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diff --git a/sync/engine/model_type_worker.h b/sync/engine/model_type_worker.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_WORKER_H_
+#define SYNC_ENGINE_MODEL_TYPE_WORKER_H_
+
+#include "base/containers/scoped_ptr_map.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/non_thread_safe.h"
+#include "sync/base/sync_export.h"
+#include "sync/engine/commit_contributor.h"
+#include "sync/engine/commit_queue.h"
+#include "sync/engine/nudge_handler.h"
+#include "sync/engine/update_handler.h"
+#include "sync/internal_api/public/base/model_type.h"
+#include "sync/internal_api/public/non_blocking_sync_common.h"
+#include "sync/internal_api/public/sync_encryption_handler.h"
+#include "sync/protocol/sync.pb.h"
+#include "sync/util/cryptographer.h"
+
+namespace base {
+class SingleThreadTaskRunner;
+}
+
+namespace syncer_v2 {
+
+class ModelTypeProcessor;
+class EntityTracker;
+
+// A smart cache for sync types that use message passing (rather than
+// transactions and the syncable::Directory) to communicate with the sync
+// thread.
+//
+// When the non-blocking sync type wants to talk with the sync server, it will
+// send a message from its thread to this object on the sync thread.  This
+// object ensures the appropriate sync server communication gets scheduled and
+// executed.  The response, if any, will be returned to the non-blocking sync
+// type's thread eventually.
+//
+// This object also has a role to play in communications in the opposite
+// direction.  Sometimes the sync thread will receive changes from the sync
+// server and deliver them here.  This object will post this information back to
+// the appropriate component on the model type's thread.
+//
+// This object does more than just pass along messages.  It understands the sync
+// protocol, and it can make decisions when it sees conflicting messages.  For
+// example, if the sync server sends down an update for a sync entity that is
+// currently pending for commit, this object will detect this condition and
+// cancel the pending commit.
+class SYNC_EXPORT ModelTypeWorker : public syncer::UpdateHandler,
+                                    public syncer::CommitContributor,
+                                    public CommitQueue,
+                                    public base::NonThreadSafe {
+ public:
+  ModelTypeWorker(syncer::ModelType type,
+                  const DataTypeState& initial_state,
+                  const UpdateResponseDataList& saved_pending_updates,
+                  scoped_ptr<syncer::Cryptographer> cryptographer,
+                  syncer::NudgeHandler* nudge_handler,
+                  scoped_ptr<ModelTypeProcessor> model_type_processor);
+  ~ModelTypeWorker() override;
+
+  syncer::ModelType GetModelType() const;
+
+  bool IsEncryptionRequired() const;
+  void UpdateCryptographer(scoped_ptr<syncer::Cryptographer> cryptographer);
+
+  // UpdateHandler implementation.
+  void GetDownloadProgress(
+      sync_pb::DataTypeProgressMarker* progress_marker) const override;
+  void GetDataTypeContext(sync_pb::DataTypeContext* context) const override;
+  syncer::SyncerError ProcessGetUpdatesResponse(
+      const sync_pb::DataTypeProgressMarker& progress_marker,
+      const sync_pb::DataTypeContext& mutated_context,
+      const SyncEntityList& applicable_updates,
+      syncer::sessions::StatusController* status) override;
+  void ApplyUpdates(syncer::sessions::StatusController* status) override;
+  void PassiveApplyUpdates(syncer::sessions::StatusController* status) override;
+
+  // CommitQueue implementation.
+  void EnqueueForCommit(const CommitRequestDataList& request_list) override;
+
+  // CommitContributor implementation.
+  scoped_ptr<syncer::CommitContribution> GetContribution(
+      size_t max_entries) override;
+
+  // Callback for when our contribution gets a response.
+  void OnCommitResponse(const CommitResponseDataList& response_list);
+
+  base::WeakPtr<ModelTypeWorker> AsWeakPtr();
+
+ private:
+  typedef base::ScopedPtrMap<std::string, scoped_ptr<EntityTracker>> EntityMap;
+
+  // Stores a single commit request in this object's internal state.
+  void StorePendingCommit(const CommitRequestData& request);
+
+  // Returns true if this type has successfully fetched all available updates
+  // from the server at least once.  Our state may or may not be stale, but at
+  // least we know that it was valid at some point in the past.
+  bool IsTypeInitialized() const;
+
+  // Returns true if this type is prepared to commit items.  Currently, this
+  // depends on having downloaded the initial data and having the encryption
+  // settings in a good state.
+  bool CanCommitItems() const;
+
+  // Initializes the parts of a commit entity that are the responsibility of
+  // this class, and not the EntityTracker.  Some fields, like the
+  // client-assigned ID, can only be set by an entity with knowledge of the
+  // entire data type's state.
+  void HelpInitializeCommitEntity(sync_pb::SyncEntity* commit_entity);
+
+  // Attempts to decrypt pending updates stored in the EntityMap.  If
+  // successful, will remove the update from the its EntityTracker and forward
+  // it to the proxy thread for application.  Will forward any new encryption
+  // keys to the proxy to trigger re-encryption if necessary.
+  void OnCryptographerUpdated();
+
+  // Attempts to decrypt the given specifics and return them in the |out|
+  // parameter.  Assumes cryptographer->CanDecrypt(specifics) returned true.
+  //
+  // Returns false if the decryption failed.  There are no guarantees about the
+  // contents of |out| when that happens.
+  //
+  // In theory, this should never fail.  Only corrupt or invalid entries could
+  // cause this to fail, and no clients are known to create such entries.  The
+  // failure case is an attempt to be defensive against bad input.
+  static bool DecryptSpecifics(syncer::Cryptographer* cryptographer,
+                               const sync_pb::EntitySpecifics& in,
+                               sync_pb::EntitySpecifics* out);
+
+  syncer::ModelType type_;
+
+  // State that applies to the entire model type.
+  DataTypeState data_type_state_;
+
+  // Pointer to the ModelTypeProcessor associated with this worker.
+  // This is NULL when no proxy is connected..
+  scoped_ptr<ModelTypeProcessor> model_type_processor_;
+
+  // A private copy of the most recent cryptographer known to sync.
+  // Initialized at construction time and updated with UpdateCryptographer().
+  // NULL if encryption is not enabled for this type.
+  scoped_ptr<syncer::Cryptographer> cryptographer_;
+
+  // Interface used to access and send nudges to the sync scheduler.  Not owned.
+  syncer::NudgeHandler* nudge_handler_;
+
+  // A map of per-entity information known to this object.
+  //
+  // When commits are pending, their information is stored here.  This
+  // information is dropped from memory when the commit succeeds or gets
+  // cancelled.
+  //
+  // This also stores some information related to received server state in
+  // order to implement reflection blocking and conflict detection.  This
+  // information is kept in memory indefinitely.  With a bit more coordination
+  // with the model thread, we could optimize this to reduce memory usage in
+  // the steady state.
+  EntityMap entities_;
+
+  base::WeakPtrFactory<ModelTypeWorker> weak_ptr_factory_;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_ENGINE_MODEL_TYPE_WORKER_H_