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/*
 * Copyright 2011-2018 Netflix, 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.netflix.servo.monitor;

import com.google.common.base.Throwables;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.collect.ImmutableList;
import com.netflix.servo.DefaultMonitorRegistry;
import com.netflix.servo.tag.TagList;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

/**
 * Utility class that dynamically creates gauges based on an arbitrary (name, tagList),
 * or {@link com.netflix.servo.monitor.MonitorConfig}
 * Gauges are automatically expired after 15 minutes of inactivity.
 */
public final class DynamicGauge implements CompositeMonitor, SpectatorMonitor {
  private static final Logger LOGGER = LoggerFactory.getLogger(DynamicGauge.class);
  private static final String DEFAULT_EXPIRATION = "15";
  private static final String DEFAULT_EXPIRATION_UNIT = "MINUTES";
  private static final String CLASS_NAME = DynamicGauge.class.getCanonicalName();
  private static final String EXPIRATION_PROP = CLASS_NAME + ".expiration";
  private static final String EXPIRATION_PROP_UNIT = CLASS_NAME + ".expirationUnit";
  private static final String INTERNAL_ID = "servoGauges";
  private static final String CACHE_MONITOR_ID = "servoGaugesCache";
  private static final MonitorConfig BASE_CONFIG = new MonitorConfig.Builder(INTERNAL_ID).build();

  private static final DynamicGauge INSTANCE = new DynamicGauge();

  private final LoadingCache gauges;
  private final CompositeMonitor cacheMonitor;

  private DynamicGauge() {
    final String expiration = System.getProperty(EXPIRATION_PROP, DEFAULT_EXPIRATION);
    final String expirationUnit = System.getProperty(EXPIRATION_PROP_UNIT, DEFAULT_EXPIRATION_UNIT);
    final long expirationValue = Long.parseLong(expiration);
    final TimeUnit expirationUnitValue = TimeUnit.valueOf(expirationUnit);

    gauges = CacheBuilder.newBuilder()
        .expireAfterAccess(expirationValue, expirationUnitValue)
        .build(new CacheLoader() {
          @Override
          public DoubleGauge load(final MonitorConfig config) throws Exception {
            return new DoubleGauge(config);
          }
        });
    cacheMonitor = Monitors.newCacheMonitor(CACHE_MONITOR_ID, gauges);
    DefaultMonitorRegistry.getInstance().register(this);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void initializeSpectator(TagList tags) {
  }

  /**
   * Set a gauge based on a given {@link MonitorConfig} by a given value.
   *
   * @param config The monitoring config
   * @param value  The amount added to the current value
   */
  public static void set(MonitorConfig config, double value) {
    INSTANCE.get(config).set(value);
  }

  /**
   * Increment a gauge specified by a name.
   */
  public static void set(String name, double value) {
    set(MonitorConfig.builder(name).build(), value);
  }

  /**
   * Set the gauge for a given name, tagList by a given value.
   */
  public static void set(String name, TagList list, double value) {
    final MonitorConfig config = MonitorConfig.builder(name).withTags(list).build();
    set(config, value);
  }

  private DoubleGauge get(MonitorConfig config) {
    try {
      return gauges.get(config);
    } catch (ExecutionException e) {
      LOGGER.error("Failed to get a gauge for {}: {}", config, e.getMessage());
      throw Throwables.propagate(e);
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public List> getMonitors() {
    final ConcurrentMap gaugesMap = gauges.asMap();
    return ImmutableList.copyOf(gaugesMap.values());
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public Long getValue() {
    return (long) gauges.asMap().size();
  }

  @Override
  public Long getValue(int pollerIndex) {
    return getValue();
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public MonitorConfig getConfig() {
    return BASE_CONFIG;
  }

  @Override
  public String toString() {
    ConcurrentMap map = gauges.asMap();
    return "DynamicGauge{"
        + "baseConfig=" + BASE_CONFIG
        + ", totalGauges=" + map.size()
        + ", gauges=" + map + '}';
  }
}