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.beam.runners.dataflow.worker;
  
  import java.util.concurrent.ConcurrentMap;
  import java.util.concurrent.ConcurrentSkipListMap;
  import org.apache.beam.sdk.metrics.MetricsContainer;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.FluentIterable;
  
  /**
   * Manages the instances of {@link MetricsContainer} that have been created for a specific context.
   *
   * <p>This class is thread-safe. In Batch mode it there is a single thread creating containers and a
   * separate thread calling {@link #getContainers()} to report tentative results. In Streaming all
   * execution threads use the same registry. Thus it supports concurrent retrieval and creation of
   * containers, as well as the ability to iterate over the containers.
   */
  public abstract class MetricsContainerRegistry<T extends MetricsContainer> {
  
    private final ConcurrentMap<String, T> containers = new ConcurrentSkipListMap<>();
  
    /** Retrieve (creating if necessary) the {@link MetricsContainer} to use with the given step. */
    public T getContainer(String stepName) {
      return containers.computeIfAbsent(stepName, unused -> createContainer(stepName));
    }
  
    /**
     * Returns all of the containers that have been created with this registry. It returns a weakly
     * consistent view of the containers that have been created in this registry. That is:
     *
     * <ul>
     *   <li>iteration may proceed concurrently with other operations
     *   <li>iteration will never throw ConcurrentModificationException
     *   <li>iteration is guaranteed to traverse elements as they existed upon construction exactly
     *       once, and may (but are not guaranteed to) reflect any modifications subsequent to
     *       construction.
     * </ul>
     */
    public FluentIterable<T> getContainers() {
      return FluentIterable.from(containers.values());
    }
  
    /** Create the container for a given step. */
    protected abstract T createContainer(String stepName);
  }
  