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      1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "net/url_request/url_request_throttler_entry.h"
      6 
      7 #include <cmath>
      8 
      9 #include "base/logging.h"
     10 #include "base/metrics/field_trial.h"
     11 #include "base/metrics/histogram.h"
     12 #include "base/rand_util.h"
     13 #include "base/strings/string_number_conversions.h"
     14 #include "base/values.h"
     15 #include "net/base/load_flags.h"
     16 #include "net/base/net_log.h"
     17 #include "net/url_request/url_request.h"
     18 #include "net/url_request/url_request_context.h"
     19 #include "net/url_request/url_request_throttler_header_interface.h"
     20 #include "net/url_request/url_request_throttler_manager.h"
     21 
     22 namespace net {
     23 
     24 const int URLRequestThrottlerEntry::kDefaultSlidingWindowPeriodMs = 2000;
     25 const int URLRequestThrottlerEntry::kDefaultMaxSendThreshold = 20;
     26 
     27 // This set of back-off parameters will (at maximum values, i.e. without
     28 // the reduction caused by jitter) add 0-41% (distributed uniformly
     29 // in that range) to the "perceived downtime" of the remote server, once
     30 // exponential back-off kicks in and is throttling requests for more than
     31 // about a second at a time.  Once the maximum back-off is reached, the added
     32 // perceived downtime decreases rapidly, percentage-wise.
     33 //
     34 // Another way to put it is that the maximum additional perceived downtime
     35 // with these numbers is a couple of seconds shy of 15 minutes, and such
     36 // a delay would not occur until the remote server has been actually
     37 // unavailable at the end of each back-off period for a total of about
     38 // 48 minutes.
     39 //
     40 // Ignoring the first couple of errors is just a conservative measure to
     41 // avoid false positives.  It should help avoid back-off from kicking in e.g.
     42 // on flaky connections.
     43 const int URLRequestThrottlerEntry::kDefaultNumErrorsToIgnore = 2;
     44 const int URLRequestThrottlerEntry::kDefaultInitialDelayMs = 700;
     45 const double URLRequestThrottlerEntry::kDefaultMultiplyFactor = 1.4;
     46 const double URLRequestThrottlerEntry::kDefaultJitterFactor = 0.4;
     47 const int URLRequestThrottlerEntry::kDefaultMaximumBackoffMs = 15 * 60 * 1000;
     48 const int URLRequestThrottlerEntry::kDefaultEntryLifetimeMs = 2 * 60 * 1000;
     49 const char URLRequestThrottlerEntry::kExponentialThrottlingHeader[] =
     50     "X-Chrome-Exponential-Throttling";
     51 const char URLRequestThrottlerEntry::kExponentialThrottlingDisableValue[] =
     52     "disable";
     53 
     54 // Returns NetLog parameters when a request is rejected by throttling.
     55 base::Value* NetLogRejectedRequestCallback(const std::string* url_id,
     56                                            int num_failures,
     57                                            int release_after_ms,
     58                                            NetLog::LogLevel /* log_level */) {
     59   base::DictionaryValue* dict = new base::DictionaryValue();
     60   dict->SetString("url", *url_id);
     61   dict->SetInteger("num_failures", num_failures);
     62   dict->SetInteger("release_after_ms", release_after_ms);
     63   return dict;
     64 }
     65 
     66 URLRequestThrottlerEntry::URLRequestThrottlerEntry(
     67     URLRequestThrottlerManager* manager,
     68     const std::string& url_id)
     69     : sliding_window_period_(
     70           base::TimeDelta::FromMilliseconds(kDefaultSlidingWindowPeriodMs)),
     71       max_send_threshold_(kDefaultMaxSendThreshold),
     72       is_backoff_disabled_(false),
     73       backoff_entry_(&backoff_policy_),
     74       manager_(manager),
     75       url_id_(url_id),
     76       net_log_(BoundNetLog::Make(
     77           manager->net_log(), NetLog::SOURCE_EXPONENTIAL_BACKOFF_THROTTLING)) {
     78   DCHECK(manager_);
     79   Initialize();
     80 }
     81 
     82 URLRequestThrottlerEntry::URLRequestThrottlerEntry(
     83     URLRequestThrottlerManager* manager,
     84     const std::string& url_id,
     85     int sliding_window_period_ms,
     86     int max_send_threshold,
     87     int initial_backoff_ms,
     88     double multiply_factor,
     89     double jitter_factor,
     90     int maximum_backoff_ms)
     91     : sliding_window_period_(
     92           base::TimeDelta::FromMilliseconds(sliding_window_period_ms)),
     93       max_send_threshold_(max_send_threshold),
     94       is_backoff_disabled_(false),
     95       backoff_entry_(&backoff_policy_),
     96       manager_(manager),
     97       url_id_(url_id) {
     98   DCHECK_GT(sliding_window_period_ms, 0);
     99   DCHECK_GT(max_send_threshold_, 0);
    100   DCHECK_GE(initial_backoff_ms, 0);
    101   DCHECK_GT(multiply_factor, 0);
    102   DCHECK_GE(jitter_factor, 0.0);
    103   DCHECK_LT(jitter_factor, 1.0);
    104   DCHECK_GE(maximum_backoff_ms, 0);
    105   DCHECK(manager_);
    106 
    107   Initialize();
    108   backoff_policy_.initial_delay_ms = initial_backoff_ms;
    109   backoff_policy_.multiply_factor = multiply_factor;
    110   backoff_policy_.jitter_factor = jitter_factor;
    111   backoff_policy_.maximum_backoff_ms = maximum_backoff_ms;
    112   backoff_policy_.entry_lifetime_ms = -1;
    113   backoff_policy_.num_errors_to_ignore = 0;
    114   backoff_policy_.always_use_initial_delay = false;
    115 }
    116 
    117 bool URLRequestThrottlerEntry::IsEntryOutdated() const {
    118   // This function is called by the URLRequestThrottlerManager to determine
    119   // whether entries should be discarded from its url_entries_ map.  We
    120   // want to ensure that it does not remove entries from the map while there
    121   // are clients (objects other than the manager) holding references to
    122   // the entry, otherwise separate clients could end up holding separate
    123   // entries for a request to the same URL, which is undesirable.  Therefore,
    124   // if an entry has more than one reference (the map will always hold one),
    125   // it should not be considered outdated.
