diff --git a/payload_state_interface.h b/payload_state_interface.h
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+++ b/payload_state_interface.h
@@ -0,0 +1,65 @@
+// Copyright (c) 2012 The Chromium OS 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 CHROMEOS_PLATFORM_UPDATE_ENGINE_PAYLOAD_STATE_INTERFACE_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_PAYLOAD_STATE_INTERFACE_H__
+
+#include <string>
+
+#include "update_engine/action_processor.h"
+
+namespace chromeos_update_engine {
+
+// Forward declaration here because we get a circular dependency if
+// we include omaha_request_action.h directly.
+struct OmahaResponse;
+
+// Describes the methods that need to be implemented by the PayloadState class.
+// This interface has been carved out to support mocking of the PayloadState
+// object.
+class PayloadStateInterface {
+ public:
+  // Sets the internal payload state based on the given Omaha response. This
+  // response could be the same or different from the one for which we've stored
+  // the internal state. If it's different, then this method resets all the
+  // internal state corresponding to the old response. Since the Omaha response
+  // has a lot of fields that are not related to payload state, it uses only
+  // a subset of the fields in the Omaha response to compare equality.
+  virtual void SetResponse(const OmahaResponse& response) = 0;
+
+  // This method should be called whenever we have completed downloading all
+  // the bytes of a payload and have verified that its size and hash match the
+  // expected values. We use this notificaiton to increment the payload attempt
+  // number so that the throttle the next attempt to download the same payload
+  // (in case there's an error in subsequent steps such as post-install)
+  // appropriately.
+  virtual void DownloadComplete() = 0;
+
+  // This method should be called whenever we receive new bytes from the
+  // network for the current payload. We use this notification to reset the
+  // failure count for a given URL since receipt of some bytes means we are
+  // able to make forward progress with the current URL.
+  virtual void DownloadProgress(size_t count) = 0;
+
+  // This method should be called whenever an update attempt fails with the
+  // given error code. We use this notification to update the payload state
+  // depending on the type of the error that happened.
+  virtual void UpdateFailed(ActionExitCode error) = 0;
+
+  // Returns the currently stored response.
+  virtual std::string GetResponse() = 0;
+
+  // Returns the payload attempt number.
+  virtual uint32_t GetPayloadAttemptNumber() = 0;
+
+  // Returns the current URL index.
+  virtual uint32_t GetUrlIndex() = 0;
+
+  // Returns the current URL's failure count.
+  virtual uint32_t GetUrlFailureCount() = 0;
+ };
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_PAYLOAD_STATE_INTERFACE_H__