Home | History | Annotate | Download | only in src 
      1 // Copyright 2015 The Weave 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 "src/backoff_entry.h"
      6 
      7 #include <algorithm>
      8 #include <cmath>
      9 #include <limits>
     10 
     11 #include <base/logging.h>
     12 #include <base/rand_util.h>
     13 
     14 namespace weave {
     15 
     16 BackoffEntry::BackoffEntry(const BackoffEntry::Policy* const policy)
     17     : policy_(policy) {
     18   DCHECK(policy_);
     19   Reset();
     20 }
     21 
     22 void BackoffEntry::InformOfRequest(bool succeeded) {
     23   if (!succeeded) {
     24     ++failure_count_;
     25     exponential_backoff_release_time_ = CalculateReleaseTime();
     26   } else {
     27     // We slowly decay the number of times delayed instead of
     28     // resetting it to 0 in order to stay stable if we receive
     29     // successes interleaved between lots of failures.  Note that in
     30     // the normal case, the calculated release time (in the next
     31     // statement) will be in the past once the method returns.
     32     if (failure_count_ > 0)
     33       --failure_count_;
     34 
     35     // The reason why we are not just cutting the release time to
     36     // ImplGetTimeNow() is on the one hand, it would unset a release
     37     // time set by SetCustomReleaseTime and on the other we would like
     38     // to push every request up to our "horizon" when dealing with
     39     // multiple in-flight requests. Ex: If we send three requests and
     40     // we receive 2 failures and 1 success. The success that follows
     41     // those failures will not reset the release time, further
     42     // requests will then need to wait the delay caused by the 2
     43     // failures.
     44     base::TimeDelta delay;
     45     if (policy_->always_use_initial_delay)
     46       delay = base::TimeDelta::FromMilliseconds(policy_->initial_delay_ms);
     47     exponential_backoff_release_time_ =
     48         std::max(ImplGetTimeNow() + delay, exponential_backoff_release_time_);
     49   }
     50 }
     51 
     52 bool BackoffEntry::ShouldRejectRequest() const {
     53   return exponential_backoff_release_time_ > ImplGetTimeNow();
     54 }
     55 
     56 base::TimeDelta BackoffEntry::GetTimeUntilRelease() const {
     57   base::TimeTicks now = ImplGetTimeNow();
     58   if (exponential_backoff_release_time_ <= now)
     59     return base::TimeDelta();
     60   return exponential_backoff_release_time_ - now;
     61 }
     62 
     63 base::TimeTicks BackoffEntry::GetReleaseTime() const {
     64   return exponential_backoff_release_time_;
     65 }
     66 
     67 void BackoffEntry::SetCustomReleaseTime(const base::TimeTicks& release_time) {
     68   exponential_backoff_release_time_ = release_time;
     69 }
     70 
     71 bool BackoffEntry::CanDiscard() const {
     72   if (policy_->entry_lifetime_ms == -1)
     73     return false;
     74 
     75   base::TimeTicks now = ImplGetTimeNow();
     76 
     77   int64_t unused_since_ms =
     78       (now - exponential_backoff_release_time_).InMilliseconds();
     79 
     80   // Release time is further than now, we are managing it.
     81   if (unused_since_ms < 0)
     82     return false;
     83 
     84   if (failure_count_ > 0) {
     85     // Need to keep track of failures until maximum back-off period
     86     // has passed (since further failures can add to back-off).
     87     return unused_since_ms >=
     88            std::max(policy_->maximum_backoff_ms, policy_->entry_lifetime_ms);
     89   }
     90 
     91   // Otherwise, consider the entry is outdated if it hasn't been used for the
     92   // specified lifetime period.
     93   return unused_since_ms >= policy_->entry_lifetime_ms;
     94 }
     95 
     96 void BackoffEntry::Reset() {
     97   failure_count_ = 0;
     98 
     99   // We leave exponential_backoff_release_time_ unset, meaning 0. We could
    100   // initialize to ImplGetTimeNow() but because it's a virtual method it's
    101   // not safe to call in the constructor (and the constructor calls Reset()).
    102   // The effects are the same, i.e. ShouldRejectRequest() will return false
    103   // right after Reset().
    104   exponential_backoff_release_time_ = base::TimeTicks();
    105 }
    106 
    107 base::TimeTicks BackoffEntry::ImplGetTimeNow() const {
    108   return base::TimeTicks::Now();
    109 }
    110 
    111 base::TimeTicks BackoffEntry::CalculateReleaseTime() const {
    112   int effective_failure_count =
    113       std::max(0, failure_count_ - policy_->num_errors_to_ignore);
    114 
    115   // If always_use_initial_delay is true, it's equivalent to
    116   // the effective_failure_count always being one greater than when it's false.
    117   if (policy_->always_use_initial_delay)
    118     ++effective_failure_count;
    119 
    120   if (effective_failure_count == 0) {
    121     // Never reduce previously set release horizon, e.g. due to Retry-After
    122     // header.
    123     return std::max(ImplGetTimeNow(), exponential_backoff_release_time_);
    124   }
    125 
    126   // The delay is calculated with this formula:
    127   // delay = initial_backoff * multiply_factor^(
    128   //     effective_failure_count - 1) * Uniform(1 - jitter_factor, 1]
    129   // Note: if the failure count is too high, |delay_ms| will become infinity
    130   // after the exponential calculation, and then NaN after the jitter is
    131   // accounted for. Both cases are handled by using CheckedNumeric<int64_t> to
    132   // perform the conversion to integers.
    133   double delay_ms = policy_->initial_delay_ms;
    134   delay_ms *= pow(policy_->multiply_factor, effective_failure_count - 1);
    135   delay_ms -= base::RandDouble() * policy_->jitter_factor * delay_ms;
    136 
    137   // Do overflow checking in microseconds, the internal unit of TimeTicks.
    138   const int64_t kTimeTicksNowUs =
    139       (ImplGetTimeNow() - base::TimeTicks()).InMicroseconds();
    140   base::internal::CheckedNumeric<int64_t> calculated_release_time_us =
    141       delay_ms + 0.5;
    142   calculated_release_time_us *= base::Time::kMicrosecondsPerMillisecond;
    143   calculated_release_time_us += kTimeTicksNowUs;
    144 
    145   const int64_t kMaxTime = std::numeric_limits<int64_t>::max();
    146   base::internal::CheckedNumeric<int64_t> maximum_release_time_us = kMaxTime;
    147   if (policy_->maximum_backoff_ms >= 0) {
    148     maximum_release_time_us = policy_->maximum_backoff_ms;
    149     maximum_release_time_us *= base::Time::kMicrosecondsPerMillisecond;
    150     maximum_release_time_us += kTimeTicksNowUs;
    151   }
    152 
    153   // Decide between maximum release time and calculated release time, accounting
    154   // for overflow with both.
    155   int64_t release_time_us =
    156       std::min(calculated_release_time_us.ValueOrDefault(kMaxTime),
    157                maximum_release_time_us.ValueOrDefault(kMaxTime));
    158 
    159   // Never reduce previously set release horizon, e.g. due to Retry-After
    160   // header.
    161   return std::max(
    162       base::TimeTicks() + base::TimeDelta::FromMicroseconds(release_time_us),
    163       exponential_backoff_release_time_);
    164 }
    165 
    166 }  // namespace weave
    167