    126   //
    127   // We considered whether to make URLRequestThrottlerEntry objects
    128   // non-refcounted, but since any means of knowing whether they are
    129   // currently in use by others than the manager would be more or less
    130   // equivalent to a refcount, we kept them refcounted.
    131   if (!HasOneRef())
    132     return false;
    133 
    134   // If there are send events in the sliding window period, we still need this
    135   // entry.
    136   if (!send_log_.empty() &&
    137       send_log_.back() + sliding_window_period_ > ImplGetTimeNow()) {
    138     return false;
    139   }
    140 
    141   return GetBackoffEntry()->CanDiscard();
    142 }
    143 
    144 void URLRequestThrottlerEntry::DisableBackoffThrottling() {
    145   is_backoff_disabled_ = true;
    146 }
    147 
    148 void URLRequestThrottlerEntry::DetachManager() {
    149   manager_ = NULL;
    150 }
    151 
    152 bool URLRequestThrottlerEntry::ShouldRejectRequest(
    153     const URLRequest& request,
    154     NetworkDelegate* network_delegate) const {
    155   bool reject_request = false;
    156   if (!is_backoff_disabled_ && !ExplicitUserRequest(request.load_flags()) &&
    157       (!network_delegate || network_delegate->CanThrottleRequest(request)) &&
    158       GetBackoffEntry()->ShouldRejectRequest()) {
    159     int num_failures = GetBackoffEntry()->failure_count();
    160     int release_after_ms =
    161         GetBackoffEntry()->GetTimeUntilRelease().InMilliseconds();
    162 
    163     net_log_.AddEvent(
    164         NetLog::TYPE_THROTTLING_REJECTED_REQUEST,
    165         base::Bind(&NetLogRejectedRequestCallback,
    166                    &url_id_, num_failures, release_after_ms));
    167 
    168     reject_request = true;
    169   }
    170 
    171   int reject_count = reject_request ? 1 : 0;
    172   UMA_HISTOGRAM_ENUMERATION(
    173       "Throttling.RequestThrottled", reject_count, 2);
    174 
    175   return reject_request;
    176 }
    177 
    178 int64 URLRequestThrottlerEntry::ReserveSendingTimeForNextRequest(
    179     const base::TimeTicks& earliest_time) {
    180   base::TimeTicks now = ImplGetTimeNow();
    181 
    182   // If a lot of requests were successfully made recently,
    183   // sliding_window_release_time_ may be greater than
    184   // exponential_backoff_release_time_.
    185   base::TimeTicks recommended_sending_time =
    186       std::max(std::max(now, earliest_time),
    187                std::max(GetBackoffEntry()->GetReleaseTime(),
    188                         sliding_window_release_time_));
    189 
    190   DCHECK(send_log_.empty() ||
    191          recommended_sending_time >= send_log_.back());
    192   // Log the new send event.
    193   send_log_.push(recommended_sending_time);
    194 
    195   sliding_window_release_time_ = recommended_sending_time;
    196 
    197   // Drop the out-of-date events in the event list.
    198   // We don't need to worry that the queue may become empty during this
    199   // operation, since the last element is sliding_window_release_time_.
    200   while ((send_log_.front() + sliding_window_period_ <=
    201           sliding_window_release_time_) ||
    202          send_log_.size() > static_cast<unsigned>(max_send_threshold_)) {
    203     send_log_.pop();
    204   }
    205 
    206   // Check if there are too many send events in recent time.
