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.runners.fnexecution.state;
  
  import java.util.Collections;
  import java.util.concurrent.CompletionStage;
  import org.apache.beam.model.fnexecution.v1.BeamFnApi;
  import org.apache.beam.model.fnexecution.v1.BeamFnApi.StateRequest;
  import org.apache.beam.model.fnexecution.v1.BeamFnApi.StateResponse;
  
  /**
   * Handler for {@link org.apache.beam.model.fnexecution.v1.BeamFnApi.StateRequest StateRequests}.
   */
  public interface StateRequestHandler {
    /**
     * Handle a {@link org.apache.beam.model.fnexecution.v1.BeamFnApi.StateRequest} asynchronously.
     *
     * <p>The handler is allowed to complete the future within the callers thread if it can be
     * completed without blocking. Otherwise the caller should delegate to another thread to perform
     * any blocking work completing the future when able.
     *
     * <p>Throwing an error during handling will complete the handler result {@link CompletionStage}
     * exceptionally.
     */
    CompletionStage<StateResponse.Builder> handle(StateRequest request) throws Exception;
  
    /** Retrieves a list of valid cache tokens. */
    default Iterable<BeamFnApi.ProcessBundleRequest.CacheToken> getCacheTokens() {
      return Collections.emptyList();
    }
  
    static StateRequestHandler unsupported() {
      return request -> {
        throw new UnsupportedOperationException(
            String.format("Cannot use an empty %s", StateRequestHandler.class.getSimpleName()));
      };
    }
  }
  