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+# Expander Plugin over gRPC
+
+Authors:
+Evan Sheng (evan.sheng@airbnb.com) @evansheng
+
+## Context and Rationale
+
+Cluster Autoscaler makes node scaling decisions based on Expander strategies, to select which node group to add nodes to.
+Currently, there are 5 expanders: random, most-pods, least-waste, price, and priority. These provide a variety of behaviors in which CA behaves to expand a cluster.
+
+However, whenever users would like to implement a customer expander they must either submit a change upstream, or fork CA and maintain their own implementation.
+Iteration time would be slow, and oftentimes, users may have a case where they want to implement custom business logic into the expander that may not be relevant to the wider community, but do not want to maintain a fork.
+
+Namely, we (Airbnb) would like to iterate on custom expander strategies (such as a weighted random expander, custom spot market strategy, etc) out of band with CA.
+To do this, we propose a solution that adds a new expander to CA, but does not break backwards compatibility.
+
+
+## Proposal
+
+We will extend CA to utilize a pluggable external expander. The design for this expander plugin is heavily based off of this [proposal](https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/proposals/plugable-provider-grpc.md) to CA, for a pluggable cloud provider interface.
+ 
+The solution will include a server acting as an external expander alongside CA, and communicate via gRPC with TLS. This expander will run in another pod, as a separate service, deployed independently of CA.
+This is depicted below.
+
+![Figure](./images/external-expander-plugin-grpc.jpg)
+
+The gRPC server must implement the API of the [expander strategy interface](https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/expander/expander.go#L50) in CA, which only has one method.
+
+
+```go
+// Strategy describes an interface for selecting the best option when scaling up
+type Strategy interface {
+BestOption(options []Option, nodeInfo map[string]*schedulerframework.NodeInfo) *Option
+}
+```
+
+The gRPC service, running as an external pod, is defined below:
+
+```protobuf
+syntax = "proto3";
+
+package clusterautoscaler.expander.v1;
+
+import "google/protobuf";
+import "k8s.io/autoscaler/cluster-autoscaler/cloudprovider/generated.proto";
+import "k8s.io/api/core/v1/generated.proto";
+
+option go_package = "v1";
+
+service Expander {
+  rpc BestOption (BestOptionRequest)
+    returns (BestOptionResponse) {}
+}
+```
+The messages used in these calls are also defined:
+```protobuf
+message BestOptionRequest {
+  repeated Option options = 1;
+  map<string, schedulerframework.NodeInfo> nodeInfo = 2;
+}
+
+message BestOptionResponse {
+ Option option = 1;
+}
+
+message Option {
+ k8s.io.autoscaler.cluster-autoscaler.cloudprovider.NodeGroup nodeGroup = 1;
+ int32 nodeCount = 2;
+ string debug = 3;
+ repeated k8s.io.api.core.v1.Pod pod = 4;
+}
+```
+
+If errors arise with the gRPC server, the client will handle these by falling back to the random expander strategy, similarly to the fallbacks of the current expanders.
+
+To communicate with the external gRPC server, CA needs new flags to expose details about the server.
+
+We’ll add a new option to the expander flag: `--expander=externalgrpc`, and inntroduce a new flag `--expander-plugin-url=https://external-grpc-url/server` to reach the gRPC server.
+
+Additionally, we’ll need to use TLS for secure communication. An additional flag `--external-expander-cert=~/path/to/cert` will be exposed to specify the path to the certificate authority bundle used to validate the TLS cert used by the server.
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