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// 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
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// specific language governing permissions and limitations
// under the License.

package org.apache.impala.util;

import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;

import org.apache.impala.service.BackendConfig;
import org.apache.impala.thrift.TNetworkAddress;
import org.apache.impala.thrift.TUpdateExecutorMembershipRequest;
import com.google.common.collect.Sets;

/**
 * Singleton class that represents a snapshot of the Impalad executor membership. Host
 * membership is determined by both IP address and hostname (to mimic the backend's
 * Scheduler). A new snapshot is created whenever the cluster membership changes
 * so that clients don't need to hold a lock while examining a snapshot.
 */
public class ExecutorMembershipSnapshot {
  // The latest instance of the ExecutorMembershipSnapshot.
  private static AtomicReference<ExecutorMembershipSnapshot> cluster_ =
      new AtomicReference<ExecutorMembershipSnapshot>(new ExecutorMembershipSnapshot());

  // The set of hosts that are members of the cluster given by hostname. Can be empty when
  // using executor groups, in which case the planner shall assume that all reads are
  // remote.
  private final Set<String> hostnames_;

  // The set of hosts that are members of the cluster given by IP address. Can be empty
  // when using executor groups, in which case the planner shall assume that all reads are
  // remote.
  private final Set<String> ipAddresses_;

  // The number of executor nodes of the cluster.
  //
  // When not using executor groups, this value reflects the number of executors in the
  // cluster. It will be equal to hostnames_.size(), except in the test minicluster where
  // multiple impalads are running on a single host.

  // When using executor groups, this value reflects the number of executors in the
  // largest healthy group. If all group become unhealthy, the backend will not send a
  // membership update and this value will retain the last non-zero value. This allows the
  // planner to work on the assumption that a healthy executor group of the same size will
  // eventually come online to execute queries.
  private final int numExecutors_;

  // Used only to construct the initial ExecutorMembershipSnapshot.
  private ExecutorMembershipSnapshot() {
    hostnames_ = Sets.newHashSet();
    ipAddresses_ = Sets.newHashSet();
    // We use 0 for the number of executors to indicate that no update from the
    // ClusterMembershipManager has arrived yet and we will return the value
    // '-num_expected_executors' in numExecutors().
    numExecutors_ = 0;
  }

  // Construct a new snapshot based on the TUpdateExecutorMembershipRequest.
  private ExecutorMembershipSnapshot(TUpdateExecutorMembershipRequest request) {
    hostnames_ = request.getHostnames();
    ipAddresses_ = request.getIp_addresses();
    // If the updates snapshot does not contain any executors we fall back to the previous
    // value. This can happen if no healthy executor groups are currently online.
    if (request.getNum_executors() > 0) {
      numExecutors_ = request.getNum_executors();
    } else {
      numExecutors_ = cluster_.get().numExecutors_;
    }
  }

  // Determine whether a host, given either by IP address or hostname, is a member of
  // this snapshot.  Returns true if it is, false otherwise.
  public boolean contains(TNetworkAddress address) {
    String host = address.getHostname();
    return ipAddresses_.contains(host) || hostnames_.contains(host);
  }

  // Return the number of executors that should be used for planning. If no executors have
  // been registered so far, this method will return a configurable default to allow the
  // planner to operated based on the expected number of executors.
  public int numExecutors() {
    if (numExecutors_ == 0) {
      return BackendConfig.INSTANCE.getBackendCfg().num_expected_executors;
    }
    return numExecutors_;
  }

  // Atomically update the singleton snapshot instance.  After the update completes,
  // all calls to getCluster() will return the new snapshot.
  public static void update(TUpdateExecutorMembershipRequest request) {
    cluster_.set(new ExecutorMembershipSnapshot(request));
  }

  // Return the current singleton snapshot instance.
  public static ExecutorMembershipSnapshot getCluster() { return cluster_.get(); }
}