com.datastax.driver.core.policies.ExponentialReconnectionPolicy Maven / Gradle / Ivy

/*
 * Copyright DataStax, Inc.
 *
 * Licensed 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 com.datastax.driver.core.policies;

import com.datastax.driver.core.Cluster;

/**
 * A reconnection policy that waits exponentially longer between each reconnection attempt (but
 * keeps a constant delay once a maximum delay is reached).
 */
public class ExponentialReconnectionPolicy implements ReconnectionPolicy {

  private final long baseDelayMs;
  private final long maxDelayMs;
  private final long maxAttempts;

  /**
   * Creates a reconnection policy waiting exponentially longer for each new attempt.
   *
   * @param baseDelayMs the base delay in milliseconds to use for the schedules created by this
   *     policy.
   * @param maxDelayMs the maximum delay to wait between two attempts.
   */
  public ExponentialReconnectionPolicy(long baseDelayMs, long maxDelayMs) {
    if (baseDelayMs < 0 || maxDelayMs < 0)
      throw new IllegalArgumentException("Invalid negative delay");
    if (baseDelayMs == 0)
      throw new IllegalArgumentException("baseDelayMs must be strictly positive");
    if (maxDelayMs < baseDelayMs)
      throw new IllegalArgumentException(
          String.format(
              "maxDelayMs (got %d) cannot be smaller than baseDelayMs (got %d)",
              maxDelayMs, baseDelayMs));

    this.baseDelayMs = baseDelayMs;
    this.maxDelayMs = maxDelayMs;

    // Maximum number of attempts after which we overflow (which is kind of theoretical anyway,
    // you'll
    // die of old age before reaching that but hey ...)
    int ceil = (baseDelayMs & (baseDelayMs - 1)) == 0 ? 0 : 1;
    this.maxAttempts = 64 - Long.numberOfLeadingZeros(Long.MAX_VALUE / baseDelayMs) - ceil;
  }

  /**
   * The base delay in milliseconds for this policy (e.g. the delay before the first reconnection
   * attempt).
   *
   * @return the base delay in milliseconds for this policy.
   */
  public long getBaseDelayMs() {
    return baseDelayMs;
  }

  /**
   * The maximum delay in milliseconds between reconnection attempts for this policy.
   *
   * @return the maximum delay in milliseconds between reconnection attempts for this policy.
   */
  public long getMaxDelayMs() {
    return maxDelayMs;
  }

  /**
   * A new schedule that used an exponentially growing delay between reconnection attempts.
   *
   * For this schedule, reconnection attempt {@code i} will be tried {@code Math.min(2^(i-1) *
   * getBaseDelayMs(), getMaxDelayMs())} milliseconds after the previous one.
   *
   * @return the newly created schedule.
   */
  @Override
  public ReconnectionSchedule newSchedule() {
    return new ExponentialSchedule();
  }

  private class ExponentialSchedule implements ReconnectionSchedule {

    private int attempts;

    @Override
    public long nextDelayMs() {

      if (attempts > maxAttempts) return maxDelayMs;

      return Math.min(baseDelayMs * (1L << attempts++), maxDelayMs);
    }
  }

  @Override
  public void init(Cluster cluster) {
    // nothing to do
  }

  @Override
  public void close() {
    // nothing to do
  }
}