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diff --git a/sync/engine/model_type_entity.h b/sync/engine/model_type_entity.h
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+++ b/sync/engine/model_type_entity.h
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+// Copyright 2014 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.
+
+#ifndef SYNC_ENGINE_MODEL_TYPE_ENTITY_H_
+#define SYNC_ENGINE_MODEL_TYPE_ENTITY_H_
+
+#include <string>
+
+#include "base/memory/scoped_ptr.h"
+#include "base/time/time.h"
+#include "sync/base/sync_export.h"
+#include "sync/engine/non_blocking_sync_common.h"
+#include "sync/protocol/sync.pb.h"
+
+namespace syncer {
+
+// This is the model thread's representation of a SyncEntity.
+//
+// The model type entity receives updates from the model itself and
+// (asynchronously) from the sync server via the sync thread.  From the point
+// of view of this class, updates from the server take precedence over local
+// changes, though the model may be given an opportunity to overwrite this
+// decision later.
+//
+// Sync will try to commit this entity's data to the sync server and local
+// storage.
+//
+// Most of the logic related to those processes live outside this class.  This
+// class helps out a bit by offering some functions to serialize its data to
+// various formats and query the entity's status.
+class SYNC_EXPORT_PRIVATE ModelTypeEntity {
+ public:
+  // Construct an instance representing a new locally-created item.
+  static scoped_ptr<ModelTypeEntity> NewLocalItem(
+      const std::string& client_tag,
+      const sync_pb::EntitySpecifics& specifics,
+      base::Time now);
+
+  // Construct an instance representing an item newly received from the server.
+  static scoped_ptr<ModelTypeEntity> FromServerUpdate(
+      const std::string& id,
+      const std::string& client_tag_hash,
+      const std::string& non_unique_name,
+      int64 version,
+      const sync_pb::EntitySpecifics& specifics,
+      bool deleted,
+      base::Time ctime,
+      base::Time mtime);
+
+  // TODO(rlarocque): Implement FromDisk constructor when we implement storage.
+
+  ~ModelTypeEntity();
+
+  // Returns true if this data is out of sync with local storage.
+  bool IsWriteRequired() const;
+
+  // Returns true if this data is out of sync with the server.
+  // A commit may or may not be in progress at this time.
+  bool IsUnsynced() const;
+
+  // Returns true if this data is out of sync with the sync thread.
+  //
+  // There may or may not be a commit in progress for this item, but there's
+  // definitely no commit in progress for this (most up to date) version of
+  // this item.
+  bool RequiresCommitRequest() const;
+
+  // Returns true if the specified update version does not contain new data.
+  bool UpdateIsReflection(int64 update_version) const;
+
+  // Returns true if the specified update version conflicts with local changes.
+  bool UpdateIsInConflict(int64 update_version) const;
+
+  // Applies an update from the sync server.
+  //
+  // Overrides any local changes.  Check UpdateIsInConflict() before calling
+  // this function if you want to handle conflicts differently.
+  void ApplyUpdateFromServer(int64 update_version,
+                             bool deleted,
+                             const sync_pb::EntitySpecifics& specifics,
+                             base::Time mtime);
+
+  // Applies a local change to this item.
+  void MakeLocalChange(const sync_pb::EntitySpecifics& specifics);
+
+  // Applies a local deletion to this item.
+  void Delete();
+
+  // Initializes a message representing this item's uncommitted state
+  // to be forwarded to the sync server for committing.
+  void InitializeCommitRequestData(CommitRequestData* request) const;
+
+  // Notes that the current version of this item has been queued for commit.
+  void SetCommitRequestInProgress();
+
+  // Receives a successful commit response.
+  //
+  // Sucssful commit responses can overwrite an item's ID.
+  //
+  // Note that the receipt of a successful commit response does not necessarily
+  // unset IsUnsynced().  If many local changes occur in quick succession, it's
+  // possible that the committed item was already out of date by the time it
+  // reached the server.
+  void ReceiveCommitResponse(const std::string& id,
+                             int64 sequence_number,
+                             int64 response_version);
+
+  // Clears any in-memory sync state associated with outstanding commits.
+  void ClearTransientSyncState();
+
+  // Clears all sync state.  Invoked when a user signs out.
+  void ClearSyncState();
+
+ private:
+  ModelTypeEntity(int64 sequence_number,
+                  int64 commit_requested_sequence_number,
+                  int64 acked_sequence_number,
+                  int64 base_version,
+                  bool is_dirty,
+                  const std::string& id,
+                  const std::string& client_tag_hash,
+                  const std::string& non_unique_name,
+                  const sync_pb::EntitySpecifics& specifics,
+                  bool deleted,
+                  base::Time ctime,
+                  base::Time mtime);
+
+  // A sequence number used to track in-progress commits.  Each local change
+  // increments this number.
+  int64 sequence_number_;
+
+  // The sequence number of the last item sent to the sync thread.
+  int64 commit_requested_sequence_number_;
+
+  // The sequence number of the last item known to be successfully committed.
+  int64 acked_sequence_number_;
+
+  // The server version on which this item is based.
+  //
+  // If there are no local changes, this is the version of the entity as we see
+  // it here.
+  //
+  // If there are local changes, this is the version of the entity on which
+  // those changes are based.
+  int64 base_version_;
+
+  // True if this entity is out of sync with local storage.
+  bool is_dirty_;
+
+  // The entity's ID.
+  //
+  // Most of the time, this is a server-assigned value.
+  //
+  // Prior to the item's first commit, we leave this value as an empty string.
+  // The initial ID for a newly created item has to meet certain uniqueness
+  // requirements, and we handle those on the sync thread.
+  std::string id_;
+
+  // A hash based on the client tag and model type.
+  // Used for various map lookups.  Should always be available.
+  std::string client_tag_hash_;
+
+  // A non-unique name associated with this entity.
+  //
+  // It is sometimes used for debugging.  It gets saved to and restored from
+  // the sync server.
+  //
+  // Its value is often related to the item's unhashed client tag, though this
+  // is not guaranteed and should not be relied on.  May be hidden when
+  // encryption is enabled.
+  std::string non_unique_name_;
+
+  // A protobuf filled with type-specific information.  Contains the most
+  // up-to-date specifics, whether it be from the server or a locally modified
+  // version.
+  sync_pb::EntitySpecifics specifics_;
+
+  // Whether or not the item is deleted.  The |specifics_| field may be empty
+  // if this flag is true.
+  bool deleted_;
+
+  // Entity creation and modification timestamps.
+  // Assigned by the client and synced by the server, though the server usually
+  // doesn't bother to inspect their values.
+  base::Time ctime_;
+  base::Time mtime_;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_ENGINE_MODEL_TYPE_ENTITY_H_