}

// LimitResponse defines how to handle requests that can not be executed right now.
// +union
message LimitResponse {
  // `type` is "Queue" or "Reject".
  // "Queue" means that requests that can not be executed upon arrival
  // are held in a queue until they can be executed or a queuing limit
  // is reached.
  // "Reject" means that requests that can not be executed upon arrival
  // are rejected.

}

// LimitResponse defines how to handle requests that can not be executed right now.
// +union
message LimitResponse {
  // `type` is "Queue" or "Reject".
  // "Queue" means that requests that can not be executed upon arrival
  // are held in a queue until they can be executed or a queuing limit
  // is reached.
  // "Reject" means that requests that can not be executed upon arrival
  // are rejected.

The Push Queue is mostly a normal queue, but it has some special logic to merge push requests for each given proxy. This results in each proxy having 0 or 1 outstanding push requests; if additional updates come in the existing push request is just expanded.

Another job polls this queue and triggers each client to start a push.

```mermaid
graph TD

  // +optional
  optional string apiVersion = 3;
}

// ExternalMetricSource indicates how to scale on a metric not associated with
// any Kubernetes object (for example length of queue in cloud
// messaging service, or QPS from loadbalancer running outside of cluster).
message ExternalMetricSource {
  // metric identifies the target metric by name and selector
  optional MetricIdentifier metric = 1;

  // API version of the referent
  // +optional
  optional string apiVersion = 3;
}

// ExternalMetricSource indicates how to scale on a metric not associated with
// any Kubernetes object (for example length of queue in cloud
// messaging service, or QPS from loadbalancer running outside of cluster).
// Exactly one "target" type should be set.
message ExternalMetricSource {
  // metricName is the name of the metric in question.

Construction should create informers (via `kclient.New`), setup a queue (via `controllers.NewQueue`), and register event handlers on the informers.
Often, these handlers are adding something to the queue like `client.AddEventHandler(controllers.ObjectHandler(queue.AddObject))`.
Construction should NOT actually start running all of these things, do I/O, or block in anyway.

}

// LimitResponse defines how to handle requests that can not be executed right now.
// +union
message LimitResponse {
  // `type` is "Queue" or "Reject".
  // "Queue" means that requests that can not be executed upon arrival
  // are held in a queue until they can be executed or a queuing limit
  // is reached.
  // "Reject" means that requests that can not be executed upon arrival
  // are rejected.

}

// LimitResponse defines how to handle requests that can not be executed right now.
// +union
message LimitResponse {
  // `type` is "Queue" or "Reject".
  // "Queue" means that requests that can not be executed upon arrival
  // are held in a queue until they can be executed or a queuing limit
  // is reached.
  // "Reject" means that requests that can not be executed upon arrival
  // are rejected.

  // +optional
  optional string apiVersion = 3;
}

// ExternalMetricSource indicates how to scale on a metric not associated with
// any Kubernetes object (for example length of queue in cloud
// messaging service, or QPS from loadbalancer running outside of cluster).
message ExternalMetricSource {
  // metric identifies the target metric by name and selector
  optional MetricIdentifier metric = 1;

  // +optional
  optional string apiVersion = 3;
}

// ExternalMetricSource indicates how to scale on a metric not associated with
// any Kubernetes object (for example length of queue in cloud
// messaging service, or QPS from loadbalancer running outside of cluster).
message ExternalMetricSource {
  // metricName is the name of the metric in question.
  optional string metricName = 1;