/*
* 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;
import org.apache.beam.sdk.annotations.Experimental;
import org.apache.beam.sdk.annotations.Experimental.Kind;
/**
* 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}.
*/
@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 implementation}.
*/
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();
}