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Beam IO for Google Cloud Pub/Sub Lite

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/*
 * Copyright 2020 Google LLC
 *
 * 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.google.cloud.pubsublite.beam;

import static com.google.cloud.pubsublite.internal.ExtractStatus.toCanonical;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.api.gax.rpc.ApiException;
import com.google.cloud.pubsublite.Offset;
import com.google.cloud.pubsublite.proto.ComputeMessageStatsResponse;
import com.google.common.base.Ticker;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.errorprone.annotations.concurrent.GuardedBy;
import java.util.concurrent.TimeUnit;
import javax.annotation.Nullable;

final class LimitingTopicBacklogReader implements TopicBacklogReader {
  private final TopicBacklogReader underlying;
  private final LoadingCache backlogCache;

  @GuardedBy("this")
  @Nullable
  private Offset currentRequestOffset = null;

  @SuppressWarnings("method.invocation.invalid")
  LimitingTopicBacklogReader(TopicBacklogReader underlying, Ticker ticker) {
    this.underlying = underlying;
    backlogCache =
        CacheBuilder.newBuilder()
            .ticker(ticker)
            .maximumSize(1)
            .expireAfterWrite(1, TimeUnit.MINUTES)
            .refreshAfterWrite(10, TimeUnit.SECONDS)
            .build(
                new CacheLoader() {
                  @Override
                  public ComputeMessageStatsResponse load(String val) {
                    return loadFromUnderlying();
                  }
                });
  }

  @SuppressWarnings("argument.type.incompatible")
  private synchronized ComputeMessageStatsResponse loadFromUnderlying() {
    return underlying.computeMessageStats(checkNotNull(currentRequestOffset));
  }

  @Override
  public synchronized ComputeMessageStatsResponse computeMessageStats(Offset offset)
      throws ApiException {
    currentRequestOffset = offset;
    try {
      // There is only a single entry in the cache.
      return backlogCache.get("cache");
    } catch (Throwable t) {
      throw toCanonical(t).underlying;
    }
  }

  @Override
  public void close() {
    underlying.close();
  }
}