Implement exponential back-off mechanism and enforce it at the URLRequestHttpJob level.

net/url_request/url_request_throttler_entry.h

+// Copyright (c) 2010 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 NET_URL_REQUEST_URL_REQUEST_THROTTLER_ENTRY_H_
+#define NET_URL_REQUEST_URL_REQUEST_THROTTLER_ENTRY_H_
+
+#include <queue>
+#include <string>
+
+#include "base/basictypes.h"
+#include "net/url_request/url_request_throttler_entry_interface.h"
+
+namespace net {
+
+// URLRequestThrottlerEntry represents an entry of URLRequestThrottlerManager.
+// It analyzes requests of a specific URL over some period of time, in order to
+// deduce the back-off time for every request.
+// The back-off algorithm consists of two parts. Firstly, exponential back-off
+// is used when receiving 5XX server errors or malformed response bodies.
+// The exponential back-off rule is enforced by URLRequestHttpJob. Any request
+// sent during the back-off period will be cancelled.
+// Secondly, a sliding window is used to count recent requests to a given
+// destination and provide guidance (to the application level only) on whether
+// too many requests have been sent and when a good time to send the next one
+// would be. This is never used to deny requests at the network level.
+class URLRequestThrottlerEntry : public URLRequestThrottlerEntryInterface {
+ public:
+  // Sliding window period.
+  static const int kDefaultSlidingWindowPeriodMs;
+
+  // Maximum number of requests allowed in sliding window period.
+  static const int kDefaultMaxSendThreshold;
+
+  // Initial delay for exponential back-off.
+  static const int kDefaultInitialBackoffMs;
+
+  // Additional constant to adjust back-off.
+  static const int kDefaultAdditionalConstantMs;
+
+  // Factor by which the waiting time will be multiplied.
+  static const double kDefaultMultiplyFactor;
+
+  // Fuzzing percentage. ex: 10% will spread requests randomly
+  // between 90%-100% of the calculated time.
+  static const double kDefaultJitterFactor;
+
+  // Maximum amount of time we are willing to delay our request.
+  static const int kDefaultMaximumBackoffMs;
+
+  // Time after which the entry is considered outdated.
+  static const int kDefaultEntryLifetimeMs;
+
+  // Name of the header that servers can use to ask clients to delay their next
+  // request.
+  static const char kRetryHeaderName[];
+
+  URLRequestThrottlerEntry();
+
+  // The life span of instances created with this constructor is set to
+  // infinite.
+  // It is only used by unit tests.
+  URLRequestThrottlerEntry(int sliding_window_period_ms,
+                           int max_send_threshold,
+                           int initial_backoff_ms,
+                           int additional_constant_ms,
+                           double multiply_factor,
+                           double jitter_factor,
+                           int maximum_backoff_ms);
+
+  // Implementation of URLRequestThrottlerEntryInterface.
+  virtual bool IsDuringExponentialBackoff() const;
+  virtual int64 ReserveSendingTimeForNextRequest(
+      const base::TimeTicks& earliest_time);
+  virtual base::TimeTicks GetExponentialBackoffReleaseTime() const;
+  virtual void UpdateWithResponse(
+      const URLRequestThrottlerHeaderInterface* response);
+  virtual void ReceivedContentWasMalformed();
+
+  // Used by the manager, returns true if the entry needs to be garbage
+  // collected.
+  bool IsEntryOutdated() const;
+
+ protected:
+  virtual ~URLRequestThrottlerEntry();
+
+  void Initialize();
+
+  // Calculates the release time for exponential back-off.
+  base::TimeTicks CalculateExponentialBackoffReleaseTime();
+
+  // Equivalent to TimeTicks::Now(), virtual to be mockable for testing purpose.
+  virtual base::TimeTicks GetTimeNow() const;
+
+  // Used internally to increase release time following a retry-after header.
+  void HandleCustomRetryAfter(const std::string& header_value);
+
+  // Used by tests.
+  void set_exponential_backoff_release_time(
+      const base::TimeTicks& release_time) {
+    exponential_backoff_release_time_ = release_time;
+  }
+
+  // Used by tests.
+  base::TimeTicks sliding_window_release_time() const {
+    return sliding_window_release_time_;
+  }
+
+  // Used by tests.
+  void set_sliding_window_release_time(const base::TimeTicks& release_time) {
+    sliding_window_release_time_ = release_time;
+  }
+
+  // Used by tests.
+  void set_failure_count(int failure_count) {
+    failure_count_ = failure_count;
+  }
+
+ private:
+  // Timestamp calculated by the exponential back-off algorithm at which we are
+  // allowed to start sending requests again.
+  base::TimeTicks exponential_backoff_release_time_;
+
+  // Number of times we encounter server errors or malformed response bodies.
+  int failure_count_;
+
+  // If true, the last request response was a failure.
+  // Note that this member can be false at the same time as failure_count_ can
+  // be greater than 0, since we gradually decrease failure_count_, instead of
+  // resetting it to 0 directly, when we receive successful responses.
+  bool latest_response_was_failure_;
+
+  // Timestamp calculated by the sliding window algorithm for when we advise
+  // clients the next request should be made, at the earliest. Advisory only,
+  // not used to deny requests.
+  base::TimeTicks sliding_window_release_time_;
+
+  // A list of the recent send events. We use them to decide whether there are
+  // too many requests sent in sliding window.
+  std::queue<base::TimeTicks> send_log_;
+
+  const base::TimeDelta sliding_window_period_;
+  const int max_send_threshold_;
+  const int initial_backoff_ms_;
+  const int additional_constant_ms_;
+  const double multiply_factor_;
+  const double jitter_factor_;
+  const int maximum_backoff_ms_;
+  // Set to -1 if the entry never expires.
+  const int entry_lifetime_ms_;
+
+  DISALLOW_COPY_AND_ASSIGN(URLRequestThrottlerEntry);
+};
+
+}  // namespace net
+
+#endif  // NET_URL_REQUEST_URL_REQUEST_THROTTLER_ENTRY_H_