recentperiod.hpp

#pragma once
 
#include <atomic>
#include <chrono>
#include <type_traits>
 
#include <userver/utils/datetime.hpp>
#include <userver/utils/fixed_array.hpp>
#include <userver/utils/statistics/fwd.hpp>
#include <userver/utils/statistics/recentperiod_detail.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils::statistics {
 
/** \brief Class maintains circular buffer of Counters
 *
 * At any time current Counter is accessible for modification via
 * GetCurrentCounter().
 * Counter can provide a Reset() member function to clear contents.
 * @see utils::statistics::Percentile
 */
template <typename Counter, typename Result,
          typename Timer = utils::datetime::SteadyClock>
class RecentPeriod {
 public:
  using Duration = typename Timer::duration;
 
  static_assert(
      (detail::kResultWantsAddFunction<Result, Counter, Duration> ||
       detail::kResultCanUseAddAssign<Result, Counter>),
      "The Result template type argument must provide either Add(Counter, "
      "Duration, Duration) function or add assignment operator");
 
  static constexpr bool kUseAddFunction =
      detail::kResultWantsAddFunction<Result, Counter, Duration>;
 
  /**
   * @param epoch_duration duration of epoch.
   * @param max_duration max duration to calculate statistics for
   *        must be multiple of epoch_duration.
   */
  RecentPeriod(Duration epoch_duration = std::chrono::seconds(5),
               Duration max_duration = std::chrono::seconds(60))
      : epoch_duration_(epoch_duration),
        max_duration_(max_duration),
        epoch_index_(0),
        items_(GetSizeForDuration(epoch_duration, max_duration)) {}
 
  Counter& GetCurrentCounter() { return items_[GetCurrentIndex()].counter; }
 
  Counter& GetPreviousCounter(int epochs_ago) {
    return items_[GetPreviousIndex(epochs_ago)].counter;
  }
 
  /** \brief Aggregates counters within given time range
   *
   * @param duration Time range. Special value Duration::min() -> use
   *        whole RecentPeriod range.
   * @param with_current_epoch  Include current (possibly unfinished) counter
   *        into aggregation
   *
   * Type Result must have method Add(Counter, Duration, Duration) or allow
   * addition of counter values
   */
  // NOLINTNEXTLINE(readability-const-return-type)
  const Result GetStatsForPeriod(Duration duration = Duration::min(),
                                 bool with_current_epoch = false) const {
    if (duration == Duration::min()) {
      duration = max_duration_;
    }
 
    Result result{};
    Duration now = Timer::now().time_since_epoch();
    Duration current_epoch = GetEpochForDuration(now);
    Duration start_epoch = current_epoch - duration;
    Duration first_epoch_duration = now - current_epoch;
    std::size_t index = epoch_index_.load();
 
    for (std::size_t i = 0; i < items_.size();
         i++, index = (index + items_.size() - 1) % items_.size()) {
      Duration epoch = items_[index].epoch;
 
      if (epoch > current_epoch) continue;
      if (epoch == current_epoch && !with_current_epoch) continue;
      if (epoch < start_epoch) break;
 
      if constexpr (kUseAddFunction) {
        Duration this_epoch_duration =
            (i == 0) ? first_epoch_duration : epoch_duration_;
 
        Duration before_this_epoch_duration = epoch - start_epoch;
        result.Add(items_[index].counter, this_epoch_duration,
                   before_this_epoch_duration);
      } else {
        result += items_[index].counter;
      }
    }
 
    return result;
  }
 
  Duration GetEpochDuration() const { return epoch_duration_; }
 
  Duration GetMaxDuration() const { return max_duration_; }
 
  void UpdateEpochIfOld() { [[maybe_unused]] auto ignore = GetCurrentIndex(); }
 
  void Reset() {
    for (auto& item : items_) {
      item.Reset();
    }
  }
 
 private:
  size_t GetCurrentIndex() const {
    while (true) {
      Duration now = Timer::now().time_since_epoch();
      Duration epoch = GetEpochForDuration(now);
      std::size_t index = epoch_index_.load();
      Duration bucket_epoch = items_[index].epoch.load();
 
      if (epoch != bucket_epoch) {
        std::size_t new_index = (index + 1) % items_.size();
 
        if (epoch_index_.compare_exchange_weak(index, new_index)) {
          items_[new_index].epoch = epoch;
          items_[(new_index + 1) % items_.size()].Reset();
          return new_index;
        }
      } else {
        return index;
      }
    }
  }
 
  std::size_t GetPreviousIndex(int epochs_ago) {
    int index = static_cast<int>(GetCurrentIndex()) - epochs_ago;
    while (index < 0) index += items_.size();
    return index % items_.size();
  }
 
  Duration GetEpochForDuration(Duration duration) const {
    auto now = std::chrono::duration_cast<Duration>(duration);
    return now - now % epoch_duration_;
  }
 
  static std::size_t GetSizeForDuration(Duration epoch_duration,
                                        Duration max_duration) {
    /* 3 = current bucket, next zero bucket and extra one to handle
       possible race. */
    return max_duration.count() / epoch_duration.count() + 3;
  }
 
  struct EpochBucket {
    static constexpr bool kUseReset = detail::kCanReset<Counter>;
    std::atomic<Duration> epoch;
    Counter counter;
 
    EpochBucket() { Reset(); }
 
    void Reset() {
      epoch = Duration::min();
      if constexpr (kUseReset) {
        counter.Reset();
      } else {
        counter = 0;
      }
    }
  };
 
  const Duration epoch_duration_;
  const Duration max_duration_;
  mutable std::atomic_size_t epoch_index_;
  mutable utils::FixedArray<EpochBucket> items_;
};
 
/// @a Writer support for @a RecentPeriod
template <typename Counter, typename Result, typename Timer>
void DumpMetric(Writer& writer,
                const RecentPeriod<Counter, Result, Timer>& recent_period) {
  writer = recent_period.GetStatsForPeriod();
}
 
/// Reset support for @a RecentPeriod
template <typename Counter, typename Result, typename Timer>
void ResetMetric(RecentPeriod<Counter, Result, Timer>& recent_period) {
  recent_period.Reset();
}
 
}  // namespace utils::statistics
 
USERVER_NAMESPACE_END