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Apache Geode provides a database-like consistency model, reliable transaction processing and a shared-nothing architecture to maintain very low latency performance with high concurrency processing

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/*
 * 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.geode.cache.query.internal.cq;

import org.apache.geode.cache.query.internal.cq.spi.CqServiceFactory;
import org.apache.geode.distributed.internal.DistributionConfig;
import org.apache.geode.internal.cache.GemFireCacheImpl;

import java.io.DataInput;
import java.io.IOException;
import java.util.Iterator;
import java.util.ServiceLoader;

public class CqServiceProvider {

  private static final CqServiceFactory factory;
  // System property to maintain the CQ event references for optimizing the updates.
  // This will allows to run the CQ query only once during update events.
  public static boolean MAINTAIN_KEYS = Boolean
      .valueOf(System.getProperty(DistributionConfig.GEMFIRE_PREFIX + "cq.MAINTAIN_KEYS", "true"))
      .booleanValue();
  /**
   * A debug flag used for testing vMotion during CQ registration
   */
  public static boolean VMOTION_DURING_CQ_REGISTRATION_FLAG = false;


  static {
    ServiceLoader loader = ServiceLoader.load(CqServiceFactory.class);
    Iterator itr = loader.iterator();
    if (!itr.hasNext()) {
      factory = null;
    } else {
      factory = itr.next();
      factory.initialize();
    }
  }

  public static CqService create(GemFireCacheImpl cache) {

    if (factory == null) {
      return new MissingCqService();
    }

    return factory.create(cache);
  }

  public static ServerCQ readCq(DataInput in) throws ClassNotFoundException, IOException {
    if (factory == null) {
      throw new IllegalStateException("CqService is not available.");
    } else {
      return factory.readCqQuery(in);
    }

  }

  private CqServiceProvider() {

  }
}