Document how to use envSecretRefName to customize S3-like object store

doc/source/servers/overview.md

@@ -28,6 +28,56 @@ spec:
 
 The `modelUri` specifies the bucket containing the saved model, in this case `gs://seldon-models/sklearn/iris`.
 
+`modeluri` supports the following three object storage providers:
+
+  * Google Cloud Storage (using `gs://`)
+  * S3-comptaible (using `s3://`)
+  * Azure Blob storage (using `https://(.+?).blob.core.windows.net/(.+)`)
+
+When using S3-comptaible object storeage provider, you can provide access credential and custom endpoint by creating a `Secret` object:
+
+```
+apiVersion: v1
+kind: Secret
+metadata:
+  name: s3-secret
+type: Opaque
+data:
+  AWS_ACCESS_KEY_ID: XXXX
+  AWS_SECRET_ACCESS_KEY: XXXX
+  S3_ENDPOINT: XXXX
+```
+
+You can create a `Secret` object from command line by
+
+```
+kubectl create secret generic s3-secret --from-literal=S3_ENDPOINT='XXXX' --from-literal=AWS_ACCESS_KEY_ID='XXXX' --from-literal=AWS_SECRET_ACCESS_KEY='XXXX'
+```
+
+and you can [read more](https://kubernetes.io/docs/concepts/configuration/secret/) about interacting with `Secret` object.
+
+And reference the `Secret` using `envSecretRefName` as shown below.
+
+```
+apiVersion: machinelearning.seldon.io/v1alpha2
+kind: SeldonDeployment
+metadata:
+  name: sklearn
+spec:
+  name: iris
+  predictors:
+  - graph:
+      children: []
+      implementation: SKLEARN_SERVER
+      modelUri: s3://seldon-models/sklearn/iris
+      envSecretRefName: s3-secret
+      name: classifier
+    name: default
+    replicas: 1
+```
+
+Alternatively, you can also create a `ServiceAccount` and attach a differently formatted `Secret` to it similar to how kfserving does it.  See kfserving documentation [on this topic](https://github.com/kubeflow/kfserving/tree/master/docs/samples/s3).  Supported annotation prefix includes `serving.kubeflow.org` and `machinelearning.seldon.io`.
+
 If you want to customize the resources for the server you can add a skeleton `Container` with the same name to your podSpecs, e.g.
 
 ```