decisions.
+
+This document describes the third approach. This approach is needed for use
+cases where scheduling decisions need to be made on resources not directly
+managed by the standard Kubernetes scheduler. The extender helps make scheduling
+decisions based on such resources. (Note that the three approaches are not
+mutually exclusive.)
+
+When scheduling a pod, the extender allows an external process to filter and
+prioritize nodes. Two separate http/https calls are issued to the extender, one
+for "filter" and one for "prioritize" actions. To use the extender, you must
+create a scheduler policy configuration file. The configuration specifies how to
+reach the extender, whether to use http or https and the timeout.
+
+```go
+// Holds the parameters used to communicate with the extender. If a verb is unspecified/empty,
+// it is assumed that the extender chose not to provide that extension.
+type ExtenderConfig struct {
+	// URLPrefix at which the extender is available
+	URLPrefix string `json:"urlPrefix"`
+	// Verb for the filter call, empty if not supported. This verb is appended to the URLPrefix when issuing the filter call to extender.
+	FilterVerb string `json:"filterVerb,omitempty"`
+	// Verb for the prioritize call, empty if not supported. This verb is appended to the URLPrefix when issuing the prioritize call to extender.
+	PrioritizeVerb string `json:"prioritizeVerb,omitempty"`
+	// The numeric multiplier for the node scores that the prioritize call generates.
+	// The weight should be a positive integer
+	Weight int `json:"weight,omitempty"`
+	// EnableHttps specifies whether https should be used to communicate with the extender
+	EnableHttps bool `json:"enableHttps,omitempty"`
+	// TLSConfig specifies the transport layer security config
+	TLSConfig *client.TLSClientConfig `json:"tlsConfig,omitempty"`
+	// HTTPTimeout specifies the timeout duration for a call to the extender. Filter timeout fails the scheduling of the pod. Prioritize
+	// timeout is ignored, k8s/other extenders priorities are used to select the node.
+	HTTPTimeout time.Duration `json:"httpTimeout,omitempty"`
+}
+```
+
+A sample scheduler policy file with extender configuration:
+
+```json
+{
+  "predicates": [
+    {
+      "name": "HostName"
+    },
+    {
+      "name": "MatchNodeSelector"
+    },
+    {
+      "name": "PodFitsResources"
+    }
+  ],
+  "priorities": [
+    {
+      "name": "LeastRequestedPriority",
+      "weight": 1
+    }
+  ],
+  "extenders": [
+    {
+      "urlPrefix": "http://127.0.0.1:12345/api/scheduler",
+      "filterVerb": "filter",
+      "enableHttps": false
+    }
+  ]
+}
+```
+
+Arguments passed to the FilterVerb endpoint on the extender are the set of nodes
+filtered through the k8s predicates and the pod. Arguments passed to the
+PrioritizeVerb endpoint on the extender are the set of nodes filtered through
+the k8s predicates and extender predicates and the pod.
+
+```go
+// ExtenderArgs represents the arguments needed by the extender to filter/prioritize
+// nodes for a pod.
+type ExtenderArgs struct {
+	// Pod being scheduled
+	Pod   api.Pod      `json:"pod"`
+	// List of candidate nodes where the pod can be scheduled
+	Nodes api.NodeList `json:"nodes"`
+}
+```
+
+The "filter" call returns a list of nodes (schedulerapi.ExtenderFilterResult). The "prioritize" call
+returns priorities for each node (schedulerapi.HostPriorityList).
+
+The "filter" call may prune the set of nodes based on its predicates. Scores
+returned by the "prioritize" call are added to the k8s scores (computed through
+its priority functions) and used for final host selection.
+
+Multiple extenders can be configured in the scheduler policy.
+
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