/*
 * Copyright DataStax, Inc.
 *
 * Licensed 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 com.datastax.oss.driver.internal.core.util.collection;

import com.datastax.oss.driver.api.core.metadata.Node;
import net.jcip.annotations.ThreadSafe;

A query plan where nodes are computed lazily, when the plan is consumed for the first time.
This class can be useful when a query plan computation is heavy but the plan has a low chance of ever being consumed, e.g. the last query plan in a CompositeQueryPlan. /**
 * A query plan where nodes are computed lazily, when the plan is consumed for the first time.
 *
 * <p>This class can be useful when a query plan computation is heavy but the plan has a low chance
 * of ever being consumed, e.g. the last query plan in a {@link CompositeQueryPlan}.
 */
@ThreadSafe
public abstract class LazyQueryPlan extends QueryPlanBase {

  private volatile Object[] nodes;

  
Computes and returns the nodes to use for this query plan.
For efficiency, the declared return type is Object[] but all elements must be instances of Node. See getNodes() for details. 
This method is guaranteed to be invoked only once, at the first call to QueryPlanBase.poll(). 
Implementors must avoid blocking calls in this method as it will be invoked on the driver's
hot path.
/**
   * Computes and returns the nodes to use for this query plan.
   *
   * <p>For efficiency, the declared return type is {@code Object[]} but all elements must be
   * instances of {@link Node}. See {@link #getNodes()} for details.
   *
   * <p>This method is guaranteed to be invoked only once, at the first call to {@link #poll()}.
   *
   * <p>Implementors must avoid blocking calls in this method as it will be invoked on the driver's
   * hot path.
   */
  protected abstract Object[] computeNodes();

  @Override
  protected Object[] getNodes() {
    if (nodes == null) {
      synchronized (this) {
        if (nodes == null) {
          nodes = computeNodes();
        }
      }
    }
    return nodes;
  }
}