Action.java

/**
 * 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.falcon.designer.primitive;

import org.apache.commons.lang.NotImplementedException;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.net.URI;
import java.util.Map;

/**
 * Actions are components of flows. They encapsulate an atomic
 * execution unit in a flow. If an action is dependent on another
 * action, then the action wont be started unless the preceding
 * action is entirely complete.
 *
 * Few examples of Actions are
 *   1. SSH Action, which executes a shell command on a remote host
 *   2. Data Transform Action, which executes a collection of data transformation
 *      as a single logical unit
 *
 * There are no restrictions imposed on an Action with respect to the input
 * or the output such as in a Transformation.
 */
public abstract class Action extends Primitive {

    protected final String type;

    protected Action(@Nonnull String type) {
        this.type = type;
    }

    @Nonnull
    public String getType() {
        return type;
    }

    /**
     * Indicates whether this action has any output or not. If this function
     * returns true, then {@link Action#outputKeys()} and {@link Action#outputFile()} ()}
     * needs to be implemented by the concrete class.
     *
     * @return - True if the action has output that any downstream control or action
     * can consume, false otherwise.
     */
    public abstract boolean hasOutput();

    /**
     * Are the keys that the action chooses to expose to the external world that
     * can either be used to manage the flow control or be used as an input in a subsequent
     * action within the context of a flow. Each key that may be output by this
     * action would have to declared along with its return type {@link java.sql.Types}.
     *
     * If an action output key isn't white listed through this with a specific type,
     * then default type is assumed {@link java.sql.Types#VARCHAR}. All output keys
     * are nullable. If null the flow control can assume default values (which may vary
     * depending on the data type. For ex: VARCHAR(""), NUMERIC(0), DATETIME(CURRENTTIME))
     *
     * @return - Output key name and its corresponding data type. If the output key
     * doesn't conform to this data type, the flow may fail at runtime or may safely assume
     * default value. The behavior is left to the implementation of the flow compiler
     * and scheduler.
     */
    @Nullable
    public Map<String, Integer> outputKeys() {
        if (hasOutput()) {
            throw new NotImplementedException(getClass() + "::outputKeys()");
        } else {
            return null;
        }
    }

    /**
     * Returns a file name as URI that can be mapped to generic hadoop file system
     * implementation.
     *
     * @return - Fully qualified file name uri as understood by the hadoop file system.
     */
    @Nullable
    public URI outputFile() {
        if (hasOutput()) {
            throw new NotImplementedException(getClass() + "::outputFile()");
        } else {
            return null;
        }
    }
}