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/*
* Copyright (C) 2013 Schlichtherle IT Services & Stimulus Software.
* All rights reserved. Use is subject to license terms.
*/
package net.java.trueupdate.manager.spec.cmd;
/**
* An interface for revertible commands with durable side effects.
* The use case is to compose revertible commands with durable side effects
* into larger revertible commands with durable side effects.
*
* For example updating an enterprise application in an application may be
* composed of a list of commands to undeploy, update and redeploy the
* enterprise application.
* Each of these commands needs to be revertible so that if for example the
* final redeployment step fails, the prior update and undeployment steps can
* get reverted in order.
* If each of the commands in the list has ACID properties, then this results
* in the composed command to have ACID properties too, and thus work similar
* to a transaction.
*
* Note that commands are generally not idempotent, that is once they have been
* successfully executed, you may not be able to successfully execute them
* again because the preconditions are no longer met.
* However, commands must be restartable, that is if {@link #perform()} has
* thrown an exception and {@link #revert()} has succeeded, then clients can
* still successfully execute the command as soon as the preconditions are met
* again.
*
* @see Commands#execute
* @see CompositeCommand
* @author Christian Schlichtherle
*/
public interface Command {
/**
* Performs this command, thereby creating some durable side effects.
* If this method throws an exception, then {@link #revert} must get called.
*/
void perform() throws Exception;
/**
* Reverts any durable side effects of the {@link #perform} method.
* If this method fails with an exception, then the state of the system may
* be corrupted.
*/
void revert() throws Exception;
}