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  package org.apache.cayenne.access.dbsync;
  
  import org.apache.cayenne.access.DataNode;
  
  import java.sql.SQLException;
  
  /**
   * @since 3.0
   */
  // Note that this object must be scoped by DataNode. An attempt to share it between nodes will result in ignoring
  // schema update for all but one node (see CAY-2125)
  public abstract class BaseSchemaUpdateStrategy implements SchemaUpdateStrategy {
  
      protected volatile boolean run;
      protected final ThreadLocal<Boolean> threadRunInProgress;
  
      public BaseSchemaUpdateStrategy() {
  
          // this barrier is needed to prevent stack overflow in the same thread
          // (getConnection/updateSchema/getConnection/...)
          this.threadRunInProgress = new ThreadLocal<>();
      }
  
      @Override
      public void updateSchema(DataNode dataNode) throws SQLException {
  
          Boolean inProgress = threadRunInProgress.get();
          if (!run && (inProgress == null || !inProgress)) {
              synchronized (this) {
                  if (!run) {
  
                      try {
                          threadRunInProgress.set(true);
                          processSchemaUpdate(dataNode);
                          run = true;
                      }
                      finally {
                          threadRunInProgress.set(false);
                      }
                  }
              }
          }
      }
  
      /**
       * @since 3.0
       */
      protected abstract void processSchemaUpdate(DataNode dataNode) throws SQLException;
  
  }
  