Request Throttler : Exponential back-off

net/request_throttler/request_throttler_entry.cc

+// 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.
+
+#include "net/request_throttler/request_throttler_entry.h"
+
+#include <cmath>
+
+#include "base/logging.h"
+#include "base/rand_util.h"
+#include "base/string_util.h"
+#include "net/request_throttler/request_throttler_header_interface.h"
+
+const int RequestThrottlerEntry::kAdditionalConstantMs = 100;
+const int RequestThrottlerEntry::kEntryLifetimeSec = 120;
+const double RequestThrottlerEntry::kJitterFactor = 0.4;
+const int RequestThrottlerEntry::kInitialBackoffMs = 700;
+const int RequestThrottlerEntry::kMaximumBackoffMs = 24 * 60 * 60 * 1000;
+const double RequestThrottlerEntry::kMultiplyFactor = 2.0;
+const char RequestThrottlerEntry::kRetryHeaderName[] = "X-Retry-After";
+
+RequestThrottlerEntry::RequestThrottlerEntry()
+    : release_time_(base::TimeTicks::Now()),
+      num_times_delayed_(0),
+      is_managed_(false) {
+  SaveState();
+}
+
+RequestThrottlerEntry::~RequestThrottlerEntry() {
+}
+
+bool RequestThrottlerEntry::IsRequestAllowed() const {
+  return (release_time_ <= GetTimeNow());
+}
+
+void RequestThrottlerEntry::UpdateWithResponse(
+    const RequestThrottlerHeaderInterface* response) {
+  SaveState();
+  if (response->GetResponseCode() >= 500) {
+    num_times_delayed_++;
+    release_time_ = std::max(CalculateReleaseTime(), release_time_);
+    is_managed_ = true;
+  } else {
+    // We slowly decay the number of times delayed instead of resetting it to 0
+    // in order to stay stable if we received lots of requests with
+    // malformed bodies at the same time.
+    if (num_times_delayed_ > 0)
+      num_times_delayed_--;
+    is_managed_ = false;
+    // The reason why we are not just cutting release_time to GetTimeNow() is
+    // on the one hand, it would unset delay put by our custom retry-after
+    // header and on the other we would like to push every request up to our
+    // "horizon" when dealing with multiple in-flight request. Ex: If we send
+    // three request and we receive 2 failures and 1 success. The success that
+    // follows those failures will not reset release time further request will
+    // then need to wait the delay caused by the 2 failures.
+    release_time_ = std::max(GetTimeNow(), release_time_);
+    if (response->GetNormalizedValue(kRetryHeaderName) != std::string())
+      HandleCustomRetryAfter(response->GetNormalizedValue(kRetryHeaderName));
+  }
+}
+
+bool RequestThrottlerEntry::IsEntryOutdated() const {
+  int64 unused_since_ms = (GetTimeNow() - release_time_).InMilliseconds();
+  int64 lifespan_ms = kEntryLifetimeSec * 1000;
+
+  // Release time is further than now, we are managing it.
+  if (unused_since_ms < 0)
+    return false;
+
+  // There are two cases. First one, when the entry is currently being managed
+  // and should not be collected unless it is older than the maximum allowed
+  // back-off. The other one, when the entry is outdated, unmanaged and should
+  // be collected.
+  if (is_managed_)
+    return unused_since_ms > kMaximumBackoffMs;
+
+  return unused_since_ms > lifespan_ms;
+}
+
+void RequestThrottlerEntry::ReceivedContentWasMalformed() {
+  // We should never revert to less back-off or else an attacker could put a
+  // malformed body in cache and replay it to decrease delay.
+  num_times_delayed_ =
+      std::max(old_values_.number_of_failed_requests, num_times_delayed_);
+  num_times_delayed_++;
+  is_managed_ = true;
+  release_time_ = std::max(CalculateReleaseTime(),
+      std::max(old_values_.release_time, release_time_));
+}
+
+base::TimeTicks RequestThrottlerEntry::CalculateReleaseTime() {
+  double delay = kInitialBackoffMs;
+  delay *= pow(kMultiplyFactor, num_times_delayed_);
+  delay += kAdditionalConstantMs;
+  delay -= base::RandDouble() * kJitterFactor * delay;
+
+  // Ensure that we do not exceed maximum delay
+  int64 delay_int = static_cast<int64>(delay + 0.5);
+  delay_int = std::min(delay_int, static_cast<int64>(kMaximumBackoffMs));
+
+  return GetTimeNow() + base::TimeDelta::FromMilliseconds(delay_int);
+}
+
+base::TimeTicks RequestThrottlerEntry::GetTimeNow() const {
+  return base::TimeTicks::Now();
+}
+
+void RequestThrottlerEntry::HandleCustomRetryAfter(
+    const std::string& header_value) {
+  // Input parameter is the number of seconds to wait in a floating point value.
+  double time_in_sec = 0;
+  bool conversion_is_ok = StringToDouble(header_value, &time_in_sec);
+
+  // Conversion of custom retry-after header value failed.
+  if (!conversion_is_ok)
+    return;
+
+  // We must use an int value later so we transform this in milliseconds.
+  int64 value_ms = static_cast<int64>(0.5 + time_in_sec * 1000);
+
+  if (kMaximumBackoffMs < value_ms || value_ms < 0)
+    return;
+
+  release_time_ = std::max(
+      (GetTimeNow() + base::TimeDelta::FromMilliseconds(value_ms)),
+      release_time_);
+}
+
+void RequestThrottlerEntry::SaveState() {
+  old_values_.release_time = release_time_;
+  old_values_.number_of_failed_requests = num_times_delayed_;
+}