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.transforms.splittabledofn;
  
  import org.apache.beam.sdk.transforms.DoFn;
  import org.joda.time.Instant;
  
  /**
   * A {@link WatermarkEstimator} which is used for estimating output watermarks of a splittable
   * {@link DoFn}. See {@link WatermarkEstimators} for commonly used watermark estimators.
   */
  public interface WatermarkEstimator<WatermarkEstimatorStateT> {
    /**
     * Return estimated output watermark. This method must return monotonically increasing watermarks
     * across instances that are constructed from prior state.
     */
    Instant currentWatermark();
  
    /**
     * Get current state of the {@link WatermarkEstimator} instance, which can be used to recreate the
     * {@link WatermarkEstimator} when processing the restriction. See {@link
     * DoFn.NewWatermarkEstimator} for additional details.
     *
     * <p>The internal state of the estimator must not be mutated by this method.
     *
     * <p>The state returned must not be mutated.
     */
    WatermarkEstimatorStateT getState();
  }
  