Proposed solution for issue Azure#732 (Azure#746)

proposals/multiple-gateways-single-cluster.md

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+# Multiple Gateways Single Cluster
+
+##### Deploying multiple ingress controller instances to the same cluster 
+
+
+### Document Purpose
+This document is a proposal to resolve the problem highlighted in [this issue](https://github.com/Azure/application-gateway-kubernetes-ingress/issues/732).
+
+  - Authors: [Garvin Casimir](https://github.com/garvinmsft)
+  - Published: February 25th, 2020
+  - Status: Open for comments
+
+### Problem Statement
+Currently the only way to partition AGIC controllers is via namespacing. Therefore, there is no way to delegate 2 ingress resources in the same namespace to 2 different controllers.
+
+### Proposed Solution
+Using the same pattern mentioned [here](https://kubernetes.github.io/ingress-nginx/user-guide/multiple-ingress/#multiple-ingress-nginx-controllers), I propose extending the ingress class annotation to allow users to partition by arbitrary keys.
+
+### Option 1: Allow any arbitrary string
+Accept entire ingress class name as a parameter in the helm chart. The danger with this approach is that it leaves the door open for someone to inadvertently enter an ingress class name already being used by a different type of ingress controller.
+
+```bash
+helm install ./helm/ingress-azure \
+     --name ingress-azure \
+     --ingress-class  arbitrary-class
+```
+
+```yaml
+kind: Ingress
+metadata:
+  name: go-server-ingress-affinity
+  namespace: test-ag
+  annotations:
+    kubernetes.io/ingress.class: arbitrary-class
+spec:
+  rules:
+  - http:
+      paths:
+      - path: /hello/
+        backend:
+          serviceName: go-server-service
+          servicePort: 80
+```
+
+### Option 2: Enforce a prefix unique to AGIC
+This option is similar to option 1 except the helm parameter is used as a suffix.
+
+```bash
+helm install ./helm/ingress-azure \
+     --name ingress-azure \
+     --ingress-suffix arbitrary-class
+```
+
+```yaml
+kind: Ingress
+metadata:
+  name: go-server-ingress-affinity
+  namespace: test-ag
+  annotations:
+    kubernetes.io/ingress.class: agic-arbitrary-class
+spec:
+  rules:
+  - http:
+      paths:
+      - path: /hello/
+        backend:
+          serviceName: go-server-service
+          servicePort: 80
+```
+