    001/*
    002 * Copyright (c) 2007-2016 Concurrent, Inc. All Rights Reserved.
    003 *
    004 * Project and contact information: http://www.cascading.org/
    005 *
    006 * This file is part of the Cascading project.
    007 *
    008 * Licensed under the Apache License, Version 2.0 (the "License");
    009 * you may not use this file except in compliance with the License.
    010 * You may obtain a copy of the License at
    011 *
    012 *     http://www.apache.org/licenses/LICENSE-2.0
    013 *
    014 * Unless required by applicable law or agreed to in writing, software
    015 * distributed under the License is distributed on an "AS IS" BASIS,
    016 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    017 * See the License for the specific language governing permissions and
    018 * limitations under the License.
    019 */
    020
    021package cascading.operation;
    022
    023import cascading.flow.FlowProcess;
    024import cascading.flow.planner.DeclaresResults;
    025import cascading.tuple.Fields;
    026
    027/**
    028 * Interface Operation is the base interface for all functions applied to {@link cascading.tuple.Tuple} streams.
    029 * <p/>
    030 * Specifically {@link Function}, {@link Filter}, {@link Aggregator}, {@link Buffer}, and {@link Assertion}.
    031 * <p/>
    032 * Use {@link BaseOperation} for a convenient way to create new Operation types.
    033 *
    034 * @see cascading.operation.BaseOperation
    035 * @see Function
    036 * @see Filter
    037 * @see Aggregator
    038 * @see Buffer
    039 * @see Assertion
    040 */
    041public interface Operation<Context> extends DeclaresResults
    042  {
    043  /** Field ANY denotes that a given Operation will take any number of argument values */
    044  int ANY = Integer.MAX_VALUE;
    045
    046  /**
    047   * The prepare method is called immediately before the current Operation instance is put into play processing Tuples.
    048   * This method should initialize any resources that can be shutdown or released in the
    049   * {@link #cleanup(cascading.flow.FlowProcess, OperationCall)} method.
    050   * <p/>
    051   * Any resources created should be stored in the {@code Context}, not as instance fields on the class.
    052   * <p/>
    053   * This method may be called more than once during the life of this instance. But it will never be called multiple times
    054   * without a cleanup invocation immediately before subsequent invocations.
    055   * <p/>
    056   * If the Flow this Operation instance belongs will execute on a remote cluster, this method will be called
    057   * cluster side, not client side.
    058   *
    059   * @param flowProcess
    060   * @param operationCall
    061   */
    062  void prepare( FlowProcess flowProcess, OperationCall<Context> operationCall );
    063
    064  /**
    065   * The flush method is called when an Operation that is caching values must empty the cache. It is called before
    066   * {@link #cleanup(cascading.flow.FlowProcess, OperationCall)} is invoked.
    067   * <p/>
    068   * It is safe to cast the {@link cascading.operation.OperationCall} to a {@link FunctionCall}, or equivalent, and
    069   * get its {@link cascading.operation.FunctionCall#getOutputCollector()}.
    070   *
    071   * @param flowProcess
    072   * @param operationCall
    073   */
    074  void flush( FlowProcess flowProcess, OperationCall<Context> operationCall );
    075
    076  /**
    077   * The cleanup method is called immediately after the current Operation instance is taken out of play processing Tuples.
    078   * This method should shutdown any resources created or initialized during the
    079   * {@link #prepare(cascading.flow.FlowProcess, OperationCall)} method.
    080   * <p/>
    081   * This method may be called more than once during the life of this instance. But it will never be called multiple times
    082   * without a prepare invocation before.
    083   * <p/>
    084   * If the Flow this Operation instance belongs will execute on a remote cluster, this method will be called
    085   * cluster side, not client side.
    086   *
    087   * @param flowProcess
    088   * @param operationCall
    089   */
    090  void cleanup( FlowProcess flowProcess, OperationCall<Context> operationCall );
    091
    092  /**
    093   * Returns the fields created by this Operation instance. If this instance is a {@link Filter}, it should always
    094   * return {@link Fields#ALL}.
    095   *
    096   * @return a Fields instance
    097   */
    098  Fields getFieldDeclaration();
    099
    100  /**
    101   * The minimum number of arguments this Operation expects from the calling {@link cascading.pipe.Each} or
    102   * {@link cascading.pipe.Every} Operator.
    103   * <p/>
    104   * Operations should be willing to receive more arguments than expected, but should ignore them if they are unused,
    105   * instead of failing.
    106   *
    107   * @return an int
    108   */
    109  int getNumArgs();
    110
    111  /**
    112   * Returns {@code true} if this Operation instance can safely execute on the same 'record' multiple
    113   * times, {@code false} otherwise.
    114   * <p/>
    115   * That is, this Operation is safe if it has no side-effects, or if it does, they are idempotent.
    116   * <p/>
    117   * If seeing the same 'record' more than once can cause errors (internally or externally),
    118   * this method must return {@code false}.
    119   *
    120   * @return a boolean
    121   */
    122  boolean isSafe();
    123  }