Helium: Initial proto and Syncable interface definition

blimp/common/proto/helium.proto

Index: blimp/common/proto/helium.proto
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Over-the-wire message definitions used for the Helium synchronization
+// protocol.
+syntax = "proto3";
+
+option optimize_for = LITE_RUNTIME;
+
+import "blimp_message.proto";
+
+package blimp.helium;
+
+// Vector clock that is used to get the partial order of changes
+// This class is the proto definition of
+// blimp/net/helium/vector_clock.h
+message VectorClock {
+  uint32 local_revision = 1;
+  uint32 remote_revision = 2;
+}
+
+// Sample proto for test purposes
+message TestChangeset {
+  int32 value1 = 1;
+  int32 value2 = 2;
+}
+
+// A "union" of the allowed types that are going to be serialized. There will
+// be one for each Helium Object that requires serialization.
+// For example: GeoLocation, EngineSettings, etc.
+message Changeset {
+  oneof data {
+    // Sample message for the test
+    TestChangeset test = 1;
+  };
+}
+
+// A message encapsulates the actual Changeset with the identifier of the
+// object and the timestamp of the checkpoint.
+message HeliumMessage {
+  // Identifies the local revision that this changeset applies to (relative
+  // to the sender), and the remote revision that the local side most recently
+  // received (i.e. an ACK, in effect).
+  VectorClock from = 1;
+  // Provides the local view of the vector-clock following application of
+  // the changeset. This allows a single changeset to collate changes across
+  // several revisions, following a break in connectivity, rather than simply
+  // re-transmitting the lost changesets.
+  VectorClock to = 2;
+
+  // Identifies the Object to which this changeset applies.
+  uint32 object_id = 3;
+
+  // The changeset that contain the actual changes
+  Changeset change = 4;
+}