content: /*
   * 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.state;
  
  /**
   * A {@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}.
   */
  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 implementation}.
     *
     * <p>The returned object should be independent of the underlying state. Any direct modification
     * of the returned object should not modify state without going through the appropriate state
     * interface, and modification to the state should not be mirrored in the returned object.
     */
    T read();
  
    /**
     * Indicate that the value will be read later.
     *
     * <p>This allows an 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();
  }
  