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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,
   * 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.
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  package org.apache.omid.tso;
  
  import org.apache.omid.tso.TSOStateManager.StateObserver;
  import org.apache.omid.tso.TSOStateManager.TSOState;
  import org.testng.annotations.BeforeMethod;
  import org.testng.annotations.Test;
  
  import java.io.IOException;
  
  import static org.mockito.Matchers.eq;
  import static org.mockito.Mockito.mock;
  import static org.mockito.Mockito.spy;
  import static org.mockito.Mockito.timeout;
  import static org.mockito.Mockito.verify;
  import static org.mockito.Mockito.when;
  import static org.testng.Assert.assertEquals;
  import static org.testng.Assert.assertTrue;
  
  public class TestTSOStateManager {
  
      private static final long INITIAL_STATE_VALUE = 1L;
      private static final long NEW_STATE_VALUE = 1000;
  
      // Mocks
      private TimestampOracle timestampOracle = mock(TimestampOracle.class);
  
      // Component under test
      private TSOStateManager stateManager = new TSOStateManagerImpl(timestampOracle);
  
      @BeforeMethod
      public void beforeMethod() {
          // Initialize the state with the one reported by the Timestamp Oracle
          when(timestampOracle.getLast()).thenReturn(INITIAL_STATE_VALUE);
      }
  
      @Test(timeOut = 10_000)
      public void testTSOServerStateInitialization() throws Exception {
  
          // Reset the state and check we get the initial state values
          TSOState initialState = stateManager.initialize();
          assertEquals(initialState.getLowWatermark(), INITIAL_STATE_VALUE);
          assertEquals(initialState.getEpoch(), INITIAL_STATE_VALUE);
          assertTrue(initialState.getLowWatermark() == initialState.getEpoch(),
                     "In this implementation low watermark should be equal to epoch");
  
          // Then, simulate a change in the state returned by the Timestamp Oracle...
          when(timestampOracle.getLast()).thenReturn(NEW_STATE_VALUE);
          // ... and again, reset the state and check we get the new values
          TSOState secondState = stateManager.initialize();
          assertEquals(secondState.getLowWatermark(), NEW_STATE_VALUE);
          assertEquals(secondState.getEpoch(), NEW_STATE_VALUE);
          assertTrue(secondState.getLowWatermark() == secondState.getEpoch(),
                     "In this implementation low watermark should be equal to epoch");
  
      }
  
      @Test(timeOut = 10_000)
      public void testObserverRegistrationAndDeregistrationForStateChanges() throws Exception {
  
          // Register observer 1 for receiving state changes
          StateObserver observer1 = spy(new DummyObserver());
          stateManager.register(observer1);
  
          // Reset the state to trigger observer notifications
          TSOState state = stateManager.initialize();
  
          // Check observer 1 was notified with the corresponding state
          verify(observer1, timeout(100).times(1)).update(eq(state));
  
          // Register observer 1 for receiving state changes
          StateObserver observer2 = spy(new DummyObserver());
          stateManager.register(observer2);
  
          // Again, reset the state to trigger observer notifications
          state = stateManager.initialize();
  
          // Check both observers were notified with the corresponding state
          verify(observer1, timeout(100).times(1)).update(eq(state));
          verify(observer2, timeout(100).times(1)).update(eq(state));
  
          // De-register observer 1
          stateManager.unregister(observer1);
 
         // Again, reset the state to trigger observer notifications
         state = stateManager.initialize();
 
         // Check only observer 2 was notified
         verify(observer1, timeout(100).times(0)).update(eq(state));
         verify(observer2, timeout(100).times(1)).update(eq(state));
     }
 
     // ----------------------------------------------------------------------------------------------------------------
     // Helper classes
     // ----------------------------------------------------------------------------------------------------------------
 
     private class DummyObserver implements StateObserver {
 
         @Override
         public void update(TSOState state) throws IOException {
         }
 
     }
 
 }