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/*
 * The MIT License
 *
 * Copyright (c) 2010, InfraDNA, Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package hudson.model.queue;

import hudson.Extension;
import hudson.ExtensionList;
import hudson.ExtensionPoint;
import hudson.model.Hudson;
import hudson.model.Node;
import hudson.model.Queue;
import hudson.model.Queue.Task;

/**
 * Vetos the execution of a task on a node
 *
 * 
 * To register your dispatcher implementations, put @{@link Extension} on your subtypes.
 *
 * @author Kohsuke Kawaguchi
 * @since 1.360
 */
public abstract class QueueTaskDispatcher implements ExtensionPoint {
    /**
     * Called whenever {@link Queue} is considering to execute the given task on a given node.
     *
     * 

     * Implementations can return null to indicate that the assignment is fine, or it can return
     * a non-null instance to block the execution of the task on the given node.
     *
     * 

     * Queue doesn't remember/cache the response from dispatchers, and instead it'll keep asking.
     * The upside of this is that it's very easy to block execution for a limited time period (
     * as you just need to return null when it's ready to execute.) The downside of this is that
     * the decision needs to be made quickly.
     *
     * 
     * Vetos are additive. When multiple {@link QueueTaskDispatcher}s are in the system,
     * the task won't run on the given node if any one of them returns a non-null value.
     * (This relationship is also the same with built-in check logic.)
     */
    public abstract CauseOfBlockage canTake(Node node, Task task);

    /**
     * All registered {@link QueueTaskDispatcher}s.
     */
    public static ExtensionList all() {
        return Hudson.getInstance().getExtensionList(QueueTaskDispatcher.class);
    }
}