code: /*
   * 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.skywalking.oap.server.core.analysis.worker;
  
  import java.util.Iterator;
  import java.util.Map;
  import java.util.concurrent.ConcurrentHashMap;
  import lombok.AccessLevel;
  import lombok.Setter;
  import org.apache.skywalking.oap.server.core.analysis.metrics.Metrics;
  import org.apache.skywalking.oap.server.library.client.request.InsertRequest;
  import org.apache.skywalking.oap.server.library.client.request.UpdateRequest;
  
  /**
   * MetricsSessionCache is a key-value cache to hold hot metric in-memory to reduce payload to pre-read.
   *
   * There are two ways to make sure metrics in-cache,
   * 1. Metrics is read from the Database through {@link MetricsPersistentWorker}.loadFromStorage
   * 2. The built {@link InsertRequest} executed successfully.
   *
   * There are two cases to remove metrics from the cache.
   * 1. The metrics expired.
   * 2. The built {@link UpdateRequest} executed failure, which could be caused
   * (1) Database error. (2) No data updated, such as the counter of update statement is 0 in JDBC.
   *
   * @since 9.4.0 Created this from MetricsPersistentWorker.sessionCache.
   */
  public class MetricsSessionCache {
      private final Map<Metrics, Metrics> sessionCache;
      @Setter(AccessLevel.PACKAGE)
      private long timeoutThreshold;
  
      public MetricsSessionCache(long timeoutThreshold) {
          // Due to the cache would be updated depending on final storage implementation,
          // the map/cache could be updated concurrently.
          // Set to ConcurrentHashMap in order to avoid HashMap deadlock.
          // Since 9.3.0
          this.sessionCache = new ConcurrentHashMap<>(100);
          this.timeoutThreshold = timeoutThreshold;
      }
  
      Metrics get(Metrics metrics) {
          return sessionCache.get(metrics);
      }
  
      public Metrics remove(Metrics metrics) {
          return sessionCache.remove(metrics);
      }
  
      public void put(Metrics metrics) {
          sessionCache.put(metrics, metrics);
      }
  
      void removeExpired() {
          Iterator<Metrics> iterator = sessionCache.values().iterator();
          long timestamp = System.currentTimeMillis();
          while (iterator.hasNext()) {
              Metrics metrics = iterator.next();
  
              if (metrics.isExpired(timestamp, timeoutThreshold)) {
                  iterator.remove();
              }
          }
      }
  }
  