Add Firestore online format specification

docs/specs/online_store_format.md

@@ -59,6 +59,34 @@ Here's an example of how the entire thing looks like:
 
 However, we'll address this issue in future versions of the protocol.
 
+## Cloud Firestore Online Store Format
+
+[Firebase data model](https://firebase.google.com/docs/firestore/data-model) is a hierarchy of documents that can contain (sub)-collections. This structure can be multiple levels deep; documents and subcollections are alternating in this hierarchy.
+
+We use the following structure to store feature data in the Firestore:
+* at the first level, there is a collection for each Feast project
+* second level, in each project-collection, there is a Firebase document for each Feature Table
+* third level, in the document for the Feature Table, there is a subcollection called `values` that contain a document per feature row. That document contains the following fields:
+  * `key` contains entity key as serialized `feast.types.EntityKey` proto
+  * `value` contains value as serialized `feast.types.Value` proto
+  * `event_ts` contains event timestamp (in the native firestore timestamp format)
+  * `created_ts` contains write timestamp (in the native firestore timestamp format)
+
+Document id for the feature document is computed by hashing entity key using murmurhash3_128 algorithm as follows:
+
+1. hash utf8-encoded entity names, sorted in alphanumeric order
+2. hash the entity values in the same order as corresponding entity names, by serializing them to bytes as follows:
+  - binary values are hashed as-is
+  - string values hashed after serializing them as utf8 string
+  - int64 and int32 hashed as little-endian byte representation (8 and 4 bytes respectively)
+  - bool hashed as 0 or 1 byte
+
+Other types of entity keys are not supported in this version of the specification, when using Cloud Firestore.
+
+**Example:**
+
+![Firestore Online Example](firebase_online_example.png)
+
 # Appendix
 
 ##### Appendix A. Value proto format.

protos/feast/types/EntityKey.proto

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2018 The Feast Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+syntax = "proto3";
+
+import "feast/types/Value.proto";
+
+package feast.types;
+
+option java_package = "feast.proto.types";
+option java_outer_classname = "EntityKeyProto";
+option go_package = "github.com/feast-dev/feast/sdk/go/protos/feast/types";
+
+message EntityKey {
+    repeated string entity_names = 1;
+    repeated feast.types.Value entity_values = 2;
+}