/*
 * 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.beam.sdk.util.state;

import org.apache.beam.sdk.annotations.Experimental;
import org.apache.beam.sdk.annotations.Experimental.Kind;

/**
 * A {@code StateContents} is produced by the read methods on all {@link State} objects.
 * Calling {@link #read} returns the associated value.
 *
 * <p>This class is similar to {@link java.util.concurrent.Future}, but each invocation of
 * {@link #read} need not return the same value.
 *
 * <p>Getting the {@code StateContents} from a read method indicates the desire to eventually
 * read a value. Depending on the runner this may or may not immediately start the read.
 *
 * @param <T> The type of value returned by {@link #read}.
 */
@Experimental(Kind.STATE)
public interface ReadableState<T> {
  /**
   * Read the current value, blocking until it is available.
   *
   * <p>If there will be many calls to {@link #read} for different state in short succession,
   * you should first call {@link #readLater} for all of them so the reads can potentially be
   * batched (depending on the underlying {@link StateInternals} implementation}.
   */
  T read();

  /**
   * Indicate that the value will be read later.
   *
   * <p>This allows a {@link StateInternals} implementation to start an asynchronous prefetch or
   * to include this state in the next batch of reads.
   *
   * @return this for convenient chaining
   */
  ReadableState<T> readLater();
}

/*
 * 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.beam.sdk.util.state;

import org.apache.beam.sdk.annotations.Experimental;
import org.apache.beam.sdk.annotations.Experimental.Kind;

/**
 * {@link State} that can be read via {@link #read()}.
 *
 * <p>Use {@link #readLater()} for marking several states for prefetching. Runners
 * can potentially batch these into one read.
 *
 * @param <T> The type of value returned by {@link #read}.
 */
@Experimental(Kind.STATE)
public interface ReadableState<T> {
  /**
   * Read the current value, blocking until it is available.
   *
   * <p>If there will be many calls to {@link #read} for different state in short succession,
   * you should first call {@link #readLater} for all of them so the reads can potentially be
   * batched (depending on the underlying {@link StateInternals} implementation}.
   */
  T read();

  /**
   * Indicate that the value will be read later.
   *
   * <p>This allows a {@link StateInternals} implementation to start an asynchronous prefetch or
   * to include this state in the next batch of reads.
   *
   * @return this for convenient chaining
   */
  ReadableState<T> readLater();
}

Fix Javadoc of ReadableState