content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package org.apache.cassandra.transport;
  
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.function.LongSupplier;
  
  import com.google.common.annotations.VisibleForTesting;
  
  import org.apache.cassandra.config.DatabaseDescriptor;
  
  import static org.apache.cassandra.utils.MonotonicClock.Global.preciseTime;
  
  /**
   * Native Queue Backpressure mechanism. If the queue fills up above {@link DatabaseDescriptor#getNativeTransportQueueMaxItemAgeThreshold()}.
   * In other words, request has been sitting more than a % of {@link DatabaseDescriptor#getNativeTransportQueueMaxItemAgeThreshold()} in the queue,
   * we start an incident.
   * <p>
   * Incident starts by marking the incident time and raising severity level to 1. Each time we observe an old item in the head of
   * the queue, we first bump then number of times we have applied the backpressure. After bumping it 10 times, we increase severity level by 1.
   * <p>
   * Backpressure delay applied to the client socket is computed by multiplying the severity level by the minimum delay.
   * <p>
   * If we have not seen old requests in the head of the queue for 1 second, we close the incident.
   * <p>
   * If the queue remains saturated for a prolonged period (meaning {@link Dispatcher#hasQueueCapacity} returns false), the amount of delay
   * will increase in proportion to the request rate as appliedTimes & severityLevel are incremented. If no new requests are considered
   * overloaded in this way for a second, the incident will be reset and so the delay will drop back down to {@link Incident#minDelayNanos()}.
   */
  public interface QueueBackpressure
  {
      QueueBackpressure NO_OP = timeUnit -> 0;
  
      QueueBackpressure DEFAULT = new QueueBackpressure()
      {
          private final AtomicReference<Incident> state = new AtomicReference<>(noBackpressure(() -> DatabaseDescriptor.getNativeTransportMinBackoffOnQueueOverload(TimeUnit.NANOSECONDS),
                                                                                               () -> DatabaseDescriptor.getNativeTransportMaxBackoffOnQueueOverload(TimeUnit.NANOSECONDS)));
  
          public long markAndGetDelay(TimeUnit timeUnit)
          {
              return state.updateAndGet(Incident::mark).delay(timeUnit);
          }
      };
  
      long markAndGetDelay(TimeUnit timeUnit);
  
      static Incident noBackpressure(LongSupplier minDelayNanos, LongSupplier maxDelayNanos)
      {
          return new Incident(minDelayNanos, maxDelayNanos,
                              -1, 0, 0);
      }
  
      @VisibleForTesting
      class Incident
      {
          private final long appliedAt;
  
          private final int severityLevel;
          private final int appliedTimes;
  
          private final LongSupplier minDelayNanos;
          private final LongSupplier maxDelayNanos;
  
          @VisibleForTesting
          public Incident(LongSupplier minDelayNanos, LongSupplier maxDelayNanos, long appliedAt, int severityLevel, int appliedTimes)
          {
              this.minDelayNanos = minDelayNanos;
              this.maxDelayNanos = maxDelayNanos;
              this.appliedAt = appliedAt;
              this.severityLevel = severityLevel;
              this.appliedTimes = appliedTimes;
          }
  
          public Incident mark()
          {
              return mark(preciseTime.now());
          }
  
          @VisibleForTesting
          public Incident mark(long now)
          {
              // Last time we have applied backpressure was over a second ago, consider this a new incident
              if (appliedAt > 0 && now - appliedAt >= TimeUnit.SECONDS.toNanos(1))
              {
                  return new Incident(minDelayNanos, maxDelayNanos, now, 1, 1);
              }
              // Continuing incident: apply backpressure but do not bump severity level yet
              else if (appliedTimes < 10)
              {
                  return new Incident(minDelayNanos, maxDelayNanos, now, severityLevel == 0 ? 1 : severityLevel, appliedTimes + 1);
              }
              //
              else
              {
                  return new Incident(minDelayNanos, maxDelayNanos, now, severityLevel + 1, 1);
              }
          }
  
          public long appliedAt()
          {
              return appliedAt;
          }
  
          public long delay(TimeUnit timeUnit)
          {
              return timeUnit.convert(Math.min(maxDelayNanos(), severityLevel * minDelayNanos()), TimeUnit.NANOSECONDS);
          }
  
          public long minDelayNanos()
          {
              return minDelayNanos.getAsLong();
          }
  
          public long maxDelayNanos()
          {
              return maxDelayNanos.getAsLong();
          }
  
          public String toString()
          {
              return "QueueBackpressure{" +
                     "appliedAgo=" + (appliedAt == -1 ? "never" : TimeUnit.NANOSECONDS.toMillis(preciseTime.now() - appliedAt)) +
                     ", severityLevel=" + severityLevel +
                     ", appliedTimes=" + appliedTimes +
                     ", currentDelay=" + delay(TimeUnit.MILLISECONDS) + "ms" +
                     '}';
          }
      }
  }