    207   if (send_log_.size() == static_cast<unsigned>(max_send_threshold_))
    208     sliding_window_release_time_ = send_log_.front() + sliding_window_period_;
    209 
    210   return (recommended_sending_time - now).InMillisecondsRoundedUp();
    211 }
    212 
    213 base::TimeTicks
    214     URLRequestThrottlerEntry::GetExponentialBackoffReleaseTime() const {
    215   // If a site opts out, it's likely because they have problems that trigger
    216   // the back-off mechanism when it shouldn't be triggered, in which case
    217   // returning the calculated back-off release time would probably be the
    218   // wrong thing to do (i.e. it would likely be too long).  Therefore, we
    219   // return "now" so that retries are not delayed.
    220   if (is_backoff_disabled_)
    221     return ImplGetTimeNow();
    222 
    223   return GetBackoffEntry()->GetReleaseTime();
    224 }
    225 
    226 void URLRequestThrottlerEntry::UpdateWithResponse(
    227     const std::string& host,
    228     const URLRequestThrottlerHeaderInterface* response) {
    229   if (IsConsideredError(response->GetResponseCode())) {
    230     GetBackoffEntry()->InformOfRequest(false);
    231   } else {
    232     GetBackoffEntry()->InformOfRequest(true);
    233 
    234     std::string throttling_header = response->GetNormalizedValue(
    235         kExponentialThrottlingHeader);
    236     if (!throttling_header.empty())
    237       HandleThrottlingHeader(throttling_header, host);
    238   }
    239 }
    240 
    241 void URLRequestThrottlerEntry::ReceivedContentWasMalformed(int response_code) {
    242   // A malformed body can only occur when the request to fetch a resource
    243   // was successful.  Therefore, in such a situation, we will receive one
    244   // call to ReceivedContentWasMalformed() and one call to
    245   // UpdateWithResponse() with a response categorized as "good".  To end
    246   // up counting one failure, we need to count two failures here against
    247   // the one success in UpdateWithResponse().
    248   //
    249   // We do nothing for a response that is already being considered an error
    250   // based on its status code (otherwise we would count 3 errors instead of 1).
    251   if (!IsConsideredError(response_code)) {
    252     GetBackoffEntry()->InformOfRequest(false);
    253     GetBackoffEntry()->InformOfRequest(false);
    254   }
    255 }
    256 
    257 URLRequestThrottlerEntry::~URLRequestThrottlerEntry() {
    258 }
    259 
    260 void URLRequestThrottlerEntry::Initialize() {
    261   sliding_window_release_time_ = base::TimeTicks::Now();
    262   backoff_policy_.num_errors_to_ignore = kDefaultNumErrorsToIgnore;
    263   backoff_policy_.initial_delay_ms = kDefaultInitialDelayMs;
    264   backoff_policy_.multiply_factor = kDefaultMultiplyFactor;
    265   backoff_policy_.jitter_factor = kDefaultJitterFactor;
    266   backoff_policy_.maximum_backoff_ms = kDefaultMaximumBackoffMs;
    267   backoff_policy_.entry_lifetime_ms = kDefaultEntryLifetimeMs;
    268   backoff_policy_.always_use_initial_delay = false;
    269 }
    270 
    271 bool URLRequestThrottlerEntry::IsConsideredError(int response_code) {
    272   // We throttle only for the status codes most likely to indicate the server
    273   // is failing because it is too busy or otherwise are likely to be
    274   // because of DDoS.
    275   //
    276   // 500 is the generic error when no better message is suitable, and
    277   //     as such does not necessarily indicate a temporary state, but
    278   //     other status codes cover most of the permanent error states.
    279   // 503 is explicitly documented as a temporary state where the server
    280   //     is either overloaded or down for maintenance.
    281   // 509 is the (non-standard but widely implemented) Bandwidth Limit Exceeded
    282   //     status code, which might indicate DDoS.
    283   //
    284   // We do not back off on 502 or 504, which are reported by gateways
    285   // (proxies) on timeouts or failures, because in many cases these requests
    286   // have not made it to the destination server and so we do not actually
    287   // know that it is down or busy.  One degenerate case could be a proxy on
    288   // localhost, where you are not actually connected to the network.
    289   return (response_code == 500 ||
    290           response_code == 503 ||
    291           response_code == 509);
    292 }
    293 
    294 base::TimeTicks URLRequestThrottlerEntry::ImplGetTimeNow() const {
    295   return base::TimeTicks::Now();
    296 }
    297 
    298 void URLRequestThrottlerEntry::HandleThrottlingHeader(
    299     const std::string& header_value,
    300     const std::string& host) {
    301   if (header_value == kExponentialThrottlingDisableValue) {
    302     DisableBackoffThrottling();
    303     if (manager_)
    304       manager_->AddToOptOutList(host);
    305   }
    306 }
    307 
    308 const BackoffEntry* URLRequestThrottlerEntry::GetBackoffEntry() const {
    309   return &backoff_entry_;
    310 }
    311 
    312 BackoffEntry* URLRequestThrottlerEntry::GetBackoffEntry() {
    313   return &backoff_entry_;
    314 }
    315 
    316 // static
    317 bool URLRequestThrottlerEntry::ExplicitUserRequest(const int load_flags) {
    318   return (load_flags & LOAD_MAYBE_USER_GESTURE) != 0;
    319 }
    320 
    321 }  // namespace net
    322