code: /*
   * SPDX-License-Identifier: Apache-2.0
   * Copyright 2018-2020 The Feast Authors
   *
   * 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
   *
   *     https://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 feast.serving.connectors.redis.retriever;
  
  import com.google.common.collect.Lists;
  import feast.proto.serving.ServingAPIProto;
  import feast.proto.storage.RedisProto;
  import feast.proto.types.ValueProto;
  import feast.serving.connectors.Feature;
  import feast.serving.connectors.OnlineRetriever;
  import feast.serving.connectors.redis.common.RedisHashDecoder;
  import feast.serving.connectors.redis.common.RedisKeyGenerator;
  import io.lettuce.core.KeyValue;
  import java.nio.ByteBuffer;
  import java.util.*;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.Future;
  import java.util.stream.Collectors;
  
  public class RedisOnlineRetriever implements OnlineRetriever {
  
    private static final String timestampPrefix = "_ts";
    private final RedisClientAdapter redisClientAdapter;
    private final EntityKeySerializer keySerializer;
    private final String project;
  
    // Number of fields in request to Redis which requires using HGETALL instead of HMGET
    public static final int HGETALL_NUMBER_OF_FIELDS_THRESHOLD = 50;
  
    public RedisOnlineRetriever(
        String project, RedisClientAdapter redisClientAdapter, EntityKeySerializer keySerializer) {
      this.project = project;
      this.redisClientAdapter = redisClientAdapter;
      this.keySerializer = keySerializer;
    }
  
    @Override
    public List<List<Feature>> getOnlineFeatures(
        List<Map<String, ValueProto.Value>> entityRows,
        List<ServingAPIProto.FeatureReferenceV2> featureReferences,
        List<String> entityNames) {
  
      List<RedisProto.RedisKeyV2> redisKeys =
          RedisKeyGenerator.buildRedisKeys(this.project, entityRows);
      return getFeaturesFromRedis(redisKeys, featureReferences);
    }
  
    private List<List<Feature>> getFeaturesFromRedis(
        List<RedisProto.RedisKeyV2> redisKeys,
        List<ServingAPIProto.FeatureReferenceV2> featureReferences) {
      // To decode bytes back to Feature
      Map<ByteBuffer, Integer> byteToFeatureIdxMap = new HashMap<>();
  
      // Serialize using proto
      List<byte[]> binaryRedisKeys =
          redisKeys.stream().map(this.keySerializer::serialize).collect(Collectors.toList());
  
      List<byte[]> retrieveFields = new ArrayList<>();
      for (int idx = 0;
          idx < featureReferences.size();
          idx++) { // eg. murmur(<featuretable_name:feature_name>)
        byte[] featureReferenceBytes =
            RedisHashDecoder.getFeatureReferenceRedisHashKeyBytes(featureReferences.get(idx));
        retrieveFields.add(featureReferenceBytes);
  
        byteToFeatureIdxMap.put(ByteBuffer.wrap(featureReferenceBytes), idx);
      }
  
      featureReferences.stream()
          .map(ServingAPIProto.FeatureReferenceV2::getFeatureViewName)
          .distinct()
          .forEach(
              table -> {
                // eg. <_ts:featuretable_name>
                byte[] featureTableTsBytes =
                    RedisHashDecoder.getTimestampRedisHashKeyBytes(table, timestampPrefix);
  
                retrieveFields.add(featureTableTsBytes);
              });
  
      List<Future<Map<byte[], byte[]>>> futures =
          Lists.newArrayListWithExpectedSize(binaryRedisKeys.size());
  
      // Number of fields that controls whether to use hmget or hgetall was discovered empirically
      // Could be potentially tuned further
      if (retrieveFields.size() < HGETALL_NUMBER_OF_FIELDS_THRESHOLD) {
        byte[][] retrieveFieldsByteArray = retrieveFields.toArray(new byte[0][]);
  
        for (byte[] binaryRedisKey : binaryRedisKeys) {
          // Access redis keys and extract features
          futures.add(
              redisClientAdapter
                  .hmget(binaryRedisKey, retrieveFieldsByteArray)
                  .thenApply(
                      list ->
                          list.stream()
                              .filter(KeyValue::hasValue)
                              .collect(Collectors.toMap(KeyValue::getKey, KeyValue::getValue)))
                  .toCompletableFuture());
        }
  
      } else {
        for (byte[] binaryRedisKey : binaryRedisKeys) {
          futures.add(redisClientAdapter.hgetall(binaryRedisKey));
        }
      }
  
      List<List<Feature>> results = Lists.newArrayListWithExpectedSize(futures.size());
      for (Future<Map<byte[], byte[]>> f : futures) {
        try {
          results.add(
              RedisHashDecoder.retrieveFeature(
                  f.get(), byteToFeatureIdxMap, featureReferences, timestampPrefix));
        } catch (InterruptedException | ExecutionException e) {
          throw new RuntimeException("Unexpected error when pulling data from Redis");
        }
      }
  
      return results;
    }
  }
  