/*
 * 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.samza.application;

import org.apache.samza.annotation.InterfaceStability;
import org.apache.samza.config.Config;
import org.apache.samza.operators.ContextManager;
import org.apache.samza.operators.MessageStream;
import org.apache.samza.operators.OutputStream;
import org.apache.samza.operators.StreamGraph;
import org.apache.samza.operators.functions.InitableFunction;
import org.apache.samza.task.StreamTask;
import org.apache.samza.task.TaskContext;

/**
 * Describes and initializes the transforms for processing message streams and generating results.
 * <p>
 * The following example removes page views older than 1 hour from the input stream:
 * <pre>{@code
 * public class PageViewCounter implements StreamApplication {
 *   public void init(StreamGraph graph, Config config) {
 *     MessageStream<PageViewEvent> pageViewEvents =
 *       graph.getInputStream("pageViewEvents", (k, m) -> (PageViewEvent) m);
 *     OutputStream<String, PageViewEvent, PageViewEvent> recentPageViewEvents =
 *       graph.getOutputStream("recentPageViewEvents", m -> m.memberId, m -> m);
 *
 *     pageViewEvents
 *       .filter(m -> m.getCreationTime() > System.currentTimeMillis() - Duration.ofHours(1).toMillis())
 *       .sendTo(filteredPageViewEvents);
 *   }
 * }
 * }</pre>
 *<p>
 * The example above can be run using an ApplicationRunner:
 * <pre>{@code
 *   public static void main(String[] args) {
 *     CommandLine cmdLine = new CommandLine();
 *     Config config = cmdLine.loadConfig(cmdLine.parser().parse(args));
 *     PageViewCounter app = new PageViewCounter();
 *     LocalApplicationRunner runner = new LocalApplicationRunner(config);
 *     runner.run(app);
 *     runner.waitForFinish();
 *   }
 * }</pre>
 * <p>
 * Implementation Notes: Currently StreamApplications are wrapped in a {@link StreamTask} during execution.
 * A new StreamApplication instance will be created and initialized when planning the execution, as well as for each
 * {@link StreamTask} instance used for processing incoming messages. Execution is synchronous and thread-safe
 * within each {@link StreamTask}.
 */
@InterfaceStability.Unstable
public interface StreamApplication {

  /**
   * Describes and initializes the transforms for processing message streams and generating results.
   * <p>
   * The {@link StreamGraph} provides access to input and output streams. Input {@link MessageStream}s can be
   * transformed into other {@link MessageStream}s or sent to an {@link OutputStream} using the {@link MessageStream}
   * operators.
   * <p>
   * Most operators accept custom functions for doing the transformations. These functions are {@link InitableFunction}s
   * and are provided the {@link Config} and {@link TaskContext} during their own initialization. The config and the
   * context can be used, for example, to create custom metrics or access durable state stores.
   * <p>
   * A shared context between {@link InitableFunction}s for different operators within a task instance can be set
   * up by providing a {@link ContextManager} using {@link StreamGraph#withContextManager}.
   *
   * @param graph the {@link StreamGraph} to get input/output streams from
   * @param config the configuration for the application
   */
  void init(StreamGraph graph, Config config);

}