the-algorithm/src/java/com/twitter/search/earlybird/common/CaughtUpMonitor.java

package com.twitter.search.earlybird.common;

import java.util.concurrent.atomic.AtomicBoolean;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.twitter.search.common.metrics.SearchCustomGauge;

/**
 * A monitor which enforces the condition that a single thread's work is caught up, and allows
 * other threads to wait to be notified when the work is complete. An AtomicBoolean ensures the
 * current status is visible to all threads.
 */
public class CaughtUpMonitor {
  private static final Logger LOG = LoggerFactory.getLogger(CaughtUpMonitor.class);

  protected final AtomicBoolean isCaughtUp = new AtomicBoolean(false);

  public CaughtUpMonitor(String statPrefix) {
    SearchCustomGauge.export(statPrefix + "_is_caught_up", () -> isCaughtUp() ? 1 : 0);
  }

  public boolean isCaughtUp() {
    return isCaughtUp.get();
  }

  /**
   * Set caught up state, and notify waiting threads if caught up.
   */
  public synchronized void setAndNotify(boolean caughtUp) {
    isCaughtUp.set(caughtUp);
    if (caughtUp) {
      // Readers are caught up, notify waiting threads
      notifyAll();
    }
  }

  /**
   * Wait using Object.wait() until caught up or until thread is interrupted.
   */
  public synchronized void resetAndWaitUntilCaughtUp() {
    LOG.info("Waiting to catch up.");
    // Explicitly set isCaughtUp to false before waiting
    isCaughtUp.set(false);
    try {
      while (!isCaughtUp()) {
        wait();
      }
    } catch (InterruptedException e) {
      LOG.error("{} was interrupted while waiting to catch up", Thread.currentThread());
    }
    LOG.info("Caught up.");
  }
}