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// Copyright 2019 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.functions;

/**
 * Represents a Cloud Function that is activated by an event and parsed into a user-supplied class.
 * The payload of the event is a JSON object, which is deserialized into a user-defined class as
 * described for Gson.
 *
 * Here is an example of an implementation that accesses the {@code messageId} property from a
 * payload that matches a user-defined {@code PubSubMessage} class:
 * 
 *
 * 
 * public class Example implements{@code BackgroundFunction} {
 *   private static final Logger logger = Logger.getLogger(Example.class.getName());
 *
 *  {@code @Override}
 *   public void accept(PubSubMessage pubSubMessage, Context context) {
 *     logger.info("Got messageId " + pubSubMessage.messageId);
 *   }
 * }
 *
 * // Where PubSubMessage is a user-defined class like this:
 * public class PubSubMessage {
 *   String data;
 *  {@code Map} attributes;
 *   String messageId;
 *   String publishTime;
 * }
 * 
 *
 * @param  the class of payload objects that this function expects.
 */
@FunctionalInterface
public interface BackgroundFunction {
  /**
   * Called to service an incoming event. This interface is implemented by user code to provide the
   * action for a given background function. If this method throws any exception (including any
   * {@link Error}) then the HTTP response will have a 500 status code.
   *
   * @param payload the payload of the event, deserialized from the original JSON string.
   * @param context the context of the event. This is a set of values that every event has,
   *     separately from the payload, such as timestamp and event type.
   * @throws Exception to produce a 500 status code in the HTTP response.
   */
  void accept(T payload, Context context) throws Exception;
}