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.cassandra.service.pager;
  
  import org.apache.cassandra.db.ConsistencyLevel;
  import org.apache.cassandra.db.ReadExecutionController;
  import org.apache.cassandra.db.filter.DataLimits;
  import org.apache.cassandra.db.EmptyIterators;
  import org.apache.cassandra.db.partitions.PartitionIterator;
  import org.apache.cassandra.exceptions.RequestExecutionException;
  import org.apache.cassandra.exceptions.RequestValidationException;
  import org.apache.cassandra.service.ClientState;
  import org.apache.cassandra.transport.Dispatcher;
  
  /**
   * Perform a query, paging it by page of a given size.
   *
   * This is essentially an iterator of pages. Each call to fetchPage() will
   * return the next page (i.e. the next list of rows) and isExhausted()
   * indicates whether there is more page to fetch. The pageSize will
   * either be in term of cells or in term of CQL3 row, depending on the
   * parameters of the command we page.
   *
   * Please note that the pager might page within rows, so there is no guarantee
   * that successive pages won't return the same row (though with different
   * columns every time).
   *
   * Also, there is no guarantee that fetchPage() won't return an empty list,
   * even if isExhausted() return false (but it is guaranteed to return an empty
   * list *if* isExhausted() return true). Indeed, isExhausted() does *not*
   * trigger a query so in some (fairly rare) case we might not know the paging
   * is done even though it is.
   */
  public interface QueryPager
  {
      QueryPager EMPTY = new QueryPager()
      {
          public ReadExecutionController executionController()
          {
              return ReadExecutionController.empty();
          }
  
          public PartitionIterator fetchPage(int pageSize, ConsistencyLevel consistency, ClientState clientState, Dispatcher.RequestTime requestTime) throws RequestValidationException, RequestExecutionException
          {
              return EmptyIterators.partition();
          }
  
          public PartitionIterator fetchPageInternal(int pageSize, ReadExecutionController executionController) throws RequestValidationException, RequestExecutionException
          {
              return EmptyIterators.partition();
          }
  
          public boolean isExhausted()
          {
              return true;
          }
  
          public int maxRemaining()
          {
              return 0;
          }
  
          public PagingState state()
          {
              return null;
          }
  
          public QueryPager withUpdatedLimit(DataLimits newLimits)
          {
              throw new UnsupportedOperationException();
          }
      };
  
      /**
       * Fetches the next page.
       *
       * @param pageSize the maximum number of elements to return in the next page.
       * @param consistency the consistency level to achieve for the query.
       * @param clientState the {@code ClientState} for the query. In practice, this can be null unless
       * {@code consistency} is a serial consistency.
       * @return the page of result.
       */
      public PartitionIterator fetchPage(int pageSize, ConsistencyLevel consistency, ClientState clientState, Dispatcher.RequestTime requestTime) throws RequestValidationException, RequestExecutionException;
  
      /**
       * Starts a new read operation.
       * <p>
       * This must be called before {@link fetchPageInternal} and passed to it to protect the read.
       * The returned object <b>must</b> be closed on all path and it is thus strongly advised to
       * use it in a try-with-ressource construction.
       *
       * @return a newly started order group for this {@code QueryPager}.
       */
      public ReadExecutionController executionController();
  
      /**
       * Fetches the next page internally (in other, this does a local query).
       *
       * @param pageSize the maximum number of elements to return in the next page.
       * @param executionController the {@code ReadExecutionController} protecting the read.
       * @return the page of result.
       */
      public PartitionIterator fetchPageInternal(int pageSize, ReadExecutionController executionController) throws RequestValidationException, RequestExecutionException;
  
      /**
       * Whether or not this pager is exhausted, i.e. whether or not a call to
       * fetchPage may return more result.
       *
       * @return whether the pager is exhausted.
       */
      public boolean isExhausted();
  
      /**
       * The maximum number of cells/CQL3 row that we may still have to return.
       * In other words, that's the initial user limit minus what we've already
       * returned (note that it's not how many we *will* return, just the upper
       * limit on it).
       */
      public int maxRemaining();
  
      /**
       * Get the current state of the pager. The state can allow to restart the
       * paging on another host from where we are at this point.
       *
       * @return the current paging state. Will return null if paging is at the
       * beginning. If the pager is exhausted, the result is undefined.
       */
      public PagingState state();
  
      /**
       * Creates a new <code>QueryPager</code> that use the new limits.
       *
       * @param newLimits the new limits
       * @return a new <code>QueryPager</code> that use the new limits
       */
      public QueryPager withUpdatedLimit(DataLimits newLimits);
  
  
      /**
       * @return true given read query is a top-k request
       */
      default boolean isTopK()
      {
          return false;
      }
  }
  