content: /*
   * Copyright 2010 Proofpoint, 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 io.airlift.discovery.client;
  
  import com.google.common.collect.ImmutableList;
  import com.google.common.util.concurrent.FutureCallback;
  import com.google.common.util.concurrent.Futures;
  import com.google.common.util.concurrent.ListenableFuture;
  import com.google.common.util.concurrent.SettableFuture;
  import io.airlift.log.Logger;
  import io.airlift.units.Duration;
  import jakarta.annotation.PostConstruct;
  
  import java.util.List;
  import java.util.concurrent.Executor;
  import java.util.concurrent.ScheduledExecutorService;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicReference;
  
  import static com.google.common.base.Preconditions.checkState;
  import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
  import static io.airlift.discovery.client.DiscoveryAnnouncementClient.DEFAULT_DELAY;
  import static java.util.Objects.requireNonNull;
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  import static java.util.concurrent.TimeUnit.SECONDS;
  
  public class CachingServiceSelector
          implements ServiceSelector
  {
      private static final Logger log = Logger.get(CachingServiceSelector.class);
  
      private final String type;
      private final String pool;
      private final DiscoveryLookupClient lookupClient;
      private final AtomicReference<ServiceDescriptors> serviceDescriptors = new AtomicReference<>();
      private final ScheduledExecutorService executor;
  
      private final ExponentialBackOff errorBackOff;
  
      private final AtomicBoolean started = new AtomicBoolean(false);
  
      public CachingServiceSelector(String type, ServiceSelectorConfig selectorConfig, DiscoveryLookupClient lookupClient, ScheduledExecutorService executor)
      {
          requireNonNull(type, "type is null");
          requireNonNull(selectorConfig, "selectorConfig is null");
          requireNonNull(lookupClient, "client is null");
          requireNonNull(executor, "executor is null");
  
          this.type = type;
          this.pool = selectorConfig.getPool();
          this.lookupClient = lookupClient;
          this.executor = executor;
          this.errorBackOff = new ExponentialBackOff(
                  new Duration(1, MILLISECONDS),
                  new Duration(1, SECONDS),
                  String.format("Discovery server connect succeeded for refresh (%s/%s)", type, pool),
                  String.format("Cannot connect to discovery server for refresh (%s/%s)", type, pool),
                  log);
      }
  
      @PostConstruct
      public void start()
      {
          if (started.compareAndSet(false, true)) {
              checkState(!executor.isShutdown(), "CachingServiceSelector has been destroyed");
  
              // if discovery is available, get the initial set of servers before starting
              try {
                  refresh().get(1, TimeUnit.SECONDS);
              }
              catch (Exception ignored) {
              }
          }
      }
  
      @Override
      public String getType()
      {
          return type;
      }
  
      @Override
      public String getPool()
      {
          return pool;
      }
  
      @Override
      public List<ServiceDescriptor> selectAllServices()
      {
          ServiceDescriptors serviceDescriptors = this.serviceDescriptors.get();
          if (serviceDescriptors == null) {
              return ImmutableList.of();
          }
          return serviceDescriptors.getServiceDescriptors();
      }
  
      @Override
      public ListenableFuture<List<ServiceDescriptor>> refresh()
      {
          ServiceDescriptors oldDescriptors = this.serviceDescriptors.get();
  
          ListenableFuture<ServiceDescriptors> future;
          if (oldDescriptors == null) {
              future = lookupClient.getServices(type, pool);
          }
          else {
              future = lookupClient.refreshServices(oldDescriptors);
          }
  
          future = chainedCallback(future, new FutureCallback<ServiceDescriptors>()
          {
              @Override
              public void onSuccess(ServiceDescriptors newDescriptors)
              {
                  serviceDescriptors.set(newDescriptors);
                  errorBackOff.success();
  
                  Duration delay = newDescriptors.getMaxAge();
                  if (delay == null) {
                      delay = DEFAULT_DELAY;
                  }
                  scheduleRefresh(delay);
              }
  
              @Override
              public void onFailure(Throwable t)
              {
                  Duration duration = errorBackOff.failed(t);
                  scheduleRefresh(duration);
              }
          }, executor);
  
          return Futures.transform(future, ServiceDescriptors::getServiceDescriptors, directExecutor());
      }
  
      private void scheduleRefresh(Duration delay)
      {
          // already stopped?  avoids rejection exception
          if (executor.isShutdown()) {
              return;
          }
          executor.schedule(this::refresh, delay.toMillis(), TimeUnit.MILLISECONDS);
      }
  
      private static <V> ListenableFuture<V> chainedCallback(
              ListenableFuture<V> future,
              final FutureCallback<? super V> callback,
              Executor executor)
      {
          final SettableFuture<V> done = SettableFuture.create();
          Futures.addCallback(future, new FutureCallback<V>()
          {
              @Override
              public void onSuccess(V result)
              {
                  try {
                      callback.onSuccess(result);
                  }
                  finally {
                      done.set(result);
                  }
              }
  
              @Override
              public void onFailure(Throwable t)
              {
                  try {
                      callback.onFailure(t);
                  }
                  finally {
                      done.setException(t);
                  }
              }
          }, executor);
          return done;
      }
  }
  