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    * Licensed to the Apache Software Foundation (ASF) under one
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    * 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,
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   * See the License for the specific language governing permissions and
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  package org.apache.hadoop.hbase;
  
  import static org.junit.Assert.assertEquals;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.MultithreadedTestUtil.TestContext;
  import org.apache.hadoop.hbase.MultithreadedTestUtil.TestThread;
  import org.apache.hadoop.hbase.master.CachedClusterId;
  import org.apache.hadoop.hbase.master.HMaster;
  import org.apache.hadoop.hbase.testclassification.MediumTests;
  import org.junit.AfterClass;
  import org.junit.BeforeClass;
  import org.junit.Test;
  import org.junit.experimental.categories.Category;
  
  @Category(MediumTests.class)
  public class TestCachedClusterId {
  
    private static final HBaseTestingUtility TEST_UTIL = new HBaseTestingUtility();
  
    private static String clusterId;
    private static HMaster activeMaster;
    private static HMaster standByMaster;
  
    private static class GetClusterIdThread extends TestThread {
      CachedClusterId cachedClusterId;
      public GetClusterIdThread(TestContext ctx, CachedClusterId clusterId) {
        super(ctx);
        cachedClusterId = clusterId;
      }
  
      @Override
      public void doWork() throws Exception {
        assertEquals(clusterId, cachedClusterId.getFromCacheOrFetch());
      }
    }
  
    @BeforeClass
    public static void setUp() throws Exception {
      TEST_UTIL.startMiniCluster(1);
      activeMaster = TEST_UTIL.getHBaseCluster().getMaster();
      clusterId = activeMaster.getClusterId();
      standByMaster = TEST_UTIL.getHBaseCluster().startMaster().getMaster();
    }
  
    @AfterClass
    public static void tearDown() throws Exception {
      TEST_UTIL.shutdownMiniCluster();
    }
  
    @Test
    public void testClusterIdMatch() {
      assertEquals(clusterId, standByMaster.getClusterId());
    }
  
    @Test
    public void testMultiThreadedGetClusterId() throws Exception {
      Configuration conf = TEST_UTIL.getConfiguration();
      CachedClusterId cachedClusterId = new CachedClusterId(conf);
      TestContext context = new TestContext(conf);
      int numThreads = 100;
      for (int i = 0; i < numThreads; i++) {
        context.addThread(new GetClusterIdThread(context, cachedClusterId));
      }
      context.startThreads();
      context.stop();
      int cacheMisses = cachedClusterId.getCacheStats();
      assertEquals(cacheMisses, 1);
    }
